Article records
https://feeds.library.caltech.edu/people/Low-S-H/article.rss
A Caltech Library Repository Feedhttp://www.rssboard.org/rss-specificationpython-feedgenenTue, 16 Apr 2024 13:53:23 +0000A new approach to service provisioning in ATM networks
https://resolver.caltech.edu/CaltechAUTHORS:LOWieeeacmtn93
Authors: {'items': [{'id': 'Low-S-H', 'name': {'family': 'Low', 'given': 'Steven L.'}, 'orcid': '0000-0001-6476-3048'}, {'id': 'Varaiya-P-P', 'name': {'family': 'Varaiya', 'given': 'Pravin P.'}}]}
Year: 1993
DOI: 10.1109/90.251913
The authors formulate and solve a problem of allocating resources among competing services differentiated by user traffic characteristics and maximum end-to-end delay. The solution leads to an alternative approach to service provisioning in an ATM network, in which the network offers directly for rent its bandwidth and buffers and users purchase freely resources to meet their desired quality. Users make their decisions based on their own traffic parameters and delay requirements and the network sets prices for those resources. The procedure is iterative in that the network periodically adjusts prices based on monitored user demand, and is decentralized in that only local information is needed for individual users to determine resource requests. The authors derive the network's adjustment scheme and the users' decision rule and establish their optimality. Since the approach does not require the network to know user traffic and delay parameters, it does not require traffic policing on the part of the network.https://authors.library.caltech.edu/records/5zskg-9ef63Anonymous credit cards and their collusion analysis
https://resolver.caltech.edu/CaltechAUTHORS:LOWieeeacmtn96
Authors: {'items': [{'id': 'Low-S-H', 'name': {'family': 'Low', 'given': 'Steven H.'}, 'orcid': '0000-0001-6476-3048'}, {'id': 'Maxemchuk-N-F', 'name': {'family': 'Maxemcheuk', 'given': 'Nicholas F.'}}, {'id': 'Paul-S', 'name': {'family': 'Paul', 'given': 'Sanjoy'}}]}
Year: 1996
DOI: 10.1109/90.556339
Communications networks are traditionally used to bring information together. They can also be used to keep information apart in order to protect personal privacy. A cryptographic protocol specifies a process by which some information is transferred among some users and hidden from others. We show how to implement anonymous credit cards using simple cryptographic protocols. We pose, and solve, a collusion problem which determines whether it is possible for a subset of users to discover information that is designed to be hidden from them during or after execution of the anonymous credit card protocol.https://authors.library.caltech.edu/records/9y97e-2he88Copyright protection for the electronic distribution of text documents
https://resolver.caltech.edu/CaltechAUTHORS:BRAprocieee99
Authors: {'items': [{'id': 'Brassil-J-T', 'name': {'family': 'Brassil', 'given': 'Jack T.'}}, {'id': 'Low-S-H', 'name': {'family': 'Low', 'given': 'Steven'}, 'orcid': '0000-0001-6476-3048'}, {'id': 'Maxemchuk-N', 'name': {'family': 'Maxemchuk', 'given': 'Nicholas F.'}}]}
Year: 1999
DOI: 10.1109/5.771071
Each copy of a text document can be made different in a nearly invisible way by repositioning or modifying the appearance of different elements of text, i.e., lines, words, or characters. A unique copy can be registered with its recipient, so that subsequent unauthorized copies that are retrieved can be traced back to the original owner.
In this paper we describe and compare several mechanisms for marking documents and several other mechanisms for decoding the marks after documents have been subjected to common types of distortion. The marks are intended to protect documents of limited value that are owned by individuals who would rather possess a legal than an illegal copy if they can be distinguished. We will describe attacks that remove the marks and countermeasures to those attacks.
An architecture is described for distributing a large number of copies without burdening the publisher with creating and transmitting the unique documents. The architecture also allows the publisher to determine the identity of a recipient who has illegally redistributed the document, without compromising the privacy of individuals who are not operating illegally.
Two experimental systems are described. One was used to distribute an issue of the IEEE JOURNAL ON SELECTED AREAS IN COMMUNICATIONS, and the second was used to mark copies of company private memoranda.https://authors.library.caltech.edu/records/k4nww-k6n74Optimization flow control -- I: Basic algorithm and convergence
https://resolver.caltech.edu/CaltechAUTHORS:LOWieeeacmtn99
Authors: {'items': [{'id': 'Low-S-H', 'name': {'family': 'Low', 'given': 'Steven L.'}, 'orcid': '0000-0001-6476-3048'}, {'id': 'Lapsley-D-E', 'name': {'family': 'Lapsley', 'given': 'David E.'}}]}
Year: 1999
DOI: 10.1109/90.811451
We propose an optimization approach to flow control where the objective is to maximize the aggregate source utility over their transmission rates. We view network links and sources as processors of a distributed computation system to solve the dual problem using a gradient projection algorithm. In this system, sources select transmission rates that maximize their own benefits, utility minus bandwidth cost, and network links adjust bandwidth prices to coordinate the sources' decisions. We allow feedback delays to be different, substantial, and time varying, and links and sources to update at different times and with different frequencies. We provide asynchronous distributed algorithms and prove their convergence in a static environment. We present measurements obtained from a preliminary prototype to illustrate the convergence of the algorithm in a slowly time-varying environment. We discuss its fairness property.https://authors.library.caltech.edu/records/wm1h1-1nq41Equilibrium bandwidth and buffer allocations for elastic traffics
https://resolver.caltech.edu/CaltechAUTHORS:LOWieeeacmtn00
Authors: {'items': [{'id': 'Low-S-H', 'name': {'family': 'Low', 'given': 'Steven L.'}, 'orcid': '0000-0001-6476-3048'}]}
Year: 2000
DOI: 10.1109/90.851983
Consider a set of users sharing a network node under an allocation scheme that provides each user with a fixed minimum and a random extra amount of bandwidth and buffer. Allocations and prices are adjusted to adapt to resource availability and user demands. Equilibrium is achieved when all users optimize their utility and demand equals supply for nonfree resources. We analyze two models of user behavior. We show that at equilibrium expected return on purchasing variable resources can be higher than that on fixed resources. Thus users must balance the marginal increase in utility due to higher return on variable resources and the marginal decrease in utility due to their variability. For the first user model we further show that at equilibrium where such tradeoff is optimized all users hold strictly positive amounts of variable bandwidth and buffer. For the second model we show that if both variable bandwidth and buffer are scarce then at equilibrium every user either holds both variable resources or none.https://authors.library.caltech.edu/records/wk0ap-hrn72Capacity of text marking channel
https://resolver.caltech.edu/CaltechAUTHORS:20170810-135348454
Authors: {'items': [{'id': 'Low-S-H', 'name': {'family': 'Low', 'given': 'Steven H.'}, 'orcid': '0000-0001-6476-3048'}, {'id': 'Maxemchuk-N-F', 'name': {'family': 'Maxemchuk', 'given': 'Nicholas F.'}}]}
Year: 2000
DOI: 10.1109/97.883364
We have proposed earlier watermarking text documents by slightly shifting certain text lines. Such a text line represents a noisy channel, and marking represents the transmission of a signal through this channel. The power of the signal represents the size of the shift and must be small for the marks to be imperceptible. We formulate the channel capacity under a constraint or individual signal power. We show that to achieve the capacity, the shifts should be normally distributed, have maximum power, and adjacent shifts should be negatively correlated.https://authors.library.caltech.edu/records/9tk6r-xy760Convergence of REM flow control at a single link
https://resolver.caltech.edu/CaltechAUTHORS:YINieeecl01
Authors: {'items': [{'id': 'Yin-Q', 'name': {'family': 'Yin', 'given': 'Qinghe'}}, {'id': 'Low-S-H', 'name': {'family': 'Low', 'given': 'Steven H.'}, 'orcid': '0000-0001-6476-3048'}]}
Year: 2001
DOI: 10.1109/4234.913159
Various TCP congestion control schemes can be interpreted as approximately carrying out a certain basic algorithm to maximize aggregate source utility, different schemes corresponding to different choices of utility functions. The basic algorithm consists of a link algorithm that updates a congestion measure based on its traffic load, and a source algorithm that adapts the source rate to congestion in its path. Though convergent, this algorithm can lead to large equilibrium backlogs. This problem can be eliminated by modifying the basic algorithm to include backlog in the update of the congestion measure. This article proves that the modified algorithm converges when the network can be modeled as a single bottleneck link. Moreover, in equilibrium, the source rates are matched to the link capacity while the buffer is cleared.https://authors.library.caltech.edu/records/9f94n-e7g45REM: active queue management
https://resolver.caltech.edu/CaltechAUTHORS:ATHieeen01
Authors: {'items': [{'id': 'Athuraliya-S', 'name': {'family': 'Athuraliya', 'given': 'Sanjeewa'}}, {'id': 'Low-S-H', 'name': {'family': 'Low', 'given': 'Steven H.'}, 'orcid': '0000-0001-6476-3048'}, {'id': 'Li-Victor-H', 'name': {'family': 'Li', 'given': 'Victor H.'}}, {'id': 'Yin-Qinghe', 'name': {'family': 'Yin', 'given': 'Qinghe'}}]}
Year: 2001
DOI: 10.1109/65.923940
We describe a new active queue management scheme, random exponential marking (REM), that aims to achieve both high utilization and negligible loss and delay in a simple and scalable manner. The key idea is to decouple the congestion measure from the performance measure such as loss, queue length, or delay. While the congestion measure indicates excess demand for bandwidth and must track the number of users, the performance measure should be stabilized around their targets independent of the number of users. We explain the design rationale behind REM and present simulation results of its performance in wireline and wireless networks.https://authors.library.caltech.edu/records/qefkj-hpb45Understanding TCP Vegas: a duality model
https://resolver.caltech.edu/CaltechAUTHORS:20161129-164526605
Authors: {'items': [{'id': 'Low-S-H', 'name': {'family': 'Low', 'given': 'Steven H.'}, 'orcid': '0000-0001-6476-3048'}, {'id': 'Peterson-L-L', 'name': {'family': 'Peterson', 'given': 'Larry'}}, {'id': 'Wang-Limin', 'name': {'family': 'Wang', 'given': 'Limin'}}]}
Year: 2001
DOI: 10.1145/384268.378787
This paper presents a model of the TCP Vegas congestion control mechanism as a distributed optimization algorithm. Doing so has three important benefits. First, it helps us gain a fundamental understanding of why TCP Vegas works, and an appreciation of its limitations. Second, it allows us to prove that Vegas stabilizes at a weighted proportionally fair allocation of network capacity when there is sufficient buffering in the network. Third, it suggests how we might use explicit feedback to allow each Vegas source to determine the optimal sending rate when there is insufficient buffering in the network. We present simulation results that validate our conclusions.https://authors.library.caltech.edu/records/99139-9yv82Internet congestion control
https://resolver.caltech.edu/CaltechAUTHORS:LOWieeecsm02
Authors: {'items': [{'id': 'Low-S-H', 'name': {'family': 'Low', 'given': 'Steven H.'}, 'orcid': '0000-0001-6476-3048'}, {'id': 'Paganini-F', 'name': {'family': 'Paganini', 'given': 'Fernando'}}, {'id': 'Doyle-J-C', 'name': {'family': 'Doyle', 'given': 'John C.'}, 'orcid': '0000-0002-1828-2486'}]}
Year: 2002
DOI: 10.1109/37.980245
This article reviews the current transmission control protocol (TCP) congestion control protocols and overviews recent advances that have brought analytical tools to this problem. We describe an optimization-based framework that provides an interpretation of various flow control mechanisms, in particular, the utility being optimized by the protocol's equilibrium structure. We also look at the dynamics of TCP and employ linear models to exhibit stability limitations in the predominant TCP versions, despite certain built-in compensations for delay. Finally, we present a new protocol that overcomes these limitations and provides stability in a way that is scalable to arbitrary networks, link capacities, and delays.https://authors.library.caltech.edu/records/yx9qg-arn69Understanding TCP Vegas: A Duality Model
https://resolver.caltech.edu/CaltechAUTHORS:20111101-161123893
Authors: {'items': [{'id': 'Low-S-H', 'name': {'family': 'Low', 'given': 'Steven H.'}, 'orcid': '0000-0001-6476-3048'}, {'id': 'Peterson-L-L', 'name': {'family': 'Peterson', 'given': 'Larry L.'}}, {'id': 'Wang-Limin', 'name': {'family': 'Wang', 'given': 'Limin'}}]}
Year: 2002
DOI: 10.1145/506147.506152
We view congestion control as a distributed primal--dual algorithm carried out by sources and links over a network to solve a global optimization problem. We describe a multilink multisource model of the TCP Vegas congestion control mechanism. The model provides a fundamental understanding of delay, fairness and loss properties of TCP Vegas. It implies that Vegas stabilizes around a weighted proportionally fair allocation of network capacity when there is sufficient buffering in the network. It clarifies the mechanism through which persistent congestion may arise and its consequences, and suggests how we might use REM active queue management to prevent it. We present simulation results that validate our conclusions.https://authors.library.caltech.edu/records/nb5vf-9r853High-density model for server allocation and placement
https://resolver.caltech.edu/CaltechAUTHORS:20160812-143232076
Authors: {'items': [{'id': 'Cameron-C-W', 'name': {'family': 'Cameron', 'given': 'Craig W.'}}, {'id': 'Low-S-H', 'name': {'family': 'Low', 'given': 'Steven H.'}, 'orcid': '0000-0001-6476-3048'}, {'id': 'Wei-Xiaoliang-David', 'name': {'family': 'Wei', 'given': 'David X.'}}]}
Year: 2002
DOI: 10.1145/511399.511354
It is well known that optimal server placement is NP-hard. We present an approximate model for the case when both clients and servers are dense, and propose a simple server allocation and placement algorithm based on high-rate vector quantization theory. The key idea is to regard the location of a request as a random variable with probability density that is proportional to the demand at that
location, and the problem of server placement as source coding, i.e., to optimally map a source value (request location) to a codeword (server location) to minimize distortion (network cost). This view has led to a joint server allocation and placement algorithm that has a time-complexity that is linear in the number of clients.
Simulations are presented to illustrate its performance.https://authors.library.caltech.edu/records/ataa9-0re45Using AQM to improve TCP performance over wireless networks
https://resolver.caltech.edu/CaltechAUTHORS:20170810-133902128
Authors: {'items': [{'id': 'Li-Victor-H', 'name': {'family': 'Li', 'given': 'Victor H.'}}, {'id': 'Liu-Zhi-Qiang', 'name': {'family': 'Liu', 'given': 'Zhi-Qiang'}}, {'id': 'Low-S-H', 'name': {'family': 'Low', 'given': 'Steven H.'}, 'orcid': '0000-0001-6476-3048'}]}
Year: 2002
DOI: 10.1117/12.473023
TCP flow control algorithms have been designed for wireline networks where congestion is measured by packet loss due to buffer overflow. However, wireless networks also suffer from significant packet losses due to bit errors and handoffs. TCP responds to all the packet losses by invoking congestion control and avoidance algorithms and this results in degraded end-to-end performance in wireless networks. In this paper, we describe an Wireless Random Exponential Marking(WREM) scheme which effectively improves TCP performance over wireless networks by decoupling loss recovery from congestion control. Moreover, WREM is capable of handling the coexistence of both ECN-Capable and Non-ECN-Capable routers. We present simulation results to show its effectiveness and compatibility.https://authors.library.caltech.edu/records/gf8sj-w0m62Optimal flow control and routing in multi-path networks
https://resolver.caltech.edu/CaltechAUTHORS:20170810-112123357
Authors: {'items': [{'id': 'Wang-Wei-Hua', 'name': {'family': 'Wang', 'given': 'Wei-Hua'}}, {'id': 'Palaniswami-M', 'name': {'family': 'Palaniswami', 'given': 'Marimuthu'}}, {'id': 'Low-S-H', 'name': {'family': 'Low', 'given': 'Steven H.'}, 'orcid': '0000-0001-6476-3048'}]}
Year: 2003
DOI: 10.1016/S0166-5316(02)00176-1
We propose two flow control algorithms for networks with multiple paths between each source–destination pair. Both are distributed algorithms over the network to maximize aggregate source utility. Algorithm 1 is a first order Lagrangian method applied to a modified objective function that has the same optimal solution as the original objective function but has a better convergence property. Algorithm 2 is based on the idea that, at optimality, only paths with the minimum price carry positive flows, and naturally decomposes the overall decision into flow control (determines total transmission rate based on minimum path price) and routing (determines how to split the flow among available paths). Both algorithms can be implemented as simply a source-based mechanism in which no link algorithm nor feedback is needed. We present numerical examples to illustrate their behavior.https://authors.library.caltech.edu/records/svz98-q0y31Maximum and asymptotic UDP throughput under CHOKe
https://resolver.caltech.edu/CaltechAUTHORS:20170104-160731603
Authors: {'items': [{'id': 'Wang-Jintao', 'name': {'family': 'Wang', 'given': 'Jiantao'}}, {'id': 'Tang-Ao', 'name': {'family': 'Tang', 'given': 'Ao'}, 'orcid': '0000-0001-6296-644X'}, {'id': 'Low-S-H', 'name': {'family': 'Low', 'given': 'Steven H.'}, 'orcid': '0000-0001-6476-3048'}]}
Year: 2003
DOI: 10.1145/885651.781038
A recently proposed active queue management, CHOKe, aims to protect TCP from UDP flows. Simulations have shown that as UDP rate increases, its bandwidth share initially rises but eventually drops. We derive an approximate model of CHOKe and show that, provided the number of TCP flows is large, the UDP bandwidth share peaks at (e+1)^-1 = 0.269 when the UDP input rate is slightly larger than the link capacity, and drops to zero as UDP input rate tends to infinity, regardless of the TCP algorithm.https://authors.library.caltech.edu/records/7shkx-rh353A duality model of TCP and queue management algorithms
https://resolver.caltech.edu/CaltechAUTHORS:LOWieeeacmtn03
Authors: {'items': [{'id': 'Low-S-H', 'name': {'family': 'Low', 'given': 'Steven H.'}, 'orcid': '0000-0001-6476-3048'}]}
Year: 2003
DOI: 10.1109/TNET.2003.815297
We propose a duality model of end-to-end congestion control and apply it to understanding the equilibrium properties of TCP and active queue management schemes. The basic idea is to regard source rates as primal variables and congestion measures as dual variables, and congestion control as a distributed primal-dual algorithm over the Internet to maximize aggregate utility subject to capacity constraints. The primal iteration is carried out by TCP algorithms such as Reno or Vegas, and the dual iteration is carried out by queue management algorithms such as DropTail, RED or REM. We present these algorithms and their generalizations, derive their utility functions, and study their interaction.https://authors.library.caltech.edu/records/f7wwm-d3n30Necessary and sufficient conditions for optimal flow control in multirate multicast networks
https://resolver.caltech.edu/CaltechAUTHORS:20170810-112123268
Authors: {'items': [{'id': 'Wang-W-H', 'name': {'family': 'Wang', 'given': 'W.-H.'}}, {'id': 'Palaniswami-M', 'name': {'family': 'Palaniswami', 'given': 'M.'}}, {'id': 'Low-S-H', 'name': {'family': 'Low', 'given': 'S. H.'}, 'orcid': '0000-0001-6476-3048'}]}
Year: 2003
DOI: 10.1049/ip-com:20030591
The authors consider the optimal flow control problem in multirate multicast networks where all receivers of the same multicast group can receive service at different rates with different QoS. The objective is to achieve the fairness transmission rates that maximise the total receiver utility under the capacity constraint of links. They first propose necessary and sufficient conditions for the optimal solution to the problem, and then derive a new optimal flow control strategy using the Lagrangian multiplier method. Like the unicast case, the basic algorithm consists of a link algorithm to update the link price, and a receiver algorithm to adapt the transmission rate according to the link prices along its path. In particular if some groups contain only one receiver and become unicast, the algorithm will degrade to their previously proposed unicast algorithm.https://authors.library.caltech.edu/records/dx89v-s8s24Linear stability of TCP/RED and a scalable control
https://resolver.caltech.edu/CaltechAUTHORS:20170810-140056148
Authors: {'items': [{'id': 'Low-S-H', 'name': {'family': 'Low', 'given': 'Steven H.'}, 'orcid': '0000-0001-6476-3048'}, {'id': 'Paganini-F', 'name': {'family': 'Paganini', 'given': 'Fernando'}}, {'id': 'Wang-Jintao', 'name': {'family': 'Wang', 'given': 'Jiantao'}}, {'id': 'Doyle-J-C', 'name': {'family': 'Doyle', 'given': 'John C.'}, 'orcid': '0000-0002-1828-2486'}]}
Year: 2003
DOI: 10.1016/S1389-1286(03)00304-9
We demonstrate that the dynamic behavior of queue and average window is determined predominantly by the stability of TCP/RED, not by AIMD probing nor noise traffic. We develop a general multi-link multi-source model for TCP/RED and derive a local stability condition in the case of a single link with heterogeneous sources. We validate our model with simulations and illustrate the stability region of TCP/RED. These results suggest that TCP/RED becomes unstable when delay increases, or more strikingly, when link capacity increases. The analysis illustrates the difficulty of setting RED parameters to stabilize TCP: they can be tuned to improve stability, but only at the cost of large queues even when they are dynamically adjusted. Finally, we present a simple distributed congestion control algorithm that maintains stability for arbitrary network delay, capacity, load and topology.https://authors.library.caltech.edu/records/x4tq8-fws05A mathematical framework for designing a low-loss, low-delay internet
https://resolver.caltech.edu/CaltechAUTHORS:20170810-135831815
Authors: {'items': [{'id': 'Low-S-H', 'name': {'family': 'Low', 'given': 'Steven H.'}, 'orcid': '0000-0001-6476-3048'}, {'id': 'Srikant-R', 'name': {'family': 'Srikant', 'given': 'R.'}}]}
Year: 2004
DOI: 10.1023/B:NETS.0000015657.07692.be
We survey some recent results on modeling, analysis and design of congestion control schemes for the Internet. Using tools from convex optimization and control theory, we show that congestion controllers can be viewed as distributed algorithms for achieving fair resource allocation among competing sources. We illustrate the use of simple mathematical models to analyze the behavior of currently deployed Internet congestion control protocols as well as to design new protocols for networks with large capacities, delays and general topology. These new protocols are designed to nearly eliminate loss and queueing delay in the Internet, yet achieving high utilization and any desired fairness.https://authors.library.caltech.edu/records/qs06e-te943Allocating commodity resources in aggregate traffic networks
https://resolver.caltech.edu/CaltechAUTHORS:20170810-111128809
Authors: {'items': [{'id': 'Duffield-N-G', 'name': {'family': 'Duffield', 'given': 'N. G.'}}, {'id': 'Low-S-H', 'name': {'family': 'Low', 'given': 'S. H.'}, 'orcid': '0000-0001-6476-3048'}]}
Year: 2004
DOI: 10.1016/j.peva.2003.10.009
We examine the relation between cost and quality in networks which carry aggregate traffic. A powerful tool in this is the burstiness or indifference curve associated with a stochastic traffic flow. We relate burstiness to quality and use this relation to explore the quality experienced by aggregated flows under various rules for allocating resources to them. An example is motivated by the controlled load service specification. We show how the imposition of costs associated with buffer space and service capacity leads to the notion of a cost-optimal allocation of resources. This defines the cheapest operating point in a network where resources are commodities to be purchased as necessary to satisfy quality requirements. We define a notion of cost-based admission control: a linear admission rule which can be based on declared or measured traffic parameters.https://authors.library.caltech.edu/records/21kc5-1ea55Understanding CHOKe: throughput and spatial characteristics
https://resolver.caltech.edu/CaltechAUTHORS:TANieeeacmtn04
Authors: {'items': [{'id': 'Tang-Ao', 'name': {'family': 'Tang', 'given': 'Ao'}, 'orcid': '0000-0001-6296-644X'}, {'id': 'Wang-Jintao', 'name': {'family': 'Wang', 'given': 'Jiantao'}}, {'id': 'Low-S-H', 'name': {'family': 'Low', 'given': 'Steven H.'}, 'orcid': '0000-0001-6476-3048'}]}
Year: 2004
DOI: 10.1109/TNET.2004.833162
A recently proposed active queue management, CHOKe, is stateless, simple to implement, yet surprisingly effective in protecting TCP from UDP flows. We present an equilibrium model of TCP/CHOKe. We prove that, provided the number of TCP flows is large, the UDP bandwidth share peaks at (e+1)/sup -1/=0.269 when UDP input rate is slightly larger than link capacity, and drops to zero as UDP input rate tends to infinity. We clarify the spatial characteristics of the leaky buffer under CHOKe that produce this throughput behavior. Specifically, we prove that, as UDP input rate increases, even though the total number of UDP packets in the queue increases, their spatial distribution becomes more and more concentrated near the tail of the queue, and drops rapidly to zero toward the head of the queue. In stark contrast to a nonleaky FIFO buffer where UDP bandwidth shares would approach 1 as its input rate increases without bound, under CHOKe, UDP simultaneously maintains a large number of packets in the queue and receives a vanishingly small bandwidth share, the mechanism through which CHOKe protects TCP flows.https://authors.library.caltech.edu/records/ykxe2-5ev09The UltraLight Project: The Network as an Integrated and Managed Resource in Grid Systems for High Energy Physics and Data Intensive Science
https://resolver.caltech.edu/CaltechCACR:2005.109
Authors: {'items': [{'id': 'Newman-H-B', 'name': {'family': 'Newman', 'given': 'Harvey'}, 'orcid': '0000-0003-0964-1480'}, {'id': 'Bunn-Julian-J', 'name': {'family': 'Bunn', 'given': 'Julian'}, 'orcid': '0000-0002-3798-298X'}, {'id': 'Cavanaugh-R', 'name': {'family': 'Cavanaugh', 'given': 'Richard'}}, {'id': 'Legrand-I', 'name': {'family': 'Legrand', 'given': 'Iosif'}}, {'id': 'Low-S-H', 'name': {'family': 'Low', 'given': 'Steven'}, 'orcid': '0000-0001-6476-3048'}, {'id': 'McKee-S', 'name': {'family': 'McKee', 'given': 'Shawn'}}, {'id': 'Nae-D', 'name': {'family': 'Nae', 'given': 'Dan'}}, {'id': 'Ravot-S', 'name': {'family': 'Ravot', 'given': 'Sylvan'}}, {'id': 'Steenberg-C-D', 'name': {'family': 'Steenberg', 'given': 'Conrad'}}, {'id': 'Su-X', 'name': {'family': 'Su', 'given': 'Xun'}}, {'id': 'Thomas-Michael', 'name': {'family': 'Thomas', 'given': 'Michael'}}, {'id': 'van-Lingen-Frank', 'name': {'family': 'van Lingen', 'given': 'Frank'}}, {'id': 'Xia-Y', 'name': {'family': 'Xia', 'given': 'Yang'}}]}
Year: 2005
We describe the NSF-funded UltraLight project. The project's goal is to meet the data-intensive computing challenges of the next generation of particle physics experiments with a comprehensive, network-focused agenda. In particular we argue that instead of treating the network traditionally, as a static, unchanging and unmanaged set of inter-computer links, we instead will use it as a dynamic, configurable, and closely monitored resource, managed end-to-end, to construct a next-generation global system able to meet the data processing, distribution, access and analysis needs of the high energy physics (HEP) community. While the initial UltraLight implementation and services architecture is being developed to serve HEP, we expect many of UltraLight's developments in the areas of networking, monitoring, management, and collaborative research, to be applicable to many fields of data intensive e-science. In this paper we give an overview of, and motivation for the UltraLight project, and provide early results within different working areas of the project.https://authors.library.caltech.edu/records/eysap-48c12FAST TCP: from theory to experiments
https://resolver.caltech.edu/CaltechAUTHORS:JINieeen05
Authors: {'items': [{'id': 'Jin-Cheng', 'name': {'family': 'Jin', 'given': 'Cheng'}}, {'id': 'Wei-Xiaoliang-David', 'name': {'family': 'Wei', 'given': 'David X.'}}, {'id': 'Low-S-H', 'name': {'family': 'Low', 'given': 'Steven H.'}, 'orcid': '0000-0001-6476-3048'}, {'id': 'Bunn-J', 'name': {'family': 'Bunn', 'given': 'Julian'}, 'orcid': '0000-0002-3798-298X'}, {'id': 'Choe-Hyojeong-D', 'name': {'family': 'Choe', 'given': 'Hyojeong D.'}}, {'id': 'Doyle-J-C', 'name': {'family': 'Doyle', 'given': 'John C.'}, 'orcid': '0000-0002-1828-2486'}, {'id': 'Newman-H-B', 'name': {'family': 'Newman', 'given': 'Harvey'}, 'orcid': '0000-0003-0964-1480'}, {'id': 'Ravot-Sylvain', 'name': {'family': 'Ravot', 'given': 'Sylvain'}}, {'id': 'Singh-Suresh', 'name': {'family': 'Singh', 'given': 'Suresh'}}, {'id': 'Paganini-Fernando', 'name': {'family': 'Paganini', 'given': 'Fernando'}}, {'id': 'Buhrmaster-Gary', 'name': {'family': 'Buhrmaster', 'given': 'Gary'}}, {'id': 'Cottrell-R-L-A', 'name': {'family': 'Cottrell', 'given': 'Les'}}, {'id': 'Martin-Oliver', 'name': {'family': 'Martin', 'given': 'Oliver'}}, {'id': 'Feng-Wu-chun', 'name': {'family': 'Feng', 'given': 'Wu-chun'}}]}
Year: 2005
DOI: 10.1109/MNET.2005.1383434
We describe a variant of TCP, called FAST, that can sustain high throughput and utilization at multigigabits per second over large distances. We present the motivation, review the background theory, summarize key features of FAST TCP, and report our first experimental results.https://authors.library.caltech.edu/records/w8fm4-xwm05Congestion control for high performance, stability, and fairness in general networks
https://resolver.caltech.edu/CaltechAUTHORS:PAGieeeacmtn05
Authors: {'items': [{'id': 'Paganini-F', 'name': {'family': 'Paganini', 'given': 'Fernando'}}, {'id': 'Wang-Zhikui', 'name': {'family': 'Wang', 'given': 'Zhikui'}}, {'id': 'Doyle-J-C', 'name': {'family': 'Doyle', 'given': 'John C.'}, 'orcid': '0000-0002-1828-2486'}, {'id': 'Low-S-H', 'name': {'family': 'Low', 'given': 'Steven H.'}, 'orcid': '0000-0001-6476-3048'}]}
Year: 2005
DOI: 10.1109/TNET.2004.842216
This paper is aimed at designing a congestion control system that scales gracefully with network capacity, providing high utilization, low queueing delay, dynamic stability, and fairness among users. The focus is on developing decentralized control laws at end-systems and routers at the level of fluid-flow models, that can provably satisfy such properties in arbitrary networks, and subsequently approximate these features through practical packet-level implementations. Two families of control laws are developed. The first "dual" control law is able to achieve the first three objectives for arbitrary networks and delays, but is forced to constrain the resource allocation policy. We subsequently develop a "primal-dual" law that overcomes this limitation and allows sources to match their steady-state preferences at a slower time-scale, provided a bound on round-trip-times is known. We develop two packet-level implementations of this protocol, using 1) ECN marking, and 2) queueing delay, as means of communicating the congestion measure from links to sources. We demonstrate using ns-2 simulations the stability of the protocol and its equilibrium features in terms of utilization, queueing and fairness, under a variety of scaling parameters.https://authors.library.caltech.edu/records/gdkjg-76417Optimization and Control of Communication Networks
https://resolver.caltech.edu/CaltechAUTHORS:20160831-160652019
Authors: {'items': [{'id': 'Chiang-Mung', 'name': {'family': 'Chiang', 'given': 'Mung'}}, {'id': 'Low-S-H', 'name': {'family': 'Low', 'given': 'Steven'}, 'orcid': '0000-0001-6476-3048'}]}
Year: 2005
DOI: 10.1145/1071690.1064244
Recently, there has been a surge in research activities that utilize the power of recent developments in nonlinear
optimization to tackle a wide scope of work in the analysis and design of communication systems, touching every
layer of the layered network architecture, and resulting in both intellectual and practical impacts significantly
beyond the earlier frameworks. These research activities are driven by both new demands in the areas of
communications and networking, and new tools emerging from optimization theory. Such tools include new
developments of powerful theories and highly efficient computational algorithms for nonlinear convex
optimization, as well as global solution methods and relaxation techniques for nonconvex optimization.
Optimization theory can be used to analyze, interpret, or design a communication system, for both forward-engineering and reverse-engineering. Over the last few years, it has been successfully applied to a wide range of
communication systems, from the high speed Internet core to wireless networks, from coding and equalization to
broadband access, and from information theory to network topology models. Some of the theoretical advances
have also been put into practice and started making visible impacts, including new versions of TCP congestion
control, power control and scheduling algorithms in wireless networks, and spectrum management in DSL
broadband access networks.
Under the theme of optimization and control of communication networks, this Hot Topic Session consists of five
invited talks covering a wide range of issues, including protocols, pricing, resource allocation, cross layer design,
traffic engineering in the Internet, optical transport networks, and wireless networks.https://authors.library.caltech.edu/records/ds913-h2v73Cross-layer optimization in TCP/IP networks
https://resolver.caltech.edu/CaltechAUTHORS:WANiatnet05
Authors: {'items': [{'id': 'Wang-Jiantao', 'name': {'family': 'Wang', 'given': 'Jiantao'}}, {'id': 'Li-Lun', 'name': {'family': 'Li', 'given': 'Lun'}}, {'id': 'Low-S-H', 'name': {'family': 'Low', 'given': 'Steven H.'}, 'orcid': '0000-0001-6476-3048'}, {'id': 'Doyle-J-C', 'name': {'family': 'Doyle', 'given': 'John C.'}, 'orcid': '0000-0002-1828-2486'}]}
Year: 2005
DOI: 10.1109/TNET.2005.850219
TCP-AQM can be interpreted as distributed primal-dual algorithms to maximize aggregate utility over source rates. We show that an equilibrium of TCP/IP, if exists, maximizes aggregate utility over both source rates and routes, provided congestion prices are used as link costs. An equilibrium exists if and only if this utility maximization problem and its Lagrangian dual have no duality gap. In this case, TCP/IP incurs no penalty in not splitting traffic across multiple paths. Such an equilibrium, however, can be unstable. It can be stabilized by adding a static component to link cost, but at the expense of a reduced utility in equilibrium. If link capacities are optimally provisioned, however, pure static routing, which is necessarily stable, is sufficient to maximize utility. Moreover single-path routing again achieves the same utility as multipath routing at optimality.https://authors.library.caltech.edu/records/n04qt-0cs93The "robust yet fragile" nature of the Internet
https://resolver.caltech.edu/CaltechAUTHORS:DOYpnas05
Authors: {'items': [{'id': 'Doyle-J-C', 'name': {'family': 'Doyle', 'given': 'John C.'}, 'orcid': '0000-0002-1828-2486'}, {'id': 'Anderson-Dav-L', 'name': {'family': 'Anderson', 'given': 'David L.'}}, {'id': 'Li-Lun', 'name': {'family': 'Li', 'given': 'Lun'}}, {'id': 'Low-S-H', 'name': {'family': 'Low', 'given': 'Steven'}, 'orcid': '0000-0001-6476-3048'}, {'id': 'Roughan-M', 'name': {'family': 'Roughan', 'given': 'Matthew'}}, {'id': 'Shalunov-S', 'name': {'family': 'Shalunov', 'given': 'Stanislav'}}, {'id': 'Tanaka-Reiko', 'name': {'family': 'Tanaka', 'given': 'Reiko'}}, {'id': 'Willinger-W', 'name': {'family': 'Willinger', 'given': 'Walter'}}]}
Year: 2005
DOI: 10.1073/pnas.0501426102
PMCID: PMC1240072
The search for unifying properties of complex networks is popular, challenging, and important. For modeling approaches that focus on robustness and fragility as unifying concepts, the Internet is an especially attractive case study, mainly because its applications are ubiquitous and pervasive, and widely available expositions exist at every level of detail. Nevertheless, alternative approaches to modeling the Internet often make extremely different assumptions and derive opposite conclusions about fundamental properties of one and the same system. Fortunately, a detailed understanding of Internet technology combined with a unique ability to measure the network means that these differences can be understood thoroughly and resolved unambiguously. This article aims to make recent results of this process accessible beyond Internet specialists to the broader scientific community and to clarify several sources of basic methodological differences that are relevant beyond either the Internet or the two specific approaches focused on here (i.e., scale-free networks and highly optimized tolerance networks).https://authors.library.caltech.edu/records/3gtmf-gww63The Ultralight project: the network as an integrated and managed resource for data-intensive science
https://resolver.caltech.edu/CaltechAUTHORS:NEWcse05
Authors: {'items': [{'id': 'Newman-H-B', 'name': {'family': 'Newman', 'given': 'Harvey'}, 'orcid': '0000-0003-0964-1480'}, {'id': 'Bunn-Julian-J', 'name': {'family': 'Bunn', 'given': 'Julian'}, 'orcid': '0000-0002-3798-298X'}, {'id': 'Legrand-I', 'name': {'family': 'Legrand', 'given': 'Iosif'}}, {'id': 'Low-S-H', 'name': {'family': 'Low', 'given': 'Steven'}, 'orcid': '0000-0001-6476-3048'}, {'id': 'Nae-D', 'name': {'family': 'Nae', 'given': 'Dan'}}, {'id': 'Ravot-S', 'name': {'family': 'Ravot', 'given': 'Sylvain'}}, {'id': 'Steenberg-C-D', 'name': {'family': 'Steenberg', 'given': 'Conrad'}}, {'id': 'Su-X', 'name': {'family': 'Su', 'given': 'Xun'}}, {'id': 'Thomas-Michael', 'name': {'family': 'Thomas', 'given': 'Michael'}}, {'id': 'van-Lingen-Frank', 'name': {'family': 'van Lingen', 'given': 'Frank'}}, {'id': 'Xia-Y', 'name': {'family': 'Xia', 'given': 'Yang'}}, {'id': 'Cavanaugh-R', 'name': {'family': 'Cavanaugh', 'given': 'Richard'}}, {'id': 'McKee-S', 'name': {'family': 'McKee', 'given': 'Shawn'}}]}
Year: 2005
DOI: 10.1109/MCSE.2005.127
Looks at the UltraLight project which treats the network interconnecting globally distributed data sets as a dynamic, configurable, and closely monitored resource to construct a next-generation system that can meet the high-energy physics community's data-processing, distribution, access, and analysis needs.https://authors.library.caltech.edu/records/8r74a-2md93Equilibrium and Fairness of Networks Shared by TCP Reno and Vegas/FAST
https://resolver.caltech.edu/CaltechAUTHORS:20191009-132246224
Authors: {'items': [{'id': 'Tang-Ao', 'name': {'family': 'Tang', 'given': 'Ao'}, 'orcid': '0000-0001-6296-644X'}, {'id': 'Wang-Jintao', 'name': {'family': 'Wang', 'given': 'Jiantao'}}, {'id': 'Hegde-S', 'name': {'family': 'Hegde', 'given': 'Sanjay'}}, {'id': 'Low-S-H', 'name': {'family': 'Low', 'given': 'Steven H.'}, 'orcid': '0000-0001-6476-3048'}]}
Year: 2005
DOI: 10.1007/s11235-005-5499-1
It has been proved theoretically that a network with heterogeneous congestion control algorithms that react to different congestion signals can have multiple equilibrium points. In this paper, we demonstrate this experimentally using TCP Reno and Vegas/FAST. We also show that any desired inter-protocol fairness is in principle achievable by an appropriate choice of Vegas/FAST parameter, and that intra-protocol fairness among flows within each protocol is unaffected by the presence of the other protocol except for a reduction in effective link capacities. Dummynet experiments and ns-2 simulations are presented to verify these results.https://authors.library.caltech.edu/records/wyde3-qrt80Counter-intuitive throughput behaviors in networks under end-to-end control
https://resolver.caltech.edu/CaltechAUTHORS:TANiatnet06
Authors: {'items': [{'id': 'Tang-Ao', 'name': {'family': 'Tang', 'given': 'Ao'}, 'orcid': '0000-0001-6296-644X'}, {'id': 'Wang-Jintao', 'name': {'family': 'Wang', 'given': 'Jiantao'}}, {'id': 'Low-S-H', 'name': {'family': 'Low', 'given': 'Steven H.'}, 'orcid': '0000-0001-6476-3048'}]}
Year: 2006
DOI: 10.1109/TNET.2006.872552
It has been shown that as long as traffic sources adapt their rates to aggregate congestion measure in their paths, they implicitly maximize certain utility. In this paper we study some counter-intuitive throughput behaviors in such networks, pertaining to whether a fair allocation is always inefficient and whether increasing capacity always raises aggregate throughput. A bandwidth allocation policy can be defined in terms of a class of utility functions parameterized by a scalar a that can be interpreted as a quantitative measure of fairness. An allocation is fair if alpha is large and efficient if aggregate throughput is large. All examples in the literature suggest that a fair allocation is necessarily inefficient. We characterize exactly the tradeoff between fairness and throughput in general networks. The characterization allows us both to produce the first counter-example and trivially explain all the previous supporting examples. Surprisingly, our counter-example has the property that a fairer allocation is always more efficient. In particular it implies that maxmin fairness can achieve a higher throughput than proportional fairness. Intuitively, we might expect that increasing link capacities always raises aggregate throughput. We show that not only can throughput be reduced when some link increases its capacity, more strikingly, it can also be reduced when all links increase their capacities by the same amount. If all links increase their capacities proportionally, however, throughput will indeed increase. These examples demonstrate the intricate interactions among sources in a network setting that are missing in a single-link topology.https://authors.library.caltech.edu/records/gqjp0-a5m72Guest Editorial: Nonlinear Optimization of Communication Systems
https://resolver.caltech.edu/CaltechAUTHORS:CHIieeejsac06
Authors: {'items': [{'id': 'Chiang-Mung', 'name': {'family': 'Chiang', 'given': 'Mung'}}, {'id': 'Low-S-H', 'name': {'family': 'Low', 'given': 'Steven H.'}, 'orcid': '0000-0001-6476-3048'}, {'id': 'Luo-Zhi-Quan', 'name': {'family': 'Luo', 'given': 'Zhi-Quan'}}, {'id': 'Shroff-N-B', 'name': {'family': 'Shroff', 'given': 'Ness B.'}}, {'id': 'Yu-Wei', 'name': {'family': 'Yu', 'given': 'Wei'}}]}
Year: 2006
DOI: 10.1109/JSAC.2006.879335
Linear programming and other classical optimization techniques have found important applications in communication systems for many decades. Recently, there has been a surge in research activities that utilize the latest developments in nonlinear optimization to tackle a much wider scope of work in the analysis and design of communication systems. These activities involve every "layer" of the protocol stack and the principles of layered network architecture itself, and have made intellectual and practical impacts significantly beyond the established frameworks of optimization of communication systems in the early 1990s. These recent results are driven by new demands in the areas of communications and networking, as well as new tools emerging from optimization theory. Such tools include the powerful theories and highly efficient computational algorithms for nonlinear convex optimization, together with global solution methods and relaxation techniques for nonconvex optimization.https://authors.library.caltech.edu/records/seeza-4qa15FAST TCP: Motivation, Architecture, Algorithms, Performance
https://resolver.caltech.edu/CaltechAUTHORS:WEIieeeacmtn06
Authors: {'items': [{'id': 'Wei-Xiaoliang-David', 'name': {'family': 'Wei', 'given': 'David X.'}}, {'id': 'Jin-Cheng', 'name': {'family': 'Jin', 'given': 'Cheng'}}, {'id': 'Low-S-H', 'name': {'family': 'Low', 'given': 'Steven H.'}, 'orcid': '0000-0001-6476-3048'}, {'id': 'Hegde-S', 'name': {'family': 'Hegde', 'given': 'Sanjay'}}]}
Year: 2006
DOI: 10.1109/TNET.2006.886335
We describe FAST TCP, a new TCP congestion control algorithm for high-speed long-latency networks, from design to implementation. We highlight the approach taken by FAST TCP to address the four difficulties which the current TCP implementation has at large windows. We describe the architecture and summarize some of the algorithms implemented in our prototype. We characterize its equilibrium and stability properties. We evaluate it experimentally in terms of throughput, fairness, stability, and responsiveness.https://authors.library.caltech.edu/records/c3vmh-t3e61Application-Oriented Flow Control: Fundamentals, Algorithms and Fairness
https://resolver.caltech.edu/CaltechAUTHORS:WANiatnet06
Authors: {'items': [{'id': 'Wang-W-H', 'name': {'family': 'Wang', 'given': 'Wei-Hua'}}, {'id': 'Palaniswami-M', 'name': {'family': 'Palaniswami', 'given': 'Marimuthu'}}, {'id': 'Low-S-H', 'name': {'family': 'Low', 'given': 'Steven H.'}, 'orcid': '0000-0001-6476-3048'}]}
Year: 2006
DOI: 10.1109/TNET.2006.886318
This paper is concerned with flow control and resource allocation problems in computer networks in which real-time applications may have hard quality of service (QoS) requirements. Recent optimal flow control approaches are unable to deal with these problems since QoS utility functions generally do not satisfy the strict concavity condition in real-time applications. For elastic traffic, we show that bandwidth allocations using the existing optimal flow control strategy can be quite unfair. If we consider different QoS requirements among network users, it may be undesirable to allocate bandwidth simply according to the traditional max-min fairness or proportional fairness. Instead, a network should have the ability to allocate bandwidth resources to various users, addressing their real utility requirements. For these reasons, this paper proposes a new distributed flow control algorithm for multiservice networks, where the application's utility is only assumed to be continuously increasing over the available bandwidth. In this, we show that the algorithm converges, and that at convergence, the utility achieved by each application is well balanced in a proportionally (or max-min) fair manner.https://authors.library.caltech.edu/records/p1t1x-6n314Layering as Optimization Decomposition: A Mathematical Theory of Network Architectures
https://resolver.caltech.edu/CaltechAUTHORS:20170810-104152899
Authors: {'items': [{'id': 'Chiang-Mung', 'name': {'family': 'Chiang', 'given': 'Mung'}}, {'id': 'Low-S-H', 'name': {'family': 'Low', 'given': 'Steven H.'}, 'orcid': '0000-0001-6476-3048'}, {'id': 'Calderbank-A-R', 'name': {'family': 'Calderbank', 'given': 'A. Robert'}}, {'id': 'Doyle-J-C', 'name': {'family': 'Doyle', 'given': 'John C.'}, 'orcid': '0000-0002-1828-2486'}]}
Year: 2007
DOI: 10.1109/JPROC.2006.887322
Network protocols in layered architectures have historically been obtained on an ad hoc basis, and many of the recent cross-layer designs are also conducted through piecemeal approaches. Network protocol stacks may instead be holistically analyzed and systematically designed as distributed solutions to some global optimization problems. This paper presents a survey of the recent efforts towards a systematic understanding of layering as optimization decomposition, where the overall communication network is modeled by a generalized network utility maximization problem, each layer corresponds to a decomposed subproblem, and the interfaces among layers are quantified as functions of the optimization variables coordinating the subproblems. There can be many alternative decompositions, leading to a choice of different layering architectures. This paper surveys the current status of horizontal decomposition into distributed computation, and vertical decomposition into functional modules such as congestion control, routing, scheduling, random access, power control, and channel coding. Key messages and methods arising from many recent works are summarized, and open issues discussed. Through case studies, it is illustrated how layering as Optimization Decomposition provides a common language to think about modularization in the face of complex, networked interactions, a unifying, top-down approach to design protocol stacks, and a mathematical theory of network architectures.https://authors.library.caltech.edu/records/x0zpr-rff66Global finite-time convergence of TCP Vegas without feedback information delay
https://resolver.caltech.edu/CaltechAUTHORS:20170810-105633790
Authors: {'items': [{'id': 'Choi-Joon-Young', 'name': {'family': 'Choi', 'given': 'Joon-Young'}}, {'id': 'Koo-Kyungmo', 'name': {'family': 'Koo', 'given': 'Kyungmo'}}, {'id': 'Lee-Jin-S', 'name': {'family': 'Lee', 'given': 'Jin S.'}}, {'id': 'Low-S-H', 'name': {'family': 'Low', 'given': 'Steven H.'}, 'orcid': '0000-0001-6476-3048'}]}
Year: 2007
We prove that TCP Vegas globally converges to its equilibrium point in finite time assuming no feedback information delay. We analyze a continuous-time TCP Vegas model with discontinuity and high nonlinearity. Using the upper right-hand derivative and applying the comparison lemma, we cope with the discontinuous signum function in the TCP Vegas model; using a change of state variables, we deal with the high nonlinearity. Although we ignore feedback information delay in analyzing the model of TCP Vegas, the simulation results illustrate that TCP Vegas in the presence of feedback information delay shows very similar dynamic trends to TCP Vegas without feedback information delay. Consequently, dynamic properties of TCP Vegas without feedback information delay can be used to estimate those of TCP Vegas in the presence of feedback information delay.https://authors.library.caltech.edu/records/qtxv6-j4718Asynchronous Distributed Averaging on Communication Networks
https://resolver.caltech.edu/CaltechAUTHORS:MEHieeeacmtn07
Authors: {'items': [{'id': 'Mehyar-M', 'name': {'family': 'Mehyar', 'given': 'Mortada'}}, {'id': 'Spanos-D-P', 'name': {'family': 'Spanos', 'given': 'Demetri'}}, {'id': 'Pongsajapan-J', 'name': {'family': 'Pongsajapan', 'given': 'John'}}, {'id': 'Low-S-H', 'name': {'family': 'Low', 'given': 'Steven H.'}, 'orcid': '0000-0001-6476-3048'}, {'id': 'Murray-R-M', 'name': {'family': 'Murray', 'given': 'Richard M.'}, 'orcid': '0000-0002-5785-7481'}]}
Year: 2007
DOI: 10.1109/TNET.2007.893226
Distributed algorithms for averaging have attracted interest in the control and sensing literature. However, previous works have not addressed some practical concerns that will arise in actual implementations on packet-switched communication networks such as the Internet. In this paper, we present several implementable algorithms that are robust to asynchronism and dynamic topology changes. The algorithms are completely distributed and do not require any global coordination. In addition, they can be proven to converge under very general asynchronous timing assumptions. Our results are verified by both simulation and experiments on Planetlab, a real-world TCP/IP network. We also present some extensions that are likely to be useful in applications.https://authors.library.caltech.edu/records/90y8e-k4732Equilibrium of Heterogeneous Congestion Control: Existence and Uniqueness
https://resolver.caltech.edu/CaltechAUTHORS:TANieeeacmtn07
Authors: {'items': [{'id': 'Tang-Ao', 'name': {'family': 'Tang', 'given': 'Ao'}, 'orcid': '0000-0001-6296-644X'}, {'id': 'Wang-Jintao', 'name': {'family': 'Wang', 'given': 'Jiantao'}}, {'id': 'Low-S-H', 'name': {'family': 'Low', 'given': 'Steven H.'}, 'orcid': '0000-0001-6476-3048'}, {'id': 'Chiang-Mung', 'name': {'family': 'Chiang', 'given': 'Mung'}}]}
Year: 2007
DOI: 10.1109/TNET.2007.893885
When heterogeneous congestion control protocols that react to different pricing signals share the same network, the resulting equilibrium may no longer be interpreted as a solution to the standard utility maximization problem. We prove the existence of equilibrium in general multiprotocol networks under mild assumptions. For almost all networks, the equilibria are locally unique, finite, and odd in number. They cannot all be locally stable unless there is a globally unique equilibrium. Finally, we show that if the price mapping functions, which map link prices to effective prices observed by the sources, are sufficiently similar, then global uniqueness is guaranteed.https://authors.library.caltech.edu/records/t9md6-zs710A Game-Theoretic Framework for Medium Access Control
https://resolver.caltech.edu/CaltechAUTHORS:CUIieeejsac08
Authors: {'items': [{'id': 'Cui-Tao', 'name': {'family': 'Cui', 'given': 'Tao'}}, {'id': 'Chen-Lijun', 'name': {'family': 'Chen', 'given': 'Lijun'}}, {'id': 'Low-S-H', 'name': {'family': 'Low', 'given': 'Steven H.'}, 'orcid': '0000-0001-6476-3048'}]}
Year: 2008
DOI: 10.1109/JSAC.2008.080909
In this paper, we generalize the random access game model, and show that it provides a general game-theoretic framework for designing contention based medium access control. We extend the random access game model to the network with multiple contention measure signals, study the design of random access games, and analyze different distributed algorithms achieving their equilibria. As examples, a series of utility functions is proposed for games achieving the maximum throughput in a network of homogeneous nodes. In a network with n traffic classes, an N-signal game model is proposed which achieves the maximum throughput under the fairness constraint among different traffic classes. In addition, the convergence of different dynamic algorithms such as best response, gradient play and Jacobi play under propagation delay and estimation error is established. Simulation results show that game model based protocols can achieve superior performance over the standard IEEE 802.11 DCF, and comparable performance as existing protocols with the best performance in literature.https://authors.library.caltech.edu/records/gv77s-5zg41An Improved Link Model for Window Flow Control and Its Application to FAST TCP
https://resolver.caltech.edu/CaltechAUTHORS:20090415-081549907
Authors: {'items': [{'id': 'Jacobsson-K', 'name': {'family': 'Jacobsson', 'given': 'Krister'}}, {'id': 'Andrew-L-L-H', 'name': {'family': 'Andrew', 'given': 'Lachlan L. H.'}}, {'id': 'Tang-Ao', 'name': {'family': 'Tang', 'given': 'Ao (Kevin)'}, 'orcid': '0000-0001-6296-644X'}, {'id': 'Low-S-H', 'name': {'family': 'Low', 'given': 'Steven H.'}, 'orcid': '0000-0001-6476-3048'}, {'id': 'Hjalmarsson-H', 'name': {'family': 'Hjalmarsson', 'given': 'Håkan'}}]}
Year: 2009
DOI: 10.1109/TAC.2009.2012986
This paper presents a link model which captures the queue dynamics in response to a change in a transmission control protocol (TCP) source's congestion window. By considering both self-clocking and the link integrator effect, the model generalizes existing models and is shown to be more accurate by both open loop and closed loop packet level simulations. It reduces to the known static link model when flows' round trip delays are identical, and approximates the standard integrator link model when there is significant cross traffic. We apply this model to the stability analysis of fast active queue management scalable TCP (FAST TCP) including its filter dynamics. Under this model, the FAST control law is linearly stable for a single bottleneck link with an arbitrary distribution of round trip delays. This result resolves the notable discrepancy between empirical observations and previous theoretical predictions. The analysis highlights the critical role of self-clocking in TCP stability, and the proof technique is new and less conservative than existing ones.https://authors.library.caltech.edu/records/3mrrd-var15End-to-end fair rate optimization in wired-cum-wireless networks
https://resolver.caltech.edu/CaltechAUTHORS:20090922-143224543
Authors: {'items': [{'id': 'Wang-Xingang', 'name': {'family': 'Wang', 'given': 'Xingang'}}, {'id': 'Kar-K', 'name': {'family': 'Kar', 'given': 'Koushik'}}, {'id': 'Low-S-H', 'name': {'family': 'Low', 'given': 'Steven H.'}, 'orcid': '0000-0001-6476-3048'}]}
Year: 2009
DOI: 10.1016/j.adhoc.2008.05.003
In this paper, we address the end-to-end rate optimization problem in a wired-cum-wireless network, where CSMA/CA based wireless LANs extend a wired backbone and provide access to mobile users. The objective is to achieve proportional fairness amongst the end-to-end sessions in the network. Since the network contains wireless links whose attainable throughput is a (non-convex and non-separable) function of MAC protocol parameters, the problem requires joint optimization at both the transport and the link layers. A dual-based algorithm is proposed in this paper to solve this cross-layer rate optimization problem. It is implemented in the distributed manner, and works at the link layer to adjust scheduling rates for the wireless links in the basic service sets, and at the transport layer to adjust end-to-end session rates. We prove rigorously that the proposed algorithm converges to the globally optimal rates. Simulation results are provided to support our conclusions.https://authors.library.caltech.edu/records/1a086-zh233Optimal job fragmentation
https://resolver.caltech.edu/CaltechAUTHORS:20161206-155547391
Authors: {'items': [{'id': 'Nair-J', 'name': {'family': 'Nair', 'given': 'Jayakrishnan'}}, {'id': 'Low-S-H', 'name': {'family': 'Low', 'given': 'Steven H.'}, 'orcid': '0000-0001-6476-3048'}]}
Year: 2009
DOI: 10.1145/1639562.1639570
It has been recently discovered that on an unreliable server, the job completion time distribution function (df) can be heavy-tailed (HT) even when job size df is light-tailed (LT) [1, 5]. A key to this phenomenon is the RESTART feature where if a job is interrupted in the middle of its processing, the entire job needs to restart from the beginning, i.e., the work that is partially completed is lost.
A standard mechanism for reducing the job completion
time in an unreliable service environment is checkpointing
[3, 4, 6]. We view checkpointing as a job fragmentation operation, where the server processes one fragment of the job at a time. If the server becomes unavailable, say due to failure, then only the work corresponding to the fragment being processed at the time of failure is lost. In this paper, we are motivated by the question: Can fragmentation 'lighten' the tail df of the completion time? In Section 3, we provide sufficient conditions on the fragmentation policy that gives rise to LT completion time so long as the job size df is LT. We then characterize the optimal fragmentation policy seeking to minimize the expected job completion time. This policy
requires a priori knowledge of the job size. We then describe a sub-optimal fragmentation policy that is blind to the job size and is provably very close to optimal. We also describe the asymptotic tail behavior of the job completion time df under both policies. Assuming the server unavailability periods are LT, both policies produce LT completion times when the job size df is LT. For the case of regularly varying job size df, the job completion time under both policies is regularly varying with the same degree - this is the lightest possible asymptotic tail behavior (in the degree sense).https://authors.library.caltech.edu/records/6tbes-vrn02Understanding XCP: Equilibrium and Fairness
https://resolver.caltech.edu/CaltechAUTHORS:20100122-142759802
Authors: {'items': [{'id': 'Andrew-L-L-H', 'name': {'family': 'Andrew', 'given': 'Lachlan L. H.'}}, {'id': 'Low-S-H', 'name': {'family': 'Low', 'given': 'Steven H.'}, 'orcid': '0000-0001-6476-3048'}, {'id': 'Wydrowski-B-P', 'name': {'family': 'Wydrowski', 'given': 'Bartek P.'}}]}
Year: 2009
DOI: 10.1109/TNET.2009.2013242
We prove that the XCP equilibrium solves a constrained max-min fairness problem by identifying it with the unique solution of a hierarchy of optimization problems, namely those solved by max-min fair allocation, but solved by XCP under an additional constraint. This constraint is due to the "bandwidth shuffling" necessary to obtain fairness. We describe an algorithm to compute this equilibrium and derive a lower and upper bound on link utilization. While XCP reduces to max-min allocation at a single link, its behavior in a network can be very different. We illustrate that the additional constraint can cause flows to receive an arbitrarily small fraction of their max-min fair allocations. We confirm these results using ns2 simulations.https://authors.library.caltech.edu/records/wcf1j-5ja56Equilibrium of Heterogeneous Congestion Control: Optimality and Stability
https://resolver.caltech.edu/CaltechAUTHORS:20100709-142523769
Authors: {'items': [{'id': 'Tang-Ao', 'name': {'family': 'Tang', 'given': 'Ao'}, 'orcid': '0000-0001-6296-644X'}, {'id': 'Wei-Xiaoliang-David', 'name': {'family': 'Wei', 'given': 'Xiaoliang (David)'}}, {'id': 'Low-S-H', 'name': {'family': 'Low', 'given': 'Steven H.'}, 'orcid': '0000-0001-6476-3048'}, {'id': 'Chiang-Mung', 'name': {'family': 'Chiang', 'given': 'Mung'}}]}
Year: 2010
DOI: 10.1109/TNET.2009.2034963
When heterogeneous congestion control protocols
that react to different pricing signals share the same network,
the current theory based on utility maximization fails to predict
the network behavior. The pricing signals can be different types
of signals such as packet loss, queueing delay, etc, or different
values of the same type of signal such as different ECN marking
values based on the same actual link congestion level. Unlike in a
homogeneous network, the bandwidth allocation now depends on
router parameters and flow arrival patterns. It can be non-unique,
suboptimal and unstable. In Tang et al. ("Equilibrium of heterogeneous
congestion control: Existence and uniqueness," IEEE/ACM
Trans. Netw., vol. 15, no. 4, pp. 824–837, Aug. 2007), existence and
uniqueness of equilibrium of heterogeneous protocols are investigated.
This paper extends the study with two objectives: analyzing
the optimality and stability of such networks and designing control
schemes to improve those properties. First, we demonstrate the
intricate behavior of a heterogeneous network through simulations
and present a framework to help understand its equilibrium
properties. Second, we propose a simple source-based algorithm
to decouple bandwidth allocation from router parameters and
flow arrival patterns by only updating a linear parameter in the
sources' algorithms on a slow timescale. It steers a network to
the unique optimal equilibrium. The scheme can be deployed
incrementally as the existing protocol needs no change and only
new protocols need to adopt the slow timescale adaptation.https://authors.library.caltech.edu/records/08rep-x3j02Random Access Game and Medium Access Control Design
https://resolver.caltech.edu/CaltechAUTHORS:20100907-110446104
Authors: {'items': [{'id': 'Chen-Lijun', 'name': {'family': 'Chen', 'given': 'Lijun'}}, {'id': 'Low-S-H', 'name': {'family': 'Low', 'given': 'Steven H.'}, 'orcid': '0000-0001-6476-3048'}, {'id': 'Doyle-J-C', 'name': {'family': 'Doyle', 'given': 'John C.'}, 'orcid': '0000-0002-1828-2486'}]}
Year: 2010
DOI: 10.1109/TNET.2010.2041066
Motivated partially by a control-theoretic viewpoint, we propose a game-theoretic model, called random access game, for contention control. We characterize Nash equilibria of random access games, study their dynamics, and propose distributed algorithms (strategy evolutions) to achieve Nash equilibria. This provides a general analytical framework that is capable of modeling a large class of system-wide quality-of-service (QoS) models via the specification of per-node utility functions, in which system-wide fairness or service differentiation can be achieved in a distributed manner as long as each node executes a contention resolution algorithm that is designed to achieve the Nash equilibrium. We thus propose a novel medium access method derived from carrier sense multiple access/collision avoidance (CSMA/CA) according to distributed strategy update mechanism achieving the Nash equilibrium of random access game. We present a concrete medium access method that adapts to a continuous contention measure called conditional collision probability, stabilizes the network into a steady state that achieves optimal throughput with targeted fairness (or service differentiation), and can decouple contention control from handling failed transmissions. In addition to guiding medium access control design, the random access game model also provides an analytical framework to understand equilibrium and dynamic properties of different medium access protocols.https://authors.library.caltech.edu/records/369d2-rk609Queue Dynamics With Window Flow Control
https://resolver.caltech.edu/CaltechAUTHORS:20101124-110950283
Authors: {'items': [{'id': 'Tang-Ao', 'name': {'family': 'Tang', 'given': 'Ao'}, 'orcid': '0000-0001-6296-644X'}, {'id': 'Andrew-L-L-H', 'name': {'family': 'Andrew', 'given': 'Lachlan L. H.'}}, {'id': 'Jacobsson-K', 'name': {'family': 'Jacobsson', 'given': 'Krister'}}, {'id': 'Johansson-K-H', 'name': {'family': 'Johansson', 'given': 'Karl H.'}}, {'id': 'Hjalmarsson-H', 'name': {'family': 'Hjalmarsson', 'given': 'Håkan'}}, {'id': 'Low-S-H', 'name': {'family': 'Low', 'given': 'Steven H.'}, 'orcid': '0000-0001-6476-3048'}]}
Year: 2010
DOI: 10.1109/TNET.2010.2047951
This paper develops a new model that describes the queueing process of a communication network when data sources use window flow control. The model takes into account the burstiness in sub-round-trip time (RTT) timescales and the instantaneous rate differences of a flow at different links. It is generic and independent of actual source flow control algorithms. Basic properties of the model and its relation to existing work are discussed. In particular, for a general network with multiple links, it is demonstrated that spatial interaction of oscillations allows queue instability to occur even when all flows have the same RTTs and maintain constant windows. The model is used to study the dynamics of delay-based congestion control algorithms. It is found that the ratios of RTTs are critical to the stability of such systems, and previously unknown modes of instability are identified. Packet-level simulations and testbed measurements are provided to verify the model and its predictions.https://authors.library.caltech.edu/records/ghc8g-zy466Nonnegative Matrix Inequalities and their Application to Nonconvex Power Control Optimization
https://resolver.caltech.edu/CaltechAUTHORS:20111025-095823883
Authors: {'items': [{'id': 'Tan-C-W', 'name': {'family': 'Tan', 'given': 'Chee Wei'}}, {'id': 'Friedland-S', 'name': {'family': 'Friedland', 'given': 'Shmuel'}}, {'id': 'Low-S-H', 'name': {'family': 'Low', 'given': 'Steven'}, 'orcid': '0000-0001-6476-3048'}]}
Year: 2011
DOI: 10.1137/090757137
Maximizing the sum rates in a multiuser Gaussian channel by power control is a nonconvex NP-hard problem that finds engineering application in code division multiple access (CDMA) wireless communication network. In this paper, we extend and apply several fundamental nonnegative matrix inequalities initiated by Friedland and Karlin in a 1975 paper to solve this nonconvex power control optimization problem. Leveraging tools such as the Perron–Frobenius theorem in nonnegative matrix theory, we (1) show that this problem in the power domain can be reformulated as an equivalent convex maximization problem over a closed unbounded convex set in the logarithmic signal-to-interference-noise ratio domain, (2) propose two relaxation techniques that utilize the reformulation problem structure and convexification by Lagrange dual relaxation to compute progressively tight bounds, and (3) propose a global optimization algorithm with ϵ-suboptimality to compute the optimal power control allocation. A byproduct of our analysis is the application of Friedland–Karlin inequalities to inverse problems in nonnegative matrix theory.https://authors.library.caltech.edu/records/4ttxb-nq422Power flow optimization using positive quadratic programming
https://resolver.caltech.edu/CaltechAUTHORS:20170810-134634427
Authors: {'items': [{'id': 'Lavei-J', 'name': {'family': 'Lavei', 'given': 'Javad'}}, {'id': 'Rantzer-A', 'name': {'family': 'Rantzer', 'given': 'Anders'}}, {'id': 'Low-S-H', 'name': {'family': 'Low', 'given': 'Stephen'}, 'orcid': '0000-0001-6476-3048'}]}
Year: 2011
DOI: 10.3182/20110828-6-IT-1002.02588
The problem to minimize power losses in an electrical network subject to voltage and power constraints is in general hard to solve. However, it has recently been discovered that semidefinite programming relaxations in many cases enable exact computation of the global optimum. Here we point out a fundamental reason for the successful relaxations, namely that the passive network components give rise to matrices with nonnegative offdiagonal entries. Recent progress on quadratic programming with Metzler matrix structure can therefore be applied.https://authors.library.caltech.edu/records/xsaq2-06862Spectrum Management in Multiuser Cognitive Wireless Networks: Optimality and Algorithm
https://resolver.caltech.edu/CaltechAUTHORS:20110302-113943034
Authors: {'items': [{'id': 'Tan-C-W', 'name': {'family': 'Tan', 'given': 'Chee Wei'}}, {'id': 'Friedland-S', 'name': {'family': 'Friedland', 'given': 'Shmuel'}}, {'id': 'Low-S-H', 'name': {'family': 'Low', 'given': 'Steven H.'}, 'orcid': '0000-0001-6476-3048'}]}
Year: 2011
DOI: 10.1109/JSAC.2011.110214
Spectrum management is used to improve performance in multiuser communication system, e.g., cognitive radio or femtocell networks, where multiuser interference can lead to data rate degradation. We study the nonconvex NP-hard problem of maximizing a weighted sum rate in a multiuser Gaussian interference channel by power control subject to affine power constraints. By exploiting the fact that this problem can be restated as an optimization problem with constraints that are spectral radii of specially crafted nonnegative matrices, we derive necessary and sufficient optimality conditions and propose a global optimization algorithm based on the outer approximation method. Central to our techniques is the use of nonnegative matrix theory, e.g., nonnegative matrix inequalities and the Perron-Frobenius theorem. We also study an inner approximation method and a relaxation method that give insights to special cases. Our techniques and algorithm can be extended to a multiple carrier system model, e.g., OFDM system or receivers with interference suppression capability.https://authors.library.caltech.edu/records/nh5he-b8w58Cross-layer design in multihop wireless networks
https://resolver.caltech.edu/CaltechAUTHORS:20110315-091124013
Authors: {'items': [{'id': 'Chen-Lijun', 'name': {'family': 'Chen', 'given': 'Lijun'}}, {'id': 'Low-S-H', 'name': {'family': 'Low', 'given': 'Steven H.'}, 'orcid': '0000-0001-6476-3048'}, {'id': 'Doyle-J-C', 'name': {'family': 'Doyle', 'given': 'John C.'}, 'orcid': '0000-0002-1828-2486'}]}
Year: 2011
DOI: 10.1016/j.comnet.2010.09.005
In this paper, we take a holistic approach to the protocol architecture design in multihop wireless networks. Our goal is to integrate various protocol layers into a rigorous framework, by regarding them as distributed computations over the network to solve some optimization problem. Different layers carry out distributed computation on different subsets of the decision variables using local information to achieve individual optimality. Taken
together, these local algorithms (with respect to different layers) achieve a global optimality. Our current theory integrates three functions—congestion control, routing and scheduling—in transport, network and link layers into a coherent framework. These three functions interact through and are regulated by congestion price so as to achieve a global optimality, even in a time-varying environment. Within this context, this model allows us to systematically derive the layering structure of the various mechanisms of different protocol layers, their interfaces, and the control information that must cross these interfaces to
achieve a certain performance and robustness.https://authors.library.caltech.edu/records/xekag-ac411Greening geographical load balancing
https://resolver.caltech.edu/CaltechAUTHORS:20161128-151734850
Authors: {'items': [{'id': 'Liu-Zhenhua', 'name': {'family': 'Liu', 'given': 'Zhenhua'}}, {'id': 'Lin-Minghong', 'name': {'family': 'Lin', 'given': 'Minghong'}}, {'id': 'Wierman-A', 'name': {'family': 'Wierman', 'given': 'Adam'}}, {'id': 'Low-S-H', 'name': {'family': 'Low', 'given': 'Steven H.'}, 'orcid': '0000-0001-6476-3048'}, {'id': 'Andrew-L-L-H', 'name': {'family': 'Andrew', 'given': 'Lachlan L. H.'}}]}
Year: 2011
Energy expenditure has become a significant fraction of data center operating costs. Recently, "geographical load balancing" has been suggested to reduce energy cost by exploiting the electricity price differences across regions. However, this reduction of cost can paradoxically increase total energy use.
This paper explores whether the geographical diversity of Internet-scale systems can additionally be used to provide environmental gains. Specifically, we explore whether geographical load balancing can encourage use of "green" renewable energy and reduce use of "brown" fossil fuel energy. We make two contributions. First, we derive two distributed algorithms for achieving optimal geographical load balancing. Second, we show that if electricity is dynamically priced in proportion to the instantaneous fraction of the total energy that is brown, then geographical load balancing provides significant reductions in brown energy use. However, the benefits depend strongly on the degree to which systems accept dynamic energy pricing and the form of pricing used.https://authors.library.caltech.edu/records/hww5w-9w763Geographical load balancing with renewables
https://resolver.caltech.edu/CaltechAUTHORS:20161128-151114708
Authors: {'items': [{'id': 'Liu-Zhenhua', 'name': {'family': 'Liu', 'given': 'Zhenhua'}}, {'id': 'Lin-Minghong', 'name': {'family': 'Lin', 'given': 'Minghong'}}, {'id': 'Wierman-A', 'name': {'family': 'Wierman', 'given': 'Adam'}}, {'id': 'Low-S-H', 'name': {'family': 'Low', 'given': 'Steven H.'}, 'orcid': '0000-0001-6476-3048'}, {'id': 'Andrew-L-L-H', 'name': {'family': 'Andrew', 'given': 'Lachlan L. H.'}}]}
Year: 2011
DOI: 10.1145/2160803.2160862
Given the significant energy consumption of data centers, improving their energy efficiency is an important social problem. However, energy efficiency is necessary but not sufficient for sustainability, which demands reduced usage of energy from fossil fuels. This paper investigates the feasibility of powering internet-scale systems using (nearly) entirely renewable energy. We perform a trace-based study to evaluate three issues related to achieving this goal: the impact of geographical load balancing, the role of storage, and the optimal mix of renewables. Our results highlight that geographical load balancing can significantly reduce the required capacity of renewable energy by using the energy more efficiently with "follow the renewables" routing. Further, our results show that small-scale storage can be useful, especially in combination with geographical load balancing, and that an optimal mix of renewables includes significantly more wind than photovoltaic solar.https://authors.library.caltech.edu/records/jz61t-zg441Zero Duality Gap in Optimal Power Flow Problem
https://resolver.caltech.edu/CaltechAUTHORS:20120227-144014854
Authors: {'items': [{'id': 'Lavaei-J', 'name': {'family': 'Lavaei', 'given': 'Javad'}}, {'id': 'Low-S-H', 'name': {'family': 'Low', 'given': 'Steven H.'}, 'orcid': '0000-0001-6476-3048'}]}
Year: 2012
DOI: 10.1109/TPWRS.2011.2160974
The optimal power flow (OPF) problem is nonconvex and generally hard to solve. In this paper, we propose a semidefinite programming (SDP) optimization, which is the dual of an equivalent form of the OPF problem. A global optimum solution to the OPF problem can be retrieved from a solution of this convex dual problem whenever the duality gap is zero. A necessary and sufficient condition is provided in this paper to guarantee the existence of no duality gap for the OPF problem. This condition is satisfied by the standard IEEE benchmark systems with 14, 30, 57, 118, and 300 buses as well as several randomly generated systems. Since this condition is hard to study, a sufficient zero-duality-gap condition is also derived. This sufficient condition holds for IEEE systems after small resistance (10^(-5) per unit) is added to every transformer that originally assumes zero resistance. We investigate this sufficient condition and justify that it holds widely in practice. The main underlying reason for the successful convexification of the OPF problem can be traced back to the modeling of transformers and transmission lines as well as the non-negativity of physical quantities such as resistance and inductance.https://authors.library.caltech.edu/records/89psq-3c794Energy-efficient congestion control
https://resolver.caltech.edu/CaltechAUTHORS:20130109-105014583
Authors: {'items': [{'id': 'Gan-Lingwen', 'name': {'family': 'Gan', 'given': 'Lingwen'}}, {'id': 'Walid-Anwar', 'name': {'family': 'Walid', 'given': 'Anwar'}}, {'id': 'Low-S-H', 'name': {'family': 'Low', 'given': 'Steven H.'}, 'orcid': '0000-0001-6476-3048'}]}
Year: 2012
DOI: 10.1145/2318857.2254770
Various link bandwidth adjustment mechanisms are being
developed to save network energy. However, their interaction
with congestion control can significantly reduce throughput,
and is not well understood. We firstly put forward an easily
implementable link dynamic bandwidth adjustment (DBA)
mechanism, and then study its iteration with congestion control.
In DBA, each link updates its bandwidth according
to an integral control law to match its average buffer size
with a target buffer size. We prove that DBA reduces link
bandwidth without sacrificing throughput-DBA only turns
off excess bandwidth-in the presence of congestion control.
Preliminary ns2 simulations confirm this result.https://authors.library.caltech.edu/records/dajde-cy387A Stochastic Power Network Calculus for Integrating Renewable Energy Sources into the Power Grid
https://resolver.caltech.edu/CaltechAUTHORS:20120807-081558429
Authors: {'items': [{'id': 'Wang-Kai', 'name': {'family': 'Wang', 'given': 'Kai'}}, {'id': 'Ciucu-F', 'name': {'family': 'Ciucu', 'given': 'Florin'}}, {'id': 'Lin-Chuang', 'name': {'family': 'Lin', 'given': 'Chuang'}}, {'id': 'Low-S-H', 'name': {'family': 'Low', 'given': 'Steven H.'}, 'orcid': '0000-0001-6476-3048'}]}
Year: 2012
DOI: 10.1109/JSAC.2012.120703
Renewable energy such as solar and wind generation will constitute an important part of the future grid. As the availability of renewable sources may not match the load, energy storage is essential for grid stability. In this paper we investigate the feasibility of integrating solar photovoltaic (PV) panels and wind turbines into the grid by also accounting for energy storage. To deal with the fluctuation in both the power supply and demand, we extend and apply stochastic network calculus to analyze the power supply reliability with various renewable energy configurations. To illustrate the validity of the model, we conduct a case study for the integration of renewable energy sources into the power system of an island off the coast of Southern California. In particular, we asses the power supply reliability in terms of the average Fraction of Time that energy is Not-Served (FTNS).https://authors.library.caltech.edu/records/mhcbm-tjn40Adaptive VAR Control for Distribution Circuits With Photovoltaic Generators
https://resolver.caltech.edu/CaltechAUTHORS:20121127-113956570
Authors: {'items': [{'id': 'Yeh-H-G', 'name': {'family': 'Yeh', 'given': 'Hen-Geul'}}, {'id': 'Gayme-D-F', 'name': {'family': 'Gayme', 'given': 'Dennice F.'}}, {'id': 'Low-S-H', 'name': {'family': 'Low', 'given': 'Steven H.'}, 'orcid': '0000-0001-6476-3048'}]}
Year: 2012
DOI: 10.1109/TPWRS.2012.2183151
We show how an adaptive control algorithm can improve the performance of distributed reactive power control in a radial distribution circuit with a high penetration of photovoltaic (PV) cells. The adaptive algorithm is designed to balance the need for power quality (voltage regulation) with the desire to minimize power loss. The adaptation law determines whether the objective function minimizes power losses or voltage regulation based on whether the voltage at each node remains close enough to the voltage at the substation. The reactive power is controlled through the inverter on the PV cells. The control signals are determined based on local instantaneous measurements of the real and reactive power at each node. We use the example of a single branch radial distribution circuit to demonstrate the ability of the adaptive scheme to effectively reduce voltage variations while simultaneously minimizing the power loss in the studied cases. Simulations verify that the adaptive schemes compares favorably with local and global schemes previously reported in the literature.https://authors.library.caltech.edu/records/xjb5s-se637Congestion Control for Multicast Flows With Network Coding
https://resolver.caltech.edu/CaltechAUTHORS:20121008-112913029
Authors: {'items': [{'id': 'Chen-Lijun', 'name': {'family': 'Chen', 'given': 'Lijun'}}, {'id': 'Ho-Tracey', 'name': {'family': 'Ho', 'given': 'Tracey'}}, {'id': 'Chiang-Mung', 'name': {'family': 'Chiang', 'given': 'Mung'}}, {'id': 'Low-S-H', 'name': {'family': 'Low', 'given': 'Steven H.'}, 'orcid': '0000-0001-6476-3048'}, {'id': 'Doyle-J-C', 'name': {'family': 'Doyle', 'given': 'John C.'}, 'orcid': '0000-0002-1828-2486'}]}
Year: 2012
DOI: 10.1109/TIT.2012.2204170
Recent advances in network coding have shown great potential for efficient information multicasting in communication networks, in terms of both network throughput and network management. In this paper, the problem of flow control at end-systems for network-coding-based multicast flows is addressed. Optimization-based models are formulated for network resource allocation, based on which two sets of decentralized controllers at sources and links/nodes for congestion control are developed for wired networks with given coding subgraphs and without given coding subgraphs, respectively. With random network coding, both sets of controllers can be implemented in a distributed manner, and work at the transport layer to adjust source rates and at network layer to carry out network coding. The convergence of the proposed controllers to the desired equilibrium operating points is proved, and numerical examples are provided to complement the theoretical analysis. The extension to wireless networks is also briefly discussed.https://authors.library.caltech.edu/records/s7xxe-f8n93Max-Min SINR Coordinated Multipoint Downlink Transmission—Duality and Algorithms
https://resolver.caltech.edu/CaltechAUTHORS:20121029-090718734
Authors: {'items': [{'id': 'Cai-Desmond-W-H', 'name': {'family': 'Cai', 'given': 'Desmond W. H.'}, 'orcid': '0000-0001-9207-1890'}, {'id': 'Quek-Tony-Q-S', 'name': {'family': 'Quek', 'given': 'Tony Q. S.'}}, {'id': 'Tan-Chee-Wei', 'name': {'family': 'Tan', 'given': 'Chee Wei'}}, {'id': 'Low-S-H', 'name': {'family': 'Low', 'given': 'Steven H.'}, 'orcid': '0000-0001-6476-3048'}]}
Year: 2012
DOI: 10.1109/TSP.2012.2208631
This paper considers the max-min weighted signal-to-interference-plus-noise ratio (SINR) problem subject to multiple weighted-sum power constraints, where the weights can represent relative power costs of serving different users. First, we study the power control problem. We apply nonlinear Perron-Frobenius theory to derive closed-form expressions for the optimal value and solution and an iterative algorithm which converges geometrically fast to the optimal solution. Then, we use the structure of the closed-form solution to show that the problem can be decoupled into subproblems each involving only one power constraint. Next, we study the multiple-input-single-output (MISO) transmit beamforming and power control problem. We use uplink-downlink duality to show that this problem can be decoupled into subproblems each involving only one power constraint. We apply this decoupling result to derive an iterative subgradient projection algorithm for the problem.https://authors.library.caltech.edu/records/14e1k-thz31Optimal Decentralized Protocol for Electric Vehicle Charging
https://resolver.caltech.edu/CaltechAUTHORS:20131003-094643635
Authors: {'items': [{'id': 'Gan-Lingwen', 'name': {'family': 'Gan', 'given': 'Lingwen'}}, {'id': 'Topcu-U', 'name': {'family': 'Topcu', 'given': 'Ufuk'}}, {'id': 'Low-S-H', 'name': {'family': 'Low', 'given': 'Steven H.'}, 'orcid': '0000-0001-6476-3048'}]}
Year: 2013
DOI: 10.1109/TPWRS.2012.2210288
We propose a decentralized algorithm to optimally schedule electric vehicle (EV) charging. The algorithm exploits the elasticity of electric vehicle loads to fill the valleys in electric load profiles. We first formulate the EV charging scheduling problem as an optimal control problem, whose objective is to impose a generalized notion of valley-filling, and study properties of optimal charging profiles. We then give a decentralized algorithm to iteratively solve the optimal control problem. In each iteration, EVs update their charging profiles according to the control signal broadcast by the utility company, and the utility company alters the control signal to guide their updates. The algorithm converges to optimal charging profiles (that are as "flat" as they can possibly be) irrespective of the specifications (e.g., maximum charging rate and deadline) of EVs, even if EVs do not necessarily update their charging profiles in every iteration, and use potentially outdated control signal when they update. Moreover, the algorithm only requires each EV solving its local problem, hence its implementation requires low computation capability. We also extend the algorithm to track a given load profile and to real-time implementation.https://authors.library.caltech.edu/records/6bhnt-fy360Multipath TCP algorithms: theory and design
https://resolver.caltech.edu/CaltechAUTHORS:20161207-173639173
Authors: {'items': [{'id': 'Peng-Qiuyu', 'name': {'family': 'Peng', 'given': 'Qiuyu'}}, {'id': 'Walid-A', 'name': {'family': 'Walid', 'given': 'Anwar'}}, {'id': 'Low-S-H', 'name': {'family': 'Low', 'given': 'Steven H.'}, 'orcid': '0000-0001-6476-3048'}]}
Year: 2013
DOI: 10.1145/2494232.2466585
Multi-path TCP (MP-TCP) has the potential to greatly improve application performance by using multiple paths transparently. We propose a fluid model for a large class of MP-TCP algorithms and identify design criteria that guarantee the existence, uniqueness, and stability of system equilibrium. We characterize algorithm parameters for TCP-friendliness and prove an inevitable tradeoff between responsiveness and friendliness. We discuss the implications of these properties on the behavior of existing algorithms and motivate a new design that generalizes existing algorithms. We use ns2 simulations to evaluate the proposed algorithm and illustrate its superior overall performance.https://authors.library.caltech.edu/records/awnxr-ytb91Exact convex relaxation for optimal power flow in distribution networks
https://resolver.caltech.edu/CaltechAUTHORS:20161025-153928779
Authors: {'items': [{'id': 'Gan-Lingwen', 'name': {'family': 'Gan', 'given': 'Lingwen'}}, {'id': 'Li-Na', 'name': {'family': 'Li', 'given': 'Na'}}, {'id': 'Low-S-H', 'name': {'family': 'Low', 'given': 'Steven H.'}, 'orcid': '0000-0001-6476-3048'}, {'id': 'Topcu-U', 'name': {'family': 'Topcu', 'given': 'Ufuk'}}]}
Year: 2013
DOI: 10.1145/2494232.2465535
The optimal power flow (OPF) problem seeks to control the power generation/consumption to minimize the generation cost, and is becoming important for distribution networks. OPF is nonconvex and a second-order cone programming (SOCP) relaxation has been proposed to solve it. We prove that after a "small" modification to OPF, the SOCP relaxation is exact under a "mild" condition. Empirical studies demonstrate that the modification to OPF is "small" and that the "mild" condition holds for all test networks, including the IEEE 13-bus test network and practical networks with high penetration of distributed generation.https://authors.library.caltech.edu/records/agta3-mdn57Branch Flow Model: Relaxations and Convexification—Part II
https://resolver.caltech.edu/CaltechAUTHORS:20130916-151203881
Authors: {'items': [{'id': 'Farivar-M', 'name': {'family': 'Farivar', 'given': 'Masoud'}}, {'id': 'Low-S-H', 'name': {'family': 'Low', 'given': 'Steven H.'}, 'orcid': '0000-0001-6476-3048'}]}
Year: 2013
DOI: 10.1109/TPWRS.2013.2255318
We propose a branch flow model for the analysis and optimization of mesh as well as radial networks. The model leads to a new approach to solving optimal power flow (OPF) that consists of two relaxation steps. The first step eliminates the voltage and current angles and the second step approximates the resulting problem by a conic program that can be solved efficiently. For radial networks, we prove that both relaxation steps are always exact, provided there are no upper bounds on loads. For mesh networks, the conic relaxation is always exact but the angle relaxation may not be exact, and we provide a simple way to determine if a relaxed solution is globally optimal. We propose convexification of mesh networks using phase shifters so that OPF for the convexified network can always be solved efficiently for an optimal solution. We prove that convexification requires phase shifters only outside a spanning tree of the network and their placement depends only on network topology, not on power flows, generation, loads, or operating constraints. Part I introduces our branch flow model, explains the two relaxation steps, and proves the conditions for exact relaxation. Part II describes convexification of mesh networks, and presents simulation results.https://authors.library.caltech.edu/records/ndxj1-n1k24Branch Flow Model: Relaxations and Convexification—Part I
https://resolver.caltech.edu/CaltechAUTHORS:20131003-153831161
Authors: {'items': [{'id': 'Farivar-M', 'name': {'family': 'Farivar', 'given': 'Masoud'}}, {'id': 'Low-S-H', 'name': {'family': 'Low', 'given': 'Steven H.'}, 'orcid': '0000-0001-6476-3048'}]}
Year: 2013
DOI: 10.1109/TPWRS.2013.2255317
We propose a branch flow model for the analysis and optimization of mesh as well as radial networks. The model leads to a new approach to solving optimal power flow (OPF) that consists of two relaxation steps. The first step eliminates the voltage and current angles and the second step approximates the resulting problem by a conic program that can be solved efficiently. For radial networks, we prove that both relaxation steps are always exact, provided there are no upper bounds on loads. For mesh networks, the conic relaxation is always exact but the angle relaxation may not be exact, and we provide a simple way to determine if a relaxed solution is globally optimal. We propose convexification of mesh networks using phase shifters so that OPF for the convexified network can always be solved efficiently for an optimal solution. We prove that convexification requires phase shifters only outside a spanning tree of the network and their placement depends only on network topology, not on power flows, generation, loads, or operating constraints. Part I introduces our branch flow model, explains the two relaxation steps, and proves the conditions for exact relaxation. Part II describes convexification of mesh networks, and presents simulation results.https://authors.library.caltech.edu/records/vsdpk-r8b62Optimal Load Control via Frequency Measurement and Neighborhood Area Communication
https://resolver.caltech.edu/CaltechAUTHORS:20131203-091918486
Authors: {'items': [{'id': 'Zhao-Changhong', 'name': {'family': 'Zhao', 'given': 'Changhong'}, 'orcid': '0000-0003-0539-8591'}, {'id': 'Topcu-U', 'name': {'family': 'Topcu', 'given': 'Ufuk'}}, {'id': 'Low-S-H', 'name': {'family': 'Low', 'given': 'Steven H.'}, 'orcid': '0000-0001-6476-3048'}]}
Year: 2013
DOI: 10.1109/TPWRS.2013.2261096
We propose a decentralized optimal load control scheme that provides contingency reserve in the presence of sudden generation drop. The scheme takes advantage of flexibility of frequency responsive loads and neighborhood area communication to solve an optimal load control problem that balances load and generation while minimizing end-use disutility of participating in load control. Local frequency measurements enable individual loads to estimate the total mismatch between load and generation. Neighborhood area communication helps mitigate effects of inconsistencies in the local estimates due to frequency measurement noise. Case studies show that the proposed scheme can balance load with generation and restore the frequency within seconds of time after a generation drop, even when the loads use a highly simplified power system model in their algorithms. We also investigate tradeoffs between the amount of communication and the performance of the proposed scheme through simulation-based experiments.https://authors.library.caltech.edu/records/wdh99-fkz82Impact of residential PV adoption on Retail Electricity Rates
https://resolver.caltech.edu/CaltechAUTHORS:20131213-111904382
Authors: {'items': [{'id': 'Cai-Desmond-W-H', 'name': {'family': 'Cai', 'given': 'Desmond W. H.'}, 'orcid': '0000-0001-9207-1890'}, {'id': 'Adlakha-S', 'name': {'family': 'Adlakha', 'given': 'Sachin'}}, {'id': 'Low-S-H', 'name': {'family': 'Low', 'given': 'Steven H.'}, 'orcid': '0000-0001-6476-3048'}, {'id': 'De-Martini-P', 'name': {'family': 'De Martini', 'given': 'Paul'}}, {'id': 'Chandy-K-M', 'name': {'family': 'Chandy', 'given': 'K. Mani'}}]}
Year: 2013
DOI: 10.1016/j.enpol.2013.07.009
The price of electricity supplied from home rooftop photo voltaic (PV) solar cells has fallen below the retail price of grid electricity in some areas. A number of residential households have an economic incentive to install rooftop PV systems and reduce their purchases of electricity from the grid. A significant portion of the costs incurred by utility companies are fixed costs which must be recovered even as consumption falls. Electricity rates must increase in order for utility companies to recover fixed costs from shrinking sales bases. Increasing rates will, in turn, result in even more economic incentives for customers to adopt rooftop PV. In this paper, we model this feedback between PV adoption and electricity rates and study its impact on future PV penetration and net-metering costs. We find that the most important parameter that determines whether this feedback has an effect is the fraction of customers who adopt PV in any year based solely on the money saved by doing so in that year, independent of the uncertainties of future years. These uncertainties include possible changes in rate structures such as the introduction of connection charges, the possibility of PV prices dropping significantly in the future, possible changes in tax incentives, and confidence in the reliability and maintainability of PV.https://authors.library.caltech.edu/records/r68eb-k0712Convex Relaxation of Optimal Power Flow — Part I: Formulations and Equivalence
https://resolver.caltech.edu/CaltechAUTHORS:20140521-104006650
Authors: {'items': [{'id': 'Low-S-H', 'name': {'family': 'Low', 'given': 'Steven H.'}, 'orcid': '0000-0001-6476-3048'}]}
Year: 2014
DOI: 10.1109/TCNS.2014.2309732
This tutorial summarizes recent advances in the convex
relaxation of the optimal power flow (OPF) problem, focusing on structural properties rather than algorithms. Part I presents two power flow models, formulates OPF and their relaxations in each model, and proves equivalence relationships among them. Part II presents sufficient conditions under which the convex relaxations are exact.https://authors.library.caltech.edu/records/436tr-60p50Greening Geographical Load Balancing
https://resolver.caltech.edu/CaltechAUTHORS:20150112-084413455
Authors: {'items': [{'id': 'Liu-Zhenhua', 'name': {'family': 'Liu', 'given': 'Zhenhua'}}, {'id': 'Lin-Minghong', 'name': {'family': 'Lin', 'given': 'Minghong'}}, {'id': 'Wierman-A', 'name': {'family': 'Wierman', 'given': 'Adam'}}, {'id': 'Low-S-H', 'name': {'family': 'Low', 'given': 'Steven'}, 'orcid': '0000-0001-6476-3048'}, {'id': 'Andrew-L-L-H', 'name': {'family': 'Andrew', 'given': 'Lachlan L. H.'}}]}
Year: 2014
DOI: 10.1109/TNET.2014.2308295
Energy expenditure has become a significant fraction of data center operating costs. Recently, "geographical load balancing" has been proposed to reduce energy cost by exploiting the electricity price differences across regions. However, this reduction of cost can paradoxically increase total energy use. We explore whether the geographical diversity of Internet-scale systems can also provide environmental gains. Specifically, we explore whether geographical load balancing can encourage use of "green" renewable energy and reduce use of "brown" fossil fuel energy. We make two contributions. First, we derive three distributed algorithms for achieving optimal geographical load balancing. Second, we show that if the price of electricity is proportional to the instantaneous fraction of the total energy that is brown, then geographical load balancing significantly reduces brown energy use. However, the benefits depend strongly on dynamic energy pricing and the form of pricing used.https://authors.library.caltech.edu/records/nqgqj-djm27Design and Stability of Load-Side Primary Frequency Control in Power Systems
https://resolver.caltech.edu/CaltechAUTHORS:20140529-094709292
Authors: {'items': [{'id': 'Zhao-Changhong', 'name': {'family': 'Zhao', 'given': 'Changhong'}, 'orcid': '0000-0003-0539-8591'}, {'id': 'Topcu-U', 'name': {'family': 'Topcu', 'given': 'Ufuk'}}, {'id': 'Li-Na', 'name': {'family': 'Li', 'given': 'Na'}}, {'id': 'Low-S-H', 'name': {'family': 'Low', 'given': 'Steven'}, 'orcid': '0000-0001-6476-3048'}]}
Year: 2014
DOI: 10.1109/TAC.2014.2298140
We present a systematic method to design ubiquitous continuous fast-acting distributed load control for primary frequency regulation in power networks, by formulating an optimal load control (OLC) problem where the objective is to minimize the aggregate cost of tracking an operating point subject to power balance over the network. We prove that the swing dynamics and the branch power flows, coupled with frequency-based load control, serve as a distributed primal-dual algorithm to solve OLC. We establish the global asymptotic stability of a multimachine network under such type of load-side primary frequency control. These results imply that the local frequency deviations on each bus convey exactly the right information about the global power imbalance for the loads to make individual decisions that turn out to be globally optimal. Simulations confirm that the proposed algorithm can rebalance power and resynchronize bus frequencies after a disturbance with significantly improved transient performance.https://authors.library.caltech.edu/records/srhae-p5128Pricing link by time
https://resolver.caltech.edu/CaltechAUTHORS:20161121-153645812
Authors: {'items': [{'id': 'Lai-Chengdi', 'name': {'family': 'Lai', 'given': 'Chengdi'}}, {'id': 'Low-S-H', 'name': {'family': 'Low', 'given': 'Steven H.'}, 'orcid': '0000-0001-6476-3048'}, {'id': 'Leung-Ka-Cheong', 'name': {'family': 'Leung', 'given': 'Ka-Cheong'}}, {'id': 'Li-Victor-O-K', 'name': {'family': 'Li', 'given': 'Victor O. K.'}}]}
Year: 2014
DOI: 10.1145/2637364.2591974
The combination of loss-based TCP and drop-tail routers often results in full buffers, creating large queueing delays. The challenge with parameter tuning and the drastic consequence of improper tuning have discouraged network administrators from enabling AQM even when routers support it. To address this problem, we propose a novel design principle for AQM, called the pricing-link-by-time (PLT) principle. PLT increases the link price as the backlog stays above a threshold β, and resets the price once the backlog goes below β. We prove that such a system exhibits cyclic behavior that is robust against changes in network environment and protocol parameters. While β approximately controls the level of backlog, the backlog dynamics are invariant for β across a wide range of values. Therefore, β can be chosen to reduce delay without undermining system performance. We validate these analytical results using packet-level simulation.https://authors.library.caltech.edu/records/h1e5n-rdr69Pricing Data Center Demand Response
https://resolver.caltech.edu/CaltechAUTHORS:20140804-133949957
Authors: {'items': [{'id': 'Liu-Zhenhua', 'name': {'family': 'Liu', 'given': 'Zhenhua'}}, {'id': 'Liu-Iris', 'name': {'family': 'Liu', 'given': 'Iris'}}, {'id': 'Low-S-H', 'name': {'family': 'Low', 'given': 'Steven'}, 'orcid': '0000-0001-6476-3048'}, {'id': 'Wierman-A', 'name': {'family': 'Wierman', 'given': 'Adam'}}]}
Year: 2014
DOI: 10.1145/2591971.2592004
Demand response is crucial for the incorporation of renewable energy into the grid. In this paper, we focus on a particularly promising industry for demand response: data centers. We use simulations to show that, not only are data centers large loads, but they can provide as much (or possibly more) flexibility as large-scale storage if given the proper incentives. However, due to the market power most data centers maintain, it is difficult to design programs that are efficient for data center demand response. To that end, we propose that prediction-based pricing is an appealing market design, and show that it outperforms more traditional supply function bidding mechanisms in situations where market power is an issue. However, prediction-based pricing may be inefficient when predictions are inaccurate, and so we provide analytic, worst-case bounds on the impact of prediction error on the efficiency of prediction-based pricing. These bounds hold even when network constraints are considered, and highlight that prediction-based pricing is surprisingly robust to prediction error.https://authors.library.caltech.edu/records/8vqy3-0p575Convex Relaxation of Optimal Power Flow - Part II: Exactness
https://resolver.caltech.edu/CaltechAUTHORS:20150922-153930585
Authors: {'items': [{'id': 'Low-S-H', 'name': {'family': 'Low', 'given': 'Steven H.'}, 'orcid': '0000-0001-6476-3048'}]}
Year: 2014
DOI: 10.1109/TCNS.2014.2323634
This tutorial summarizes recent advances in the convex relaxation of the optimal power flow (OPF) problem, focusing on structural properties rather than algorithms. Part I presents two power flow models, formulates OPF and their relaxations in each model, and proves equivalence relations among them. Part II presents sufficient conditions under which the convex relaxations are exact.https://authors.library.caltech.edu/records/jdcgw-gqx82Optimal Power Flow in Direct Current Networks
https://resolver.caltech.edu/CaltechAUTHORS:20141205-135843283
Authors: {'items': [{'id': 'Gan-Lingwen', 'name': {'family': 'Gan', 'given': 'Lingwen'}}, {'id': 'Low-S-H', 'name': {'family': 'Low', 'given': 'Steven H.'}, 'orcid': '0000-0001-6476-3048'}]}
Year: 2014
DOI: 10.1109/TPWRS.2014.2313514
The optimal power flow (OPF) problem determines power generations/demands that minimize a certain objective such as generation cost or power loss. It is non-convex and NP-hard in general. In this paper, we study the OPF problem in direct current (DC) networks. A second-order cone programming (SOCP) relaxation is considered for solving the OPF problem. We prove that the SOCP relaxation is exact if either 1) voltage upper bounds do not bind; or 2) voltage upper bounds are uniform and power injection lower bounds are negative. Based on 1), a modified OPF problem is proposed, whose corresponding SOCP is guaranteed to be exact. We also prove that SOCP has at most one optimal solution if it is exact. Finally, we discuss how to improve numerical stability and how to include line constraints.https://authors.library.caltech.edu/records/zf7qf-8jb24Buffering Dynamics and Stability of Internet Congestion Controllers
https://resolver.caltech.edu/CaltechAUTHORS:20150109-091116665
Authors: {'items': [{'id': 'Sojoudi-S', 'name': {'family': 'Sojoudi', 'given': 'Somayeh'}}, {'id': 'Low-S-H', 'name': {'family': 'Low', 'given': 'Steven H.'}, 'orcid': '0000-0001-6476-3048'}, {'id': 'Doyle-J-C', 'name': {'family': 'Doyle', 'given': 'John C.'}, 'orcid': '0000-0002-1828-2486'}]}
Year: 2014
DOI: 10.1109/TNET.2013.2287198
Many existing fluid-flow models of the Internet congestion
control algorithms make simplifying assumptions on the
effects of buffers on the data flows. In particular, they assume that the flow rate of a TCP flow at every link in its path is equal to the original source rate. However, a fluid flow in practice is modified by the queueing processes on its path, so that an intermediate link will generally not see the original source rate. In this paper, a more accurate model is derived for the behavior of the
network under a congestion controller, which takes into account the effect of buffering on output flows. It is shown how this model can be deployed for some well-known service disciplines such as first-in–first-out and generalized weighted fair queueing. Based on the derived model, the dual and primal-dual algorithms are studied
under the common pricing mechanisms, and it is shown that these algorithms can become unstable. Sufficient conditions are provided to guarantee the stability of the dual and primal-dual algorithms. Finally, a new pricing mechanism is proposed under which these congestion control algorithms are both stable.https://authors.library.caltech.edu/records/2qaks-zjf17Exact Convex Relaxation of Optimal Power Flow in Radial Networks
https://resolver.caltech.edu/CaltechAUTHORS:20150203-084513340
Authors: {'items': [{'id': 'Gan-Lingwen', 'name': {'family': 'Gan', 'given': 'Lingwen'}}, {'id': 'Li-Na', 'name': {'family': 'Li', 'given': 'Na'}}, {'id': 'Topcu-U', 'name': {'family': 'Topcu', 'given': 'Ufuk'}}, {'id': 'Low-S-H', 'name': {'family': 'Low', 'given': 'Steven H.'}, 'orcid': '0000-0001-6476-3048'}]}
Year: 2015
DOI: 10.1109/TAC.2014.2332712
The optimal power flow (OPF) problem determines a network operating point that minimizes a certain objective such as generation cost or power loss. It is nonconvex. We prove that a global optimum of OPF can be obtained by solving a second-order cone program, under a mild condition after shrinking the OPF feasible set slightly, for radial power networks. The condition can be checked a priori, and holds for the IEEE 13, 34, 37, 123-bus networks and two real-world networks.https://authors.library.caltech.edu/records/7byye-f8609Equivalent relaxations of optimal power flow
https://resolver.caltech.edu/CaltechAUTHORS:20150112-101936328
Authors: {'items': [{'id': 'Bose-Subhonmesh', 'name': {'family': 'Bose', 'given': 'Subhonmesh'}, 'orcid': '0000-0002-3445-4479'}, {'id': 'Low-S-H', 'name': {'family': 'Low', 'given': 'Steven H.'}, 'orcid': '0000-0001-6476-3048'}, {'id': 'Teeraratkul-T', 'name': {'family': 'Teeraratkul', 'given': 'Thanchanok'}}, {'id': 'Hassibi-B', 'name': {'family': 'Hassibi', 'given': 'Babak'}}]}
Year: 2015
DOI: 10.1109/TAC.2014.2357112
Several convex relaxations of the optimal power flow (OPF) problem have recently been developed using both bus injection models and branch flow models. In this paper, we prove relations among three convex relaxations: a semidefinite relaxation that computes a full matrix, a chordal relaxation based on a chordal extension of the network graph, and a second-order cone relaxation that computes the smallest partial matrix. We prove a bijection between the feasible sets of the OPF in the bus injection model and the branch flow model, establishing the equivalence of these two models and their second-order cone relaxations. Our results imply that, for radial networks, all these relaxations are equivalent and one should always solve the second-order cone relaxation. For mesh networks, the semidefinite relaxation and the chordal relaxation are equally tight and both are strictly tighter than the second-order cone relaxation. Therefore, for mesh networks, one should either solve the chordal relaxation or the SOCP relaxation, trading off tightness and the required computational effort. Simulations are used to illustrate these results.https://authors.library.caltech.edu/records/cm86v-dwe23Feeder Reconfiguration in Distribution Networks Based on Convex Relaxation of OPF
https://resolver.caltech.edu/CaltechAUTHORS:20150112-094953179
Authors: {'items': [{'id': 'Peng-Qiuyu', 'name': {'family': 'Peng', 'given': 'Qiuyu'}}, {'id': 'Tang-Yujie', 'name': {'family': 'Tang', 'given': 'Yujie'}, 'orcid': '0000-0002-4921-8372'}, {'id': 'Low-S-H', 'name': {'family': 'Low', 'given': 'Steven H.'}, 'orcid': '0000-0001-6476-3048'}]}
Year: 2015
DOI: 10.1109/TPWRS.2014.2356513
The feeder reconfiguration problem chooses the on/off status of the switches in a distribution network in order to minimize a certain cost such as power loss. It is a mixed-integer nonlinear program and, hence, hard to solve. In this paper, we propose a heuristic algorithm that is based on the recently developed convex relaxation of the ac optimal power flow problem. The algorithm is computationally efficient and scales linearly with the number of redundant lines. It requires neither parameter tuning nor initialization for different networks. It successfully computes an optimal configuration on all four networks we have tested. Moreover, we have proved that the algorithm solves the feeder reconfiguration problem optimally under certain conditions for the case where only a single redundant line needs to be opened. We also propose a more computationally efficient algorithm and show that it incurs a loss in optimality of less than 3% on the four test networks.https://authors.library.caltech.edu/records/59t7c-rdd16Quadratically Constrained Quadratic Programs on Acyclic Graphs With Application to Power Flow
https://resolver.caltech.edu/CaltechAUTHORS:20170810-100823060
Authors: {'items': [{'id': 'Bose-Subhonmesh', 'name': {'family': 'Bose', 'given': 'Subhonmesh'}, 'orcid': '0000-0002-3445-4479'}, {'id': 'Gayme-D-F', 'name': {'family': 'Gayme', 'given': 'Dennice F.'}}, {'id': 'Chandy-K-M', 'name': {'family': 'Chandy', 'given': 'K. Mani'}}, {'id': 'Low-S-H', 'name': {'family': 'Low', 'given': 'Steven H.'}, 'orcid': '0000-0001-6476-3048'}]}
Year: 2015
DOI: 10.1109/TCNS.2015.2401172
This paper proves that nonconvex quadratically constrained quadratic programs can be solved in polynomial time when their underlying graph is acyclic, provided the constraints satisfy a certain technical condition. We demonstrate this theory on optimal power-flow problems over tree networks.https://authors.library.caltech.edu/records/5h1p1-dnz43On Channel Failures, File Fragmentation Policies, and Heavy-Tailed Completion Times
https://resolver.caltech.edu/CaltechAUTHORS:20160317-122351101
Authors: {'items': [{'id': 'Nair-J', 'name': {'family': 'Nair', 'given': 'Jayakrishnan'}}, {'id': 'Andreasson-M', 'name': {'family': 'Andreasson', 'given': 'Martin'}}, {'id': 'Andrew-L-L-H', 'name': {'family': 'Andrew', 'given': 'Lachlan L. H.'}}, {'id': 'Low-S-H', 'name': {'family': 'Low', 'given': 'Steven H.'}, 'orcid': '0000-0001-6476-3048'}, {'id': 'Doyle-J-C', 'name': {'family': 'Doyle', 'given': 'John C.'}, 'orcid': '0000-0002-1828-2486'}]}
Year: 2016
DOI: 10.1109/TNET.2014.2375920
It has been recently discovered that heavy-tailed completion times can result from protocol interaction even when file sizes are light-tailed. A key to this phenomenon is the use of a restart policy where if the file is interrupted before it is completed, it needs to restart from the beginning. In this paper, we show that fragmenting a file into pieces whose sizes are either bounded or independently chosen after each interruption guarantees light-tailed completion time as long as the file size is light-tailed; i.e., in this case, heavy-tailed completion time can only originate from heavy-tailed file sizes. If the file size is heavy-tailed, then the completion time is necessarily heavy-tailed. For this case, we show that when the file size distribution is regularly varying, then under independent or bounded fragmentation, the completion time tail distribution function is asymptotically bounded above by that of the original file size stretched by a constant factor. We then prove that if the distribution of times between interruptions has nondecreasing failure rate, the expected completion time is minimized by dividing the file into equal-sized fragments; this optimal fragment size is unique but depends on the file size. We also present a simple blind fragmentation policy where the fragment sizes are constant and independent of the file size and prove that it is asymptotically optimal. Both these policies are also shown to have desirable completion time tail behavior. Finally, we bound the error in expected completion time due to error in modeling of the failure process.https://authors.library.caltech.edu/records/jzwt1-qyv02Multipath TCP: Analysis, Design, and Implementation
https://resolver.caltech.edu/CaltechAUTHORS:20150109-090127092
Authors: {'items': [{'id': 'Peng-Qiuyu', 'name': {'family': 'Peng', 'given': 'Qiuyu'}}, {'id': 'Walid-A', 'name': {'family': 'Walid', 'given': 'Anwar'}}, {'id': 'Hwang-Jaehyun', 'name': {'family': 'Hwang', 'given': 'Jaehyun'}}, {'id': 'Low-S-H', 'name': {'family': 'Low', 'given': 'Steven H.'}, 'orcid': '0000-0001-6476-3048'}]}
Year: 2016
DOI: 10.1109/TNET.2014.2379698
Multipath TCP (MP-TCP) has the potential to greatly improve application performance by using multiple paths transparently. We propose a fluid model for a large class of MP-TCP algorithms and identify design criteria that guarantee the existence, uniqueness, and stability of system equilibrium. We clarify how algorithm parameters impact TCP-friendliness, responsiveness, and window oscillation and demonstrate an inevitable tradeoff among these properties. We discuss the implications of these properties on the behavior of existing algorithms and motivate our algorithm Balia (balanced linked adaptation), which generalizes existing algorithms and strikes a good balance among TCP-friendliness, responsiveness, and window oscillation. We have implemented Balia in the Linux kernel. We use our prototype to compare the new algorithm to existing MP-TCP algorithms.https://authors.library.caltech.edu/records/2c1mb-gkg87Demand Response With Capacity Constrained Supply Function Bidding
https://resolver.caltech.edu/CaltechAUTHORS:20160408-101533727
Authors: {'items': [{'id': 'Xu-Yunjian', 'name': {'family': 'Xu', 'given': 'Yunjian'}}, {'id': 'Li-Na', 'name': {'family': 'Li', 'given': 'Na'}}, {'id': 'Low-S-H', 'name': {'family': 'Low', 'given': 'Steven H.'}, 'orcid': '0000-0001-6476-3048'}]}
Year: 2016
DOI: 10.1109/TPWRS.2015.2421932
We study the problem faced by an operator who aims to allocate a certain amount of load adjustment (either load reduction or increment) to multiple consumers so as to minimize the aggregate consumer disutility. We propose and analyze a simple uniform-price market mechanism where every consumer submits a single bid to choose a supply function from a group of parameterized ones. These parameterized supply functions are designed to ensure that every consumer's load adjustment is within an exogenous capacity limit that is determined by the current power system operating condition. We show that the proposed mechanism yields bounded efficiency loss at a Nash equilibrium. In particular, the proposed mechanism is shown to achieve approximate social optimality at a Nash equilibrium, if the total capacity limit excluding the consumer with the largest one is much larger than the total amount of load to be adjusted. We complement our analysis through numerical case studies.https://authors.library.caltech.edu/records/38eaz-zmz59An Online Gradient Algorithm for Optimal Power Flow on Radial Networks
https://resolver.caltech.edu/CaltechAUTHORS:20160426-075434882
Authors: {'items': [{'id': 'Gan-Lingwen', 'name': {'family': 'Gan', 'given': 'Lingwen'}}, {'id': 'Low-S-H', 'name': {'family': 'Low', 'given': 'Steven H.'}, 'orcid': '0000-0001-6476-3048'}]}
Year: 2016
DOI: 10.1109/JSAC.2016.2525598
We propose an online algorithm for solving optimal power flow (OPF) problems on radial networks where the controllable devices continuously interact with the network that implicitly computes a power flow solution given a control action. Collectively the controllable devices and the network implement a gradient projection algorithm for the OPF problem in real time. The key design feature that enables this approach is that the intermediate iterates of our algorithm always satisfy power flow equations and operational constraints. This is achieved by explicitly exploiting the network to implicitly solve power flow equations for us in real time at scale. We prove that the proposed algorithm converges to the set of local optima and provide sufficient conditions under which it converges to a global optimum. We derive an upper bound on the suboptimality gap of any local optimum. This bound suggests that any local minimum is almost as good as any strictly feasible point. We explain how to greatly reduce the gradient computation in each iteration by using approximate gradient derived from linearized power flow equations. Numerical results on test networks, ranging from 42-bus to 1990-bus, show a great speedup over a second-order cone relaxation method with negligible difference in objective values.https://authors.library.caltech.edu/records/hb1hg-80v18Optimal Charging Schedule for a Battery Switching Station Serving Electric Buses
https://resolver.caltech.edu/CaltechAUTHORS:20160829-152749308
Authors: {'items': [{'id': 'You-Pengcheng', 'name': {'family': 'You', 'given': 'Pengcheng'}, 'orcid': '0000-0003-1532-8773'}, {'id': 'Yang-Zaiyue', 'name': {'family': 'Yang', 'given': 'Zaiyue'}, 'orcid': '0000-0002-8288-3833'}, {'id': 'Zhang-Yongmin', 'name': {'family': 'Zhang', 'given': 'Yongmin'}}, {'id': 'Low-S-H', 'name': {'family': 'Low', 'given': 'Steven H.'}, 'orcid': '0000-0001-6476-3048'}, {'id': 'Sun-Youxian', 'name': {'family': 'Sun', 'given': 'Youxian'}}]}
Year: 2016
DOI: 10.1109/TPWRS.2015.2487273
We propose a model of a battery switching station (BSS) for electric buses (EBs) that captures the predictability of bus operation. We schedule battery charging in the BSS so that every EB arrives to find a battery ready for switching. We develop an efficient algorithm to compute an optimal schedule. It uses dual decomposition to decouple the charging decisions at different charging boxes so that independent subproblems can be solved in parallel at individual charging boxes, making the algorithm inherently scalable as the size of the BSS grows. We propose a direct projection method that solves these subproblems rapidly. Numerical results illustrate that the proposed approach is far more efficient and scalable than generic algorithms and existing solvers.https://authors.library.caltech.edu/records/5ch6g-0n827An Efficient and Incentive Compatible Mechanism for Wholesale Electricity Markets
https://resolver.caltech.edu/CaltechAUTHORS:20170111-125209937
Authors: {'items': [{'id': 'Xu-Yunjian', 'name': {'family': 'Xu', 'given': 'Yunjian'}}, {'id': 'Low-S-H', 'name': {'family': 'Low', 'given': 'Steven H.'}, 'orcid': '0000-0001-6476-3048'}]}
Year: 2017
DOI: 10.1109/TSG.2015.2483523
Being widely used in many deregulated wholesale electricity markets, the locational marginal pricing (LMP) mechanism is known to achieve social optimality in a competitive market. When profit-maximizing generators act strategically to manipulate prices; however, LMP may lead to high loss of economic efficiency. In this paper, we apply the Vickrey-Clarke–Groves (VCG) mechanism to wholesale electricity markets. We show that the VCG mechanism minimizes the total cost at a truth-telling dominant strategy equilibrium. We establish an important comparative result that the VCG mechanism always results in higher per-unit electricity prices than the LMP mechanism under any given set of reported supply curves. Numerical results show that the difference between the per-unit prices resulting from the two mechanisms is negligibly small (about 4%) in the IEEE 14-bus and 30-bus test systems. Finally, we apply the VCG mechanism to a day-ahead setting with start-up cost (of conventional generators) and intermittent renewable generation. We show that the VCG mechanism induces the truth-telling behavior of conventional generators in dominant strategies and yields each conventional generator a non-negative expected profit.https://authors.library.caltech.edu/records/hc1fy-1t885Monotonicity Properties and Spectral Characterization of Power Redistribution in Cascading Failures
https://resolver.caltech.edu/CaltechAUTHORS:20180409-162520984
Authors: {'items': [{'id': 'Guo-Linqi', 'name': {'family': 'Guo', 'given': 'Linqi'}}, {'id': 'Liang-Chen', 'name': {'family': 'Liang', 'given': 'Chen'}}, {'id': 'Low-S-H', 'name': {'family': 'Low', 'given': 'Steven H.'}, 'orcid': '0000-0001-6476-3048'}]}
Year: 2017
DOI: 10.1145/3152042.3152074
In this work, we apply spectral graph theory methods to study the monotonicity and structural properties of power redistribution in a cascading failure process. We demonstrate that in contrast to the lack of monotonicity in physical domain, there is a rich collection of monotonicity one can explore in the spectral domain, leading to a systematic way to define topological metrics that are monotonic. It is further shown that many useful quantities in cascading failure analysis can be unified into a spectral inner product, which itself is related to graphical properties of the transmission network. Such graphical interpretations precisely capture the Kirchhoff's law expressed in terms of graph structural properties and gauge the impact of a line when it is tripped.https://authors.library.caltech.edu/records/8fan2-dq173Load-side Frequency Regulation with Limited Control Coverage
https://resolver.caltech.edu/CaltechAUTHORS:20180409-162519432
Authors: {'items': [{'id': 'Pang-John-Z-F', 'name': {'family': 'Pang', 'given': 'John Z. F.'}, 'orcid': '0000-0002-6485-7922'}, {'id': 'Guo-Linqi', 'name': {'family': 'Guo', 'given': 'Linqi'}}, {'id': 'Low-S-H', 'name': {'family': 'Low', 'given': 'Steven H.'}, 'orcid': '0000-0001-6476-3048'}]}
Year: 2017
DOI: 10.1145/3152042.3152071
Increasing renewable energy increases uncertainty in energy systems. As a consequence, generator-side control for frequency regulation, impacted by the slow reaction of generators to meet urgent needs, may no longer suffice. With increasing integration of smart appliances which are able to sense, communicate and control, load-side control can help alleviate the aforementioned problem as it reacts fast and helps to localize disturbances. However, almost all existing methods for optimal load-side control require full information control coverage in the system. Framing the problem as an optimization problem and applying saddle-point dynamics, we obtain a control law that rebalances power and asymptotically stabilizes frequency after a disturbance. We generalize previous work to design a controller which only requires partial control coverage over all nodes, yet still achieves secondary frequency control. We verify these results via simulation.https://authors.library.caltech.edu/records/4wtxc-vg851Battery Swapping Assignment for Electric Vehicles
https://resolver.caltech.edu/CaltechAUTHORS:20180409-142619118
Authors: {'items': [{'id': 'You-Pengcheng', 'name': {'family': 'You', 'given': 'Pengcheng'}, 'orcid': '0000-0003-1532-8773'}, {'id': 'Sun-Youxian', 'name': {'family': 'Sun', 'given': 'Youxian'}}, {'id': 'Pang-John-Z-F', 'name': {'family': 'Pang', 'given': 'John'}, 'orcid': '0000-0002-6485-7922'}, {'id': 'Low-S-H', 'name': {'family': 'Low', 'given': 'Steven'}, 'orcid': '0000-0001-6476-3048'}, {'id': 'Chen-Minghua', 'name': {'family': 'Chen', 'given': 'Minghua'}}]}
Year: 2017
DOI: 10.1145/3152042.3152068
This paper formulates a multi-period optimal station assignment problem for electric vehicle (EV) battery swapping that takes into account both temporal and spatial couplings. The goal is to reduce the total EV cost and station congestion due to temporary shortage in supply of available batteries. We show that the problem is reducible to the minimum weight perfect bipartite matching problem. This leads to an efficient solution based on the Hungarian algorithm. Numerical results suggest that the proposed solution provides a significant improvement over a greedy heuristic that assigns EVs to nearest stations.https://authors.library.caltech.edu/records/5kh3w-dkz90Real-time Optimal Power Flow
https://resolver.caltech.edu/CaltechAUTHORS:20170517-150114536
Authors: {'items': [{'id': 'Tang-Yujie', 'name': {'family': 'Tang', 'given': 'Yujie'}, 'orcid': '0000-0002-4921-8372'}, {'id': 'Dvijotham-K', 'name': {'family': 'Dvijotham', 'given': 'Krishnamurthy'}, 'orcid': '0000-0002-1328-4677'}, {'id': 'Low-S-H', 'name': {'family': 'Low', 'given': 'Steven'}, 'orcid': '0000-0001-6476-3048'}]}
Year: 2017
DOI: 10.1109/TSG.2017.2704922
Future power networks are expected to incorporate a large number of distributed energy resources, which introduce randomness and fluctuations as well as fast control capabilities. But traditional optimal power flow methods are only appropriate for applications that operate on a slow timescale. In this paper, we build on recent work to develop a real-time algorithm for AC optimal power flow, based on quasi-Newton methods. The algorithm uses second order information to provide suboptimal solutions on a fast timescale, and can be shown to track the optimal power flow solution when the estimated second order information is sufficiently accurate. We also give a specific implementation based on L-BFGS-B method, and show by simulation that the proposed algorithm has good performance and is computationally efficient.https://authors.library.caltech.edu/records/m23ne-7bw40Optimal Placement of Energy Storage in Distribution Networks
https://resolver.caltech.edu/CaltechAUTHORS:20170613-162646711
Authors: {'items': [{'id': 'Tang-Yujie', 'name': {'family': 'Tang', 'given': 'Yujie'}, 'orcid': '0000-0002-4921-8372'}, {'id': 'Low-S-H', 'name': {'family': 'Low', 'given': 'Steven H.'}, 'orcid': '0000-0001-6476-3048'}]}
Year: 2017
DOI: 10.1109/TSG.2017.2711921
We study the problem of optimal placement and capacity of energy storage devices in a distribution network to minimize total energy loss. A continuous tree with linearized DistFlow model is developed to model the distribution network. We analyze structural properties of the optimal solution when all loads have the same shape. We prove that it is optimal to place storage devices near the leaves of the network away from the substation, and that the scaled storage capacity monotonically increases towards the leaves. Moreover, under optimal storage placement, the locational marginal value of storage is equalized wherever nonzero storage is deployed and increases from the substation towards any leaf node over places where there is zero storage deployment. We illustrate through simulations that these structural properties are robust in that they hold approximately when some of our modeling assumptions are relaxed.https://authors.library.caltech.edu/records/7j9nx-td052Guest Editorial: Distributed Control and Efficient Optimization Methods for Smart Grid
https://resolver.caltech.edu/CaltechAUTHORS:20180119-140349830
Authors: {'items': [{'id': 'Lavaei-J', 'name': {'family': 'Lavaei', 'given': 'Javad'}}, {'id': 'Low-S-H', 'name': {'family': 'Low', 'given': 'Steven'}, 'orcid': '0000-0001-6476-3048'}, {'id': 'Baldick-R', 'name': {'family': 'Baldick', 'given': 'Ross'}}, {'id': 'Zhang-Baosen', 'name': {'family': 'Zhang', 'given': 'Baosen'}}, {'id': 'Molzahn-D-K', 'name': {'family': 'Molzahn', 'given': 'Daniel'}}, {'id': 'Dörfler-F', 'name': {'family': 'Dörfler', 'given': 'Florian'}}, {'id': 'Sandberg-H', 'name': {'family': 'Sandberg', 'given': 'Henrik'}}]}
Year: 2017
DOI: 10.1109/TSG.2017.2758907
Smart grid proactively uses the state-of-the-art technologies in communications, computing, and control to improve efficiency, reliability, sustainability, and stability of the electrical grid. In particular, distribution networks are expected to undergo dramatic changes by incorporating a large number of sensors and thousands of controllable devices such as distributed generators, batteries and flexible loads. To be able to efficiently operate such complex large-scale systems, new sets of control and optimization tools should be developed. On a slow time scale, optimization theory plays a major role in solving various large-scale decision-making problems for future power transmission and distribution systems. One major challenge is the design of computational methods for handling fairly detailed power system models that often include continuous and discrete nonlinearities. It is also important to develop distributed computation techniques to shift the computation from a centralized platform to many computing devices.https://authors.library.caltech.edu/records/0c4at-y0z55Distributed Control and Efficient Optimization Methods for Smart Grid
https://resolver.caltech.edu/CaltechAUTHORS:20171109-112814794
Authors: {'items': [{'id': 'Lavaei-J', 'name': {'family': 'Lavaei', 'given': 'Javad'}}, {'id': 'Low-S-H', 'name': {'family': 'Low', 'given': 'Steven'}, 'orcid': '0000-0001-6476-3048'}, {'id': 'Baldick-R', 'name': {'family': 'Baldick', 'given': 'Ross'}}, {'id': 'Zhang-Baosen', 'name': {'family': 'Zhang', 'given': 'Baosen'}}, {'id': 'Molzahn-D-K', 'name': {'family': 'Molzahn', 'given': 'Daniel'}}, {'id': 'Dörfler-F', 'name': {'family': 'Dörfler', 'given': 'Florian'}}, {'id': 'Sandberg-H', 'name': {'family': 'Sandberg', 'given': 'Henrik'}}]}
Year: 2017
DOI: 10.1109/TSG.2017.2758907
Smart grid proactively uses the state-of-the-art technologies in communications, computing, and control to improve efficiency, reliability, sustainability, and stability of the electrical grid. In particular, distribution networks are expected to undergo dramatic changes by incorporating a large number of sensors and thousands of controllable devices such as distributed generators, batteries and flexible loads. To be able to efficiently operate such complex large-scale systems, new sets of control and optimization tools should be developed. On a slow time scale, optimization theory plays a major role in solving various large-scale decision-making problems for future power transmission and distribution systems. One major challenge is the design of computational methods for handling fairly detailed power system models that often include continuous and discrete nonlinearities. It is also important to develop distributed computation techniques to shift the computation from a centralized platform to many computing devices.https://authors.library.caltech.edu/records/s2bk8-gcd08A Survey of Distributed Optimization and Control Algorithms for Electric Power Systems
https://resolver.caltech.edu/CaltechAUTHORS:20170726-151527403
Authors: {'items': [{'id': 'Molzahn-D-K', 'name': {'family': 'Molzahn', 'given': 'Daniel K.'}}, {'id': 'Dörfler-F', 'name': {'family': 'Dörfler', 'given': 'Florian'}}, {'id': 'Sandberg-H', 'name': {'family': 'Sandberg', 'given': 'Henrik'}}, {'id': 'Low-S-H', 'name': {'family': 'Low', 'given': 'Steven H.'}, 'orcid': '0000-0001-6476-3048'}, {'id': 'Chakrabarti-S', 'name': {'family': 'Chakrabarti', 'given': 'Sambuddha'}}, {'id': 'Baldick-R', 'name': {'family': 'Baldick', 'given': 'Ross'}}, {'id': 'Lavaei-J', 'name': {'family': 'Lavaei', 'given': 'Javad'}}]}
Year: 2017
DOI: 10.1109/TSG.2017.2720471
Historically, centrally computed algorithms have been the primary means of power system optimization and control. With increasing penetrations of distributed energy resources requiring optimization and control of power systems with many controllable devices, distributed algorithms have been the subject of significant research interest. This paper surveys the literature of distributed algorithms with applications to optimization and control of power systems. In particular, this paper reviews distributed algorithms for offline solution of optimal power flow (OPF) problems as well as online algorithms for real-time solution of OPF, optimal frequency control, optimal voltage control, and optimal wide-area control problems.https://authors.library.caltech.edu/records/e7pd5-zer96Optimal Load-side Control for Frequency Regulation in Smart Grids
https://resolver.caltech.edu/CaltechAUTHORS:20171101-115726975
Authors: {'items': [{'id': 'Mallada-E', 'name': {'family': 'Mallada', 'given': 'Enrique'}, 'orcid': '0000-0003-1568-1833'}, {'id': 'Zhao-Changhong', 'name': {'family': 'Zhao', 'given': 'Changhong'}, 'orcid': '0000-0003-0539-8591'}, {'id': 'Low-S-H', 'name': {'family': 'Low', 'given': 'Steven'}, 'orcid': '0000-0001-6476-3048'}]}
Year: 2017
DOI: 10.1109/TAC.2017.2713529
Frequency control rebalances supply and demand while maintaining the network state within operational margins. It is implemented using fast ramping reserves that are expensive and wasteful, and which are expected to become increasingly necessary with the current acceleration of renewable penetration. The most promising solution to this problem is the use of demand response, i.e., load participation in frequency control. Yet it is still unclear how to efficiently integrate load participation without introducing instabilities and violating operational constraints. In this paper, we present a comprehensive load-side frequency control mechanism that can maintain the grid within operational constraints. In particular, our controllers can rebalance supply and demand after disturbances, restore the frequency to its nominal value, and preserve interarea power flows. Furthermore, our controllers are distributed (unlike the currently implemented frequency control), can allocate load updates optimally, and can maintain line flows within thermal limits. We prove that such a distributed load-side control is globally asymptotically stable and robust to unknown load parameters. We illustrate its effectiveness through simulations.https://authors.library.caltech.edu/records/p6tpf-bzs41Distributed Optimal Power Flow Algorithm for Radial Networks, I: Balanced Single Phase Case
https://resolver.caltech.edu/CaltechAUTHORS:20160711-103700266
Authors: {'items': [{'id': 'Peng-Qiuyu', 'name': {'family': 'Peng', 'given': 'Qiuyu'}}, {'id': 'Low-S-H', 'name': {'family': 'Low', 'given': 'Steven H.'}, 'orcid': '0000-0001-6476-3048'}]}
Year: 2018
DOI: 10.1109/TSG.2016.2546305
The optimal power flow (OPF) problem determines a network operating point that minimizes a certain objective such as generation cost or power loss. Traditionally, OPF is solved in a centralized manner. With increasing penetration of renewable energy in distribution system, we need faster and distributed solutions for real-time feedback control. This is difficult due to the nonlinearity of the power flow equations. In this paper, we propose a solution for balanced radial networks. It exploits recent results that suggest solving for a globally optimal solution of OPF over a radial network through the second-order cone program relaxation. Our distributed algorithm is based on alternating direction method of multiplier (ADMM), but unlike standard ADMM-based distributed OPF algorithms that require solving optimization subproblems using iterative method, our decomposition allows us to derive closed form solutions for these subproblems, greatly speeding up each ADMM iteration. We illustrate the scalability of the proposed algorithm by simulating it on a real-world 2065-bus distribution network.https://authors.library.caltech.edu/records/pvjsj-k7t94Profit Maximizing Planning and Control of Battery Energy Storage Systems for Primary Frequency Control
https://resolver.caltech.edu/CaltechAUTHORS:20170810-105250761
Authors: {'items': [{'id': 'Zhang-Ying-Jun', 'name': {'family': 'Zhang', 'given': 'Ying Jun'}}, {'id': 'Zhao-Changhong', 'name': {'family': 'Zhao', 'given': 'Changhong'}, 'orcid': '0000-0003-0539-8591'}, {'id': 'Tang-Wanrong', 'name': {'family': 'Tang', 'given': 'Wanrong'}}, {'id': 'Low-S-H', 'name': {'family': 'Low', 'given': 'Steven H.'}, 'orcid': '0000-0001-6476-3048'}]}
Year: 2018
DOI: 10.1109/TSG.2016.2562672
We consider a two-level profit-maximizing strategy, including planning and control, for battery energy storage system (BESS) owners that participate in the primary frequency control (PFC) market. Specifically, the optimal BESS control minimizes the operating cost by keeping the state of charge (SoC) in an optimal range. Through rigorous analysis, we prove that the optimal BESS control is a "state-invariant" strategy in the sense that the optimal SoC range does not vary with the state of the system. As such, the optimal control strategy can be computed offline once and for all with very low complexity. Regarding the BESS planning, we prove that the the minimum operating cost is a decreasing convex function of the BESS energy capacity. This leads to the optimal BESS sizing that strikes a balance between the capital investment and operating cost. Our work here provides a useful theoretical framework for understanding the planning and control strategies that maximize the economic benefits of BESSs in ancillary service markets.https://authors.library.caltech.edu/records/xjzab-d1m36The Role of Convexity on Saddle-Point Dynamics: Lyapunov Function and Robustness
https://resolver.caltech.edu/CaltechAUTHORS:20171204-104144520
Authors: {'items': [{'id': 'Cherukuri-A', 'name': {'family': 'Cherukuri', 'given': 'Ashish'}, 'orcid': '0000-0002-7609-5080'}, {'id': 'Mallada-E', 'name': {'family': 'Mallada', 'given': 'Enrique'}, 'orcid': '0000-0003-1568-1833'}, {'id': 'Low-S-H', 'name': {'family': 'Low', 'given': 'Steven'}, 'orcid': '0000-0001-6476-3048'}, {'id': 'Cortés-J', 'name': {'family': 'Cortés', 'given': 'Jorge'}}]}
Year: 2018
DOI: 10.1109/TAC.2017.2778689
This paper studies the projected saddle-point dynamics associated to a convex–concave function, which we term saddle function. The dynamics consists of gradient descent of the saddle function in variables corresponding to convexity and (projected) gradient ascent in variables corresponding to concavity. We examine the role that the local and/or global nature of the convexity–concavity properties of the saddle function plays in guaranteeing convergence and robustness of the dynamics. Under the assumption that the saddle function is twice continuously differentiable, we provide a novel characterization of the omega-limit set of the trajectories of this dynamics in terms of the diagonal blocks of the Hessian. Using this characterization, we establish global asymptotic convergence of the dynamics under local strong convexity–concavity of the saddle function. When strong convexity–concavity holds globally, we establish three results. First, we identify a Lyapunov function (that decreases strictly along the trajectory) for the projected saddle-point dynamics when the saddle function corresponds to the Lagrangian of a general constrained convex optimization problem. Second, for the particular case when the saddle function is the Lagrangian of an equality-constrained optimization problem, we show input-to-state stability (ISS) of the saddle-point dynamics by providing an ISS Lyapunov function. Third, we use the latter result to design an opportunistic state-triggered implementation of the dynamics. Various examples illustrate our results.https://authors.library.caltech.edu/records/jkd5v-pne67Optimal Power Flow in Stand-alone DC Microgrids
https://resolver.caltech.edu/CaltechAUTHORS:20180208-073227198
Authors: {'items': [{'id': 'Li-Jia', 'name': {'family': 'Li', 'given': 'Jia'}}, {'id': 'Liu-Feng', 'name': {'family': 'Liu', 'given': 'Feng'}, 'orcid': '0000-0003-2279-2558'}, {'id': 'Wang-Zhaojian', 'name': {'family': 'Wang', 'given': 'Zhaojian'}, 'orcid': '0000-0002-4998-6339'}, {'id': 'Low-S-H', 'name': {'family': 'Low', 'given': 'Steven'}, 'orcid': '0000-0001-6476-3048'}, {'id': 'Mei-Shengwei', 'name': {'family': 'Mei', 'given': 'Shengwei'}, 'orcid': '0000-0002-2757-5977'}]}
Year: 2018
DOI: 10.1109/TPWRS.2018.2801280
Direct-current microgrids (DC-MGs) can operate in either grid-connected or stand-alone mode. In particular, stand-alone DC-MG has many distinct applications. However, the optimal power flow problem (OPF) of a stand-alone DC-MG is inherently non-convex. In this paper, the OPF of DC-MG is investigated considering convex relaxation based on second-order cone programming. Mild assumptions are proposed to guarantee the exactness of relaxation, which only require uniform nodal voltage upper bounds and positive network loss. It is revealed that the exactness of second-order conic (SOC) relaxation of DC networks does not rely on topology or operating mode of DC networks, and an optimal solution must be unique if it exists. If line constraints are considered, the exactness of SOC relaxation may not hold. In this regard, two heuristic methods are proposed to give approximate solutions. Numerical experiments confirm the theoretic results.https://authors.library.caltech.edu/records/br9qh-8x868Failure Localization in Power Systems via Tree Partitions
https://resolver.caltech.edu/CaltechAUTHORS:20190128-124902110
Authors: {'items': [{'id': 'Guo-Linqi', 'name': {'family': 'Guo', 'given': 'Linqi'}}, {'id': 'Liang-Chen', 'name': {'family': 'Liang', 'given': 'Chen'}}, {'id': 'Zocca-A', 'name': {'family': 'Zocca', 'given': 'Alessandro'}, 'orcid': '0000-0001-6585-4785'}, {'id': 'Low-S-H', 'name': {'family': 'Low', 'given': 'Steven H.'}, 'orcid': '0000-0001-6476-3048'}, {'id': 'Wierman-A', 'name': {'family': 'Wierman', 'given': 'Adam'}}]}
Year: 2018
DOI: 10.1145/3305218.3305247
Cascading failures in power systems propagate non-locally, making the control and mitigation of outages extremely hard. In this work, we use the emerging concept of the tree partition of transmission networks to provide an analytical characterization of line failure localizability in transmission systems. Our results rigorously formalize the well-known intuition that failures cannot cross bridges, and reveal a finer-grained concept that encodes more precise information on failure propagation within tree-partition regions. Specifically, when a non-bridge line is tripped, the impact of this failure only propagates within components of the tree partition defined by the bridges. In contrast, when a bridge line is tripped, the impact of this failure propagates globally across the network, affecting the power flow on all remaining lines. This characterization suggests that it is possible to improve the system robustness by temporarily switching off certain transmission lines, so as to create more, smaller components in the tree partition; thus spatially localizing line failures and making the grid less vulnerable to large outages.https://authors.library.caltech.edu/records/pk4s9-k5586Failure Localization in Power Systems via Tree Partitions
https://resolver.caltech.edu/CaltechAUTHORS:20190128-094206311
Authors: {'items': [{'id': 'Guo-Linqi', 'name': {'family': 'Guo', 'given': 'Linqi'}}, {'id': 'Liang-Chen', 'name': {'family': 'Liang', 'given': 'Chen'}}, {'id': 'Zocca-A', 'name': {'family': 'Zocca', 'given': 'Alessandro'}, 'orcid': '0000-0001-6585-4785'}, {'id': 'Low-S-H', 'name': {'family': 'Low', 'given': 'Steven H.'}, 'orcid': '0000-0001-6476-3048'}, {'id': 'Wierman-A', 'name': {'family': 'Wierman', 'given': 'Adam'}}]}
Year: 2018
DOI: 10.1145/3305218.3305240
Cascading failures in power systems propagate non-locally, making the control and mitigation of outages extremely hard. In this work, we use the emerging concept of the tree partition of transmission networks to provide an analytical characterization of line failure localizability in transmission systems. Our results rigorously establish the well perceived intuition in power community that failures cannot cross bridges, and reveal a finer-grained concept that encodes more precise information on failure propagations within tree-partition regions. Specifically, when a non-bridge line is tripped, the impact of this failure only propagates within well-defined components, which we refer to as cells, of the tree partition defined by the bridges. In contrast, when a bridge line is tripped, the impact of this failure propagates globally across the network, affecting the power flow on all remaining transmission lines. This characterization suggests that it is possible to improve the system robustness by temporarily switching off certain transmission lines, so as to create more, smaller components in the tree partition; thus spatially localizing line failures and making the grid less vulnerable to large-scale outages. We illustrate this approach using the IEEE 118-bus test system and demonstrate that switching off a negligible portion of transmission lines allows the impact of line failures to be significantly more localized without substantial changes in line congestion.https://authors.library.caltech.edu/records/hfpa5-89z53Distributed plug-and-play optimal generator and load control for power system frequency regulation
https://resolver.caltech.edu/CaltechAUTHORS:20180327-084750901
Authors: {'items': [{'id': 'Zhao-Changhong', 'name': {'family': 'Zhao', 'given': 'Changhong'}, 'orcid': '0000-0003-0539-8591'}, {'id': 'Mallada-E', 'name': {'family': 'Mallada', 'given': 'Enrique'}, 'orcid': '0000-0003-1568-1833'}, {'id': 'Low-S-H', 'name': {'family': 'Low', 'given': 'Steven H.'}, 'orcid': '0000-0001-6476-3048'}, {'id': 'Bialek-J', 'name': {'family': 'Bialek', 'given': 'Janusz'}}]}
Year: 2018
DOI: 10.1016/j.ijepes.2018.03.014
A distributed control scheme, which can be implemented on generators and controllable loads in a plug-and-play manner, is proposed for power system frequency regulation. The proposed scheme is based on local measurements, local computation, and neighborhood information exchanges over a communication network with an arbitrary (but connected) topology. In the event of a sudden change in generation or load, the proposed scheme can restore the nominal frequency and the reference inter-area power flows, while minimizing the total cost of control for participating generators and loads. Power network stability under the proposed control is proved with a relatively realistic model which includes nonlinear power flow and a generic (potentially nonlinear or high-order) turbine-governor model, and further with first- and second-order turbine-governor models as special cases. In simulations, the proposed control scheme shows a comparable performance to the existing automatic generation control (AGC) when implemented only on the generator side, and demonstrates better dynamic characteristics than AGC when each scheme is implemented on both generators and controllable loads. Simulation results also show robustness of the proposed scheme to communication link failure.https://authors.library.caltech.edu/records/37nnq-6dv36Scheduling of EV Battery Swapping, II: Distributed Solutions
https://resolver.caltech.edu/CaltechAUTHORS:20171221-153230416
Authors: {'items': [{'id': 'You-Pengcheng', 'name': {'family': 'You', 'given': 'Pengcheng'}, 'orcid': '0000-0003-1532-8773'}, {'id': 'Low-S-H', 'name': {'family': 'Low', 'given': 'Steven H.'}, 'orcid': '0000-0001-6476-3048'}, {'id': 'Zhang-Liang', 'name': {'family': 'Zhang', 'given': 'Liang'}, 'orcid': '0000-0003-3115-1752'}, {'id': 'Deng-Ruilong', 'name': {'family': 'Deng', 'given': 'Ruilong'}, 'orcid': '0000-0002-8158-150X'}, {'id': 'Giannakis-G-B', 'name': {'family': 'Giannakis', 'given': 'Georgios B.'}, 'orcid': '0000-0002-0196-0260'}, {'id': 'Sun-Youxian', 'name': {'family': 'Sun', 'given': 'Youxian'}}, {'id': 'Yang-Zaiyue', 'name': {'family': 'Yang', 'given': 'Zaiyue'}, 'orcid': '0000-0002-8288-3833'}]}
Year: 2018
DOI: 10.1109/TCNS.2017.2774012
In Part I of this paper, we formulate an optimal scheduling problem for battery swapping that assigns to each electric vehicle (EV) a best station to swap its depleted battery based on its current location and state of charge. The schedule aims to minimize a weighted sum of EVs' travel distance and electricity generation cost over both station assignments and power flow variables, subject to EV range constraints, grid operational constraints, and ac power flow equations. We propose there a centralized solution based on second-order cone programming relaxation of optimal power flow and generalized Benders decomposition that is applicable when global information is available. In this paper, we propose two distributed solutions based on the alternating direction method of multipliers and dual decomposition, respectively, that are suitable for systems where the distribution grid, stations, and EVs are managed by separate entities. Our algorithms allow these entities to make individual decisions, but coordinate through privacy-preserving information exchanges to solve a convex relaxation of the global problem. We present simulation results to show that both algorithms converge quickly to a solution that is close to optimum after discretization.https://authors.library.caltech.edu/records/2azc2-xcq26Scheduling of EV Battery Swapping, I: Centralized Solution
https://resolver.caltech.edu/CaltechAUTHORS:20171221-154244882
Authors: {'items': [{'id': 'You-Pengcheng', 'name': {'family': 'You', 'given': 'Pengcheng'}, 'orcid': '0000-0003-1532-8773'}, {'id': 'Low-S-H', 'name': {'family': 'Low', 'given': 'Steven H.'}, 'orcid': '0000-0001-6476-3048'}, {'id': 'Tushar-W', 'name': {'family': 'Tushar', 'given': 'Wayes'}, 'orcid': '0000-0003-1055-7200'}, {'id': 'Geng-Guangchao', 'name': {'family': 'Geng', 'given': 'Guangchao'}, 'orcid': '0000-0002-3626-0766'}, {'id': 'Yuen-Chau', 'name': {'family': 'Yuen', 'given': 'Chau'}, 'orcid': '0000-0002-9307-2120'}, {'id': 'Yang-Zaiyue', 'name': {'family': 'Yang', 'given': 'Zaiyue'}, 'orcid': '0000-0002-8288-3833'}, {'id': 'Sun-Youxian', 'name': {'family': 'Sun', 'given': 'Youxian'}}]}
Year: 2018
DOI: 10.1109/TCNS.2017.2773025
We formulate an optimal scheduling problem for battery swapping that assigns to each electric vehicle (EV) a best battery station to swap its depleted battery based on its current location and state of charge. The schedule aims to minimize a weighted sum of EVs' travel distance and electricity generation cost over both station assignments and power flow variables, subject to EV range constraints, grid operational constraints, and ac power flow equations. To deal with the nonconvexity of power flow equations and the binary nature of station assignments, we propose a solution based on second-order cone programming (SOCP) relaxation of optimal power flow and generalized Benders decomposition. When the SOCP relaxation is exact, this approach computes a global optimum. We evaluate the performance of the proposed algorithm through simulations. The algorithm requires global information and is suitable for cases where the distribution grid, battery stations, and EVs are managed centrally by the same operator. In Part II of this paper, we develop distributed solutions for cases where they are operated by different organizations that do not share private information.https://authors.library.caltech.edu/records/9302n-ajh28A Primal-Dual Gradient Method for Time-Varying Optimization with Application to Power Systems
https://resolver.caltech.edu/CaltechAUTHORS:20190128-132503883
Authors: {'items': [{'id': 'Tang-Yujie', 'name': {'family': 'Tang', 'given': 'Yujie'}, 'orcid': '0000-0002-4921-8372'}, {'id': "Dall'Anese-E", 'name': {'family': "Dall'Anese", 'given': 'Emiliano'}}, {'id': 'Bernstein-A', 'name': {'family': 'Bernstein', 'given': 'Andrey'}}, {'id': 'Low-S-H', 'name': {'family': 'Low', 'given': 'S. H.'}, 'orcid': '0000-0001-6476-3048'}]}
Year: 2018
DOI: 10.1145/3308897.3308939
We consider time-varying nonconvex optimization problems where the objective function and the feasible set vary over discrete time. This sequence of optimization problems induces a trajectory of Karush-Kuhn-Tucker (KKT) points. We present a class of regularized primal-dual gradient algorithms that track the KKT trajectory. These algorithms are feedback-based algorithms, where analytical models for system state or constraints are replaced with actual measurements. We present conditions for the proposed algorithms to achieve bounded tracking error when the cost and constraint functions are twice continuously differentiable. We discuss their practical implications and illustrate their applications in power systems through numerical simulations.https://authors.library.caltech.edu/records/r2smp-y7309Unified Distributed Control of Stand-alone DC Microgrids
https://resolver.caltech.edu/CaltechAUTHORS:20171006-110526562
Authors: {'items': [{'id': 'Wang-Zhaojian', 'name': {'family': 'Wang', 'given': 'Zhaojian'}, 'orcid': '0000-0002-4998-6339'}, {'id': 'Liu-Feng', 'name': {'family': 'Liu', 'given': 'Feng'}, 'orcid': '0000-0003-2279-2558'}, {'id': 'Chen-Ying', 'name': {'family': 'Chen', 'given': 'Ying'}}, {'id': 'Low-S-H', 'name': {'family': 'Low', 'given': 'Steven H.'}, 'orcid': '0000-0001-6476-3048'}, {'id': 'Mei-Shengwei', 'name': {'family': 'Mei', 'given': 'Shengwei'}, 'orcid': '0000-0002-2757-5977'}]}
Year: 2019
DOI: 10.1109/TSG.2017.2757498
Stand-alone direct current (dc) microgrids may belong to different owners and adopt various control strategies. This brings great challenge to its optimal operation due to the difficulty of implementing a unified control. This paper addresses the distributed optimal control of dc microgrids, which intends to break the restriction of diversity to some extent. First, we formulate the optimal power flow problem of stand-alone dc microgrids as an exact second-order cone program and prove the uniqueness of the optimal solution. Then a dynamic solving algorithm based on primal–dual decomposition method is proposed, the convergence of which is proved theoretically as well as the optimality of its equilibrium point. It should be stressed that the algorithm can provide control commands for the three types of microgrids: 1) power control; 2) voltage control; and 3) droop control. This implies that each microgrid does not need to change its original control strategy in practice, which is less influenced by the diversity of microgrids. Moreover, the control commands for power controlled and voltage controlled microgrids satisfy generation limits and voltage limits in both transient process and steady state. Finally, a six-microgrid dc system based on the microgrid benchmark is adopted to validate the effectiveness and plug-n-play property of our designs.https://authors.library.caltech.edu/records/fzxec-gxq21Distributed Optimal Frequency Control Considering a Nonlinear Network-Preserving Model
https://resolver.caltech.edu/CaltechAUTHORS:20180809-152132536
Authors: {'items': [{'id': 'Wang-Zhaojian', 'name': {'family': 'Wang', 'given': 'Zhaojian'}, 'orcid': '0000-0002-4998-6339'}, {'id': 'Liu-Feng', 'name': {'family': 'Liu', 'given': 'Feng'}, 'orcid': '0000-0003-2279-2558'}, {'id': 'Pang-John-Z-F', 'name': {'family': 'Pang', 'given': 'John Z. F.'}, 'orcid': '0000-0002-6485-7922'}, {'id': 'Low-S-H', 'name': {'family': 'Low', 'given': 'Steven H.'}, 'orcid': '0000-0001-6476-3048'}, {'id': 'Mei-Shengwei', 'name': {'family': 'Mei', 'given': 'Shengwei'}, 'orcid': '0000-0002-2757-5977'}]}
Year: 2019
DOI: 10.1109/TPWRS.2018.2861941
This paper addresses the distributed optimal frequency control of power systems considering a network-preserving model with nonlinear power flows and excitation voltage dynamics. Salient features of the proposed distributed control strategy are fourfold, first, nonlinearity is considered to cope with large disturbances, second, only a part of generators are controllable, third, no load measurement is required, fourth, communication connectivity is required only for the controllable generators. To this end, benefiting from the concept of "virtual load demand," we first design the distributed controller for the controllable generators by leveraging the primal-dual decomposition technique. We then propose a method to estimate the virtual load demand of each controllable generator based on local frequencies. We derive incremental passivity conditions for the uncontrollable generators. Finally, we prove that the closed-loop system is asymptotically stable and its equilibrium attains the optimal solution to the associated economic dispatch problem. Simulations, including small and large-disturbance scenarios, are carried on the New England system, demonstrating the effectiveness of our design.https://authors.library.caltech.edu/records/kf8w3-9fn59Distributed Frequency Control with Operational Constraints, Part II: Network Power Balance
https://resolver.caltech.edu/CaltechAUTHORS:20170726-155035857
Authors: {'items': [{'id': 'Wang-Zhaojian', 'name': {'family': 'Wang', 'given': 'Zhaojian'}, 'orcid': '0000-0002-4998-6339'}, {'id': 'Liu-Feng', 'name': {'family': 'Liu', 'given': 'Feng'}, 'orcid': '0000-0003-2279-2558'}, {'id': 'Low-S-H', 'name': {'family': 'Low', 'given': 'Steven H.'}, 'orcid': '0000-0001-6476-3048'}, {'id': 'Zhao-Changhong', 'name': {'family': 'Zhao', 'given': 'Changhong'}, 'orcid': '0000-0003-0539-8591'}, {'id': 'Mei-Shengwei', 'name': {'family': 'Mei', 'given': 'Shengwei'}, 'orcid': '0000-0002-2757-5977'}]}
Year: 2019
DOI: 10.1109/TSG.2017.2731811
In Part I of this paper, we propose a decentralized optimal frequency control of multi-area power system with operational constraints, where the tie-line powers remain unchanged in the steady state and the power mismatch is balanced within individual control areas. In Part II of this paper, we propose a distributed controller for optimal frequency control in the network power balance case, where the power mismatch is balanced over the whole system. With the proposed controller, the tie-line powers remain within the acceptable range at equilibrium, while the regulation capacity constraints are satisfied both at equilibrium and during transient. It is revealed that the closed-loop system with the proposed controller carries out primal–dual updates with saturation for solving an associated optimization problem. To cope with discontinuous dynamics of the closed-loop system, we deploy the invariance principle for nonpathological Lyapunov function to prove its asymptotic stability. Simulation results are provided to show the effectiveness of our controller.https://authors.library.caltech.edu/records/7dpz2-fda60Distributed Frequency Control with Operational Constraints, Part I: Per-Node Power Balance
https://resolver.caltech.edu/CaltechAUTHORS:20170726-160828931
Authors: {'items': [{'id': 'Wang-Zhaojian', 'name': {'family': 'Wang', 'given': 'Zhaojian'}, 'orcid': '0000-0002-4998-6339'}, {'id': 'Liu-Feng', 'name': {'family': 'Liu', 'given': 'Feng'}, 'orcid': '0000-0003-2279-2558'}, {'id': 'Low-S-H', 'name': {'family': 'Low', 'given': 'Steven H.'}, 'orcid': '0000-0001-6476-3048'}, {'id': 'Zhao-Changhong', 'name': {'family': 'Zhao', 'given': 'Changhong'}, 'orcid': '0000-0003-0539-8591'}, {'id': 'Mei-Shengwei', 'name': {'family': 'Mei', 'given': 'Shengwei'}, 'orcid': '0000-0002-2757-5977'}]}
Year: 2019
DOI: 10.1109/TSG.2017.2731810
This paper addresses the distributed optimal frequency control of multi-area power system with operational constraints, including the regulation capacity of individual control area and the power limits on tie-lines. Both generators and controllable loads are utilized to recover nominal frequencies while minimizing regulation cost. We study two control modes: 1) the per-node balance mode and 2) the network balance mode. In Part I of this paper, we only consider the per-node balance case, where we derive a completely decentralized strategy without the need for communication between control areas. It can adapt to unknown load disturbance. The tie-line powers are restored after load disturbance, while the regulation capacity constraints are satisfied both at equilibrium and during transient. We show that the closed-loop systems with the proposed control strategies carry out primal-dual updates for solving the associated centralized frequency optimization problems. We further prove the closed-loop systems are asymptotically stable and converge to the unique optimal solution of the centralized frequency optimization problems and their duals. Finally, we present simulation results to demonstrate the effectiveness of our design. In Part II of this paper, we address the network power balance case, where transmission congestions are managed continuously.https://authors.library.caltech.edu/records/4d9wj-z6b42System level synthesis
https://resolver.caltech.edu/CaltechAUTHORS:20190513-131853183
Authors: {'items': [{'id': 'Anderson-James-D', 'name': {'family': 'Anderson', 'given': 'James'}, 'orcid': '0000-0002-2832-8396'}, {'id': 'Doyle-J-C', 'name': {'family': 'Doyle', 'given': 'John C.'}, 'orcid': '0000-0002-1828-2486'}, {'id': 'Low-S-H', 'name': {'family': 'Low', 'given': 'Steven H.'}, 'orcid': '0000-0001-6476-3048'}, {'id': 'Matni-N', 'name': {'family': 'Matni', 'given': 'Nikolai'}, 'orcid': '0000-0003-4936-3921'}]}
Year: 2019
DOI: 10.1016/j.arcontrol.2019.03.006
This article surveys the System Level Synthesis framework, which presents a novel perspective on constrained robust and optimal controller synthesis for linear systems. We show how SLS shifts the controller synthesis task from the design of a controller to the design of the entire closed loop system, and highlight the benefits of this approach in terms of scalability and transparency. We emphasize two particular applications of SLS, namely large-scale distributed optimal control and robust control. In the case of distributed control, we show how SLS allows for localized controllers to be computed, extending robust and optimal control methods to large-scale systems under practical and realistic assumptions. In the case of robust control, we show how SLS allows for novel design methodologies that, for the first time, quantify the degradation in performance of a robust controller due to model uncertainty – such transparency is key in allowing robust control methods to interact, in a principled way, with modern techniques from machine learning and statistical inference. Throughout, we emphasize practical and efficient computational solutions, and demonstrate our methods on easy to understand case studies.https://authors.library.caltech.edu/records/w427c-j6093On Identification of Distribution Grids
https://resolver.caltech.edu/CaltechAUTHORS:20190201-153603443
Authors: {'items': [{'id': 'Ardakanian-O', 'name': {'family': 'Ardakanian', 'given': 'Omid'}, 'orcid': '0000-0002-6711-5502'}, {'id': 'Wong-Vincent-W-S', 'name': {'family': 'Wong', 'given': 'Vincent W. S.'}, 'orcid': '0000-0003-3821-4365'}, {'id': 'Dobbe-R', 'name': {'family': 'Dobbe', 'given': 'Roel'}, 'orcid': '0000-0003-4633-7023'}, {'id': 'Low-S-H', 'name': {'family': 'Low', 'given': 'Steven H.'}, 'orcid': '0000-0001-6476-3048'}, {'id': 'von-Meier-A', 'name': {'family': 'von Meier', 'given': 'Alexandra'}, 'orcid': '0000-0003-4675-752X'}, {'id': 'Tomlin-C-J', 'name': {'family': 'Tomlin', 'given': 'Claire J.'}, 'orcid': '0000-0003-3192-3185'}, {'id': 'Yuan-Ye', 'name': {'family': 'Yuan', 'given': 'Ye'}, 'orcid': '0000-0001-7858-0437'}]}
Year: 2019
DOI: 10.1109/tcns.2019.2891002
Large-scale integration of distributed energy resources into distribution feeders necessitates careful control of their operation through power flow analysis. While the knowledge of the distribution system model is crucial for this analysis, it is often unavailable or outdated. The recent introduction of synchrophasor technology in low-voltage distribution grids has created ample opportunity to learn this model from high-precision, time-synchronized measurements of voltage and current phasors at various locations. This paper focuses on joint estimation of admittance parameters and topology of a polyphase distribution network from the available telemetry data via the lasso, a method for regression shrinkage and selection. We propose tractable convex programs capable of tackling the low-rank structure of the distribution system and develop an online algorithm for early detection and localization of critical events that induce a change in the admittance matrix. The efficacy of these techniques is corroborated through power flow studies on four three-phase radial distribution systems serving real and synthetic household demands.https://authors.library.caltech.edu/records/zrs5v-8dg96Guest Editorial: Special Issue on Analysis, Control and Optimization of Energy Networks
https://resolver.caltech.edu/CaltechAUTHORS:20190822-152029926
Authors: {'items': [{'id': 'Chertkov-M', 'name': {'family': 'Chertkov', 'given': 'Michael'}}, {'id': 'Jovanovic-M', 'name': {'family': 'Jovanovic', 'given': 'Mihailo'}}, {'id': 'Lesieutre-B', 'name': {'family': 'Lesieutre', 'given': 'Bernie'}}, {'id': 'Low-S-H', 'name': {'family': 'Low', 'given': 'Steven'}, 'orcid': '0000-0001-6476-3048'}, {'id': 'Van-Hentenryck-P', 'name': {'family': 'Van Hentenryck', 'given': 'Pascal'}}, {'id': 'Wehenkel-L', 'name': {'family': 'Wehenkel', 'given': 'Louis'}}]}
Year: 2019
DOI: 10.1109/TCNS.2019.2936194
The deployment of new technologies, e.g., renewable generation and electric vehicles, is rapidly transforming power networks by blurring the previously distinct spatio-temporal scales that many traditional approaches rely on for designing, analyzing and operating power grids. Other energy systems, such as natural gas systems, are undergoing similar revolutionary transformations. This trend can be characterized as a disruptive surge in complexity that challenges design and operation, but also offers opportunities to deliver unprecedented efficiency and reliability.https://authors.library.caltech.edu/records/t47ck-mp716Distributed Load-Side Control: Coping with Variation of Renewable Generations
https://resolver.caltech.edu/CaltechAUTHORS:20190627-102755990
Authors: {'items': [{'id': 'Wang-Zhaojian', 'name': {'family': 'Wang', 'given': 'Zhaojian'}, 'orcid': '0000-0002-4998-6339'}, {'id': 'Mei-Shengwei', 'name': {'family': 'Mei', 'given': 'Shengwei'}, 'orcid': '0000-0002-2757-5977'}, {'id': 'Liu-Feng', 'name': {'family': 'Liu', 'given': 'Feng'}, 'orcid': '0000-0003-2279-2558'}, {'id': 'Low-S-H', 'name': {'family': 'Low', 'given': 'Steven H.'}, 'orcid': '0000-0001-6476-3048'}, {'id': 'Yang-Peng', 'name': {'family': 'Yang', 'given': 'Peng'}}]}
Year: 2019
DOI: 10.1016/j.automatica.2019.108556
This paper addresses the distributed frequency control problem in a multi-area power system taking into account of unknown time-varying power imbalance. Particularly, fast controllable loads are utilized to restore system frequency under changing power imbalance in an optimal manner. The imbalanced power causing frequency deviation is decomposed into three parts: a known constant part, an unknown low-frequency variation and a high-frequency residual. The known steady part is usually the prediction of power imbalance. The variation may result from the fluctuation of renewable resources, electric vehicle charging, etc., which is usually unknown to operators. The high-frequency residual is also unknown and treated as an external disturbance. Correspondingly, in this paper, we resolve the following three problems in different timescales: (1) allocate the steady part of power imbalance economically; (2) mitigate the effect of unknown low-frequency power variation locally; (3) attenuate unknown high-frequency disturbances. To this end, a distributed controller combining consensus method with adaptive internal model control is proposed. We first prove that the closed-loop system is asymptotically stable and converges to the optimal solution of an optimization problem if the external disturbance is not included. We then prove that the power variation can be mitigated accurately. Furthermore, we show that the closed-loop system is robust against both parameter uncertainty and external disturbances. The New England system is used to verify the efficacy of our design.https://authors.library.caltech.edu/records/p982t-ha254Learning Graphs from Linear Measurements: Fundamental Trade-offs and Applications
https://resolver.caltech.edu/CaltechAUTHORS:20200228-131223093
Authors: {'items': [{'id': 'Li-Tongxin', 'name': {'family': 'Li', 'given': 'Tongxin'}, 'orcid': '0000-0002-9806-8964'}, {'id': 'Werner-Lucien', 'name': {'family': 'Werner', 'given': 'Lucien'}}, {'id': 'Low-S-H', 'name': {'family': 'Low', 'given': 'Steven'}, 'orcid': '0000-0001-6476-3048'}]}
Year: 2020
DOI: 10.1109/tsipn.2020.2975368
We consider a specific graph learning task: reconstructing a symmetric matrix that represents an underlying graph using linear measurements. We present a sparsity characterization for distributions of random graphs (that are allowed to contain high-degree nodes), based on which we study fundamental trade-offs between the number of measurements, the complexity of the graph class, and the probability of error. We first derive a necessary condition on the number of measurements. Then, by considering a three-stage recovery scheme, we give a sufficient condition for recovery. Furthermore, assuming the measurements are Gaussian IID, we prove upper and lower bounds on the (worst-case) sample complexity for both noisy and noiseless recovery. In the special cases of the uniform distribution on trees with n nodes and the Erdős-Rényi (n,p) class, the fundamental trade-offs are tight up to multiplicative factors with noiseless measurements. In addition, for practical applications, we design and implement a polynomial-time (in n ) algorithm based on the three-stage recovery scheme. Experiments show that the heuristic algorithm outperforms basis pursuit on star graphs. We apply the heuristic algorithm to learn admittance matrices in electric grids. Simulations for several canonical graph classes and IEEE power system test cases demonstrate the effectiveness and robustness of the proposed algorithm for parameter reconstruction.https://authors.library.caltech.edu/records/kxenq-ds892Privacy-preserving Energy Scheduling for Smart Grid with Renewables
https://resolver.caltech.edu/CaltechAUTHORS:20200327-122618326
Authors: {'items': [{'id': 'Yang-Kai', 'name': {'family': 'Yang', 'given': 'Kai'}, 'orcid': '0000-0002-5983-198X'}, {'id': 'Jiang-Libin', 'name': {'family': 'Jiang', 'given': 'Libin'}}, {'id': 'Low-S-H', 'name': {'family': 'Low', 'given': 'Steven H.'}, 'orcid': '0000-0001-6476-3048'}, {'id': 'Liu-Sijia', 'name': {'family': 'Liu', 'given': 'Sijia'}}]}
Year: 2020
DOI: 10.1109/access.2020.2983110
We consider joint demand response and power procurement to optimize the average social welfare of a smart power grid system with renewable sources. The renewable sources such as wind and solar energy are intermittent and fluctuate rapidly. As a consequence, the demand response algorithm needs to be executed in real time to ensure the stability of a smart grid system with renewable sources. We develop a demand response algorithm that converges to the optimal solution with superlinear rates of convergence. In the simulation studies, the proposed algorithm converges roughly thirty time faster than the traditional subgradient algorithm. In addition, it is fully distributed and can be realized either synchronously or in asynchronous manner, which eases practical deployment.https://authors.library.caltech.edu/records/p04xn-v4194An Energy Sharing Game With Generalized Demand Bidding: Model and Properties
https://resolver.caltech.edu/CaltechAUTHORS:20190626-132932886
Authors: {'items': [{'id': 'Chen-Yue', 'name': {'family': 'Chen', 'given': 'Yue'}, 'orcid': '0000-0002-7594-7587'}, {'id': 'Mei-Shengwei', 'name': {'family': 'Mei', 'given': 'Shengwei'}, 'orcid': '0000-0002-2757-5977'}, {'id': 'Zhou-Fengyu', 'name': {'family': 'Zhou', 'given': 'Fengyu'}, 'orcid': '0000-0002-2639-6491'}, {'id': 'Low-S-H', 'name': {'family': 'Low', 'given': 'Steven H.'}, 'orcid': '0000-0001-6476-3048'}, {'id': 'Wei-Wei', 'name': {'family': 'Wei', 'given': 'Wei'}, 'orcid': '0000-0002-1018-7708'}, {'id': 'Liu-Feng', 'name': {'family': 'Liu', 'given': 'Feng'}, 'orcid': '0000-0003-2279-2558'}]}
Year: 2020
DOI: 10.1109/tsg.2019.2946602
This paper proposes a novel energy sharing mechanism for prosumers who can produce and consume. Different from most existing works, the role of individual prosumer as a seller or buyer in our model is endogenously determined. Several desirable properties of the proposed mechanism are proved based on a generalized game-theoretic model. We show that the Nash equilibrium exists and is the unique solution of an equivalent convex optimization problem. The sharing price at the Nash equilibrium equals to the average marginal disutility of all prosumers. We also prove that every prosumer has the incentive to participate in the sharing market, and prosumers' total cost decreases with increasing absolute value of price sensitivity. Furthermore, the Nash equilibrium approaches the social optimal as the number of prosumers grows, and competition can improve social welfare.https://authors.library.caltech.edu/records/g9dmq-51371Separation event-constrained optimal power flow to enhance resilience in low-inertia power systems
https://resolver.caltech.edu/CaltechAUTHORS:20210112-144610987
Authors: {'items': [{'id': 'Püschel-Løvengreen-Sebastián', 'name': {'family': 'Püschel-Løvengreen', 'given': 'Sebastián'}, 'orcid': '0000-0002-0248-9618'}, {'id': 'Ghazavi-Dozein-Mehdi', 'name': {'family': 'Ghazavi Dozein', 'given': 'Mehdi'}, 'orcid': '0000-0003-2574-7082'}, {'id': 'Low-S-H', 'name': {'family': 'Low', 'given': 'Steven'}, 'orcid': '0000-0001-6476-3048'}, {'id': 'Mancarella-Pierluigi', 'name': {'family': 'Mancarella', 'given': 'Pierluigi'}, 'orcid': '0000-0002-9247-1402'}]}
Year: 2020
DOI: 10.1016/j.epsr.2020.106678
Weakly interconnected power systems face separation risk during extreme weather events or due to cascading failures. These contingencies may become particularly severe in low-inertia grids, where the sub-regions resulting after system split need to procure enough resources to respond to low (importing areas) and high (exporting areas) frequency conditions. This paper introduces a separation event-constrained optimal power flow (SECOPF) with considerations of inertia and frequency response resources allocation in each balancing area to be capable to withstand, besides the largest generation and load contingencies, also the loss of selected interconnectors. The model, cast as mixed integer linear program, is tested on a reduced version of the Australian power system and validated via dynamic simulation. The results show how the system's frequency resilience to extreme events can be effectively enhanced for both high- and low-frequency conditions that arise in different areas after an interconnector's trip . This is achieved by co-optimising the interconnector flow magnitudes along with the distribution of frequency response resources and inertia levels in each area.https://authors.library.caltech.edu/records/b7nyk-ajc03Pricing EV charging service with demand charge
https://resolver.caltech.edu/CaltechAUTHORS:20210107-153400550
Authors: {'items': [{'id': 'Lee-Zachary-J', 'name': {'family': 'Lee', 'given': 'Zachary J.'}, 'orcid': '0000-0002-5358-2388'}, {'id': 'Pang-John-Z-F', 'name': {'family': 'Pang', 'given': 'John Z. F.'}, 'orcid': '0000-0002-6485-7922'}, {'id': 'Low-S-H', 'name': {'family': 'Low', 'given': 'Steven H.'}, 'orcid': '0000-0001-6476-3048'}]}
Year: 2020
DOI: 10.1016/j.epsr.2020.106694
Pricing electric vehicle (EV) charging services is difficult when the electricity tariff includes both time-of-use energy cost and demand charge based on peak power draw. In this paper, we propose a pricing scheme that assigns a session-specific energy price to each charging session at the end of the billing period. The session price precisely captures the costs of energy, demand charge, and infrastructure congestion for which that session is responsible in that month while optimizing the trade-off between inexpensive time-of-use pricing and peak power draw. While our pricing scheme is calculated offline at the end of the billing period, we propose an online scheduling algorithm based on model predictive control to determine charging rates for each EV in real-time. We provide theoretical justification for our proposal and support it with simulations using real data collected from charging facilities at Caltech and JPL. Our simulation results suggest that the online algorithm can approximate the offline optimal reasonably well, e.g., the cost paid by the operator in the online setting is higher than the offline optimal cost by 9.2% and 6.5% at Caltech and JPL respectively. In the case of JPL, congestion rents are enough to cover this increase in costs, while at Caltech, this results in a negligible average loss of $18 per month.https://authors.library.caltech.edu/records/8hkmq-h5806Classification of electric vehicle charging time series with selective clustering
https://resolver.caltech.edu/CaltechAUTHORS:20210112-144610886
Authors: {'items': [{'id': 'Sun-Chengxi', 'name': {'family': 'Sun', 'given': 'Chenxi'}}, {'id': 'Li-Tongxin', 'name': {'family': 'Li', 'given': 'Tongxin'}, 'orcid': '0000-0002-9806-8964'}, {'id': 'Low-S-H', 'name': {'family': 'Low', 'given': 'Steven H.'}, 'orcid': '0000-0001-6476-3048'}, {'id': 'Li-Victor-O-K', 'name': {'family': 'Li', 'given': 'Victor O. K.'}}]}
Year: 2020
DOI: 10.1016/j.epsr.2020.106695
We develop a novel iterative clustering method for classifying time series of EV charging rates based on their "tail features". Our method first extracts tails from a diversity of charging time series that have different lengths, contain missing data, and are distorted by scheduling algorithms and measurement noise. The charging tails are then clustered into a small number of types whose representatives are then used to improve tail extraction. This process iterates until it converges. We apply our method to ACN-Data, a fine-grained EV charging dataset recently made publicly available, to illustrate its effectiveness and potential applications.https://authors.library.caltech.edu/records/hgwjy-e8s47An integrated approach for failure mitigation & localization in power systems
https://resolver.caltech.edu/CaltechAUTHORS:20200707-103725840
Authors: {'items': [{'id': 'Liang-Chen', 'name': {'family': 'Liang', 'given': 'Chen'}}, {'id': 'Guo-Linqi', 'name': {'family': 'Guo', 'given': 'Linqi'}}, {'id': 'Zocca-Alessandro', 'name': {'family': 'Zocca', 'given': 'Alessandro'}, 'orcid': '0000-0001-6585-4785'}, {'id': 'Yu-Shuyue', 'name': {'family': 'Yu', 'given': 'Shuyue'}}, {'id': 'Low-S-H', 'name': {'family': 'Low', 'given': 'Steven H.'}, 'orcid': '0000-0001-6476-3048'}, {'id': 'Wierman-A', 'name': {'family': 'Wierman', 'given': 'Adam'}}]}
Year: 2021
DOI: 10.48550/arXiv.2004.10401
The transmission grid is often comprised of several control areas that are connected by multiple tie lines in a mesh structure for reliability. It is also well-known that line failures can propagate non-locally and redundancy can exacerbate cascading. In this paper, we propose an integrated approach to grid reliability that (i) judiciously switches off a small number of tie lines so that the control areas are connected in a tree structure; and (ii) leverages a unified frequency control paradigm to provide congestion management in real time. Even though the proposed topology reduces redundancy, the integration of tree structure at regional level and real-time congestion management can provide stronger guarantees on failure localization and mitigation. We illustrate our approach on the IEEE 39-bus network and evaluate its performance on the IEEE 118-bus, 179-bus, 200-bus and 240-bus networks with various network congestion conditions. Simulations show that, compared with the traditional approach, our approach not only prevents load shedding in more failure scenarios, but also incurs smaller amounts of load loss in scenarios where load shedding is inevitable. Moreover, generators under our approach adjust their operations more actively and efficiently in a local manner.https://authors.library.caltech.edu/records/0hpmn-9pp50A Note on Branch Flow Models with Line Shunts
https://resolver.caltech.edu/CaltechAUTHORS:20201015-152732679
Authors: {'items': [{'id': 'Zhou-Fengyu', 'name': {'family': 'Zhou', 'given': 'Fengyu'}, 'orcid': '0000-0002-2639-6491'}, {'id': 'Low-S-H', 'name': {'family': 'Low', 'given': 'Steven H.'}, 'orcid': '0000-0001-6476-3048'}]}
Year: 2021
DOI: 10.1109/tpwrs.2020.3029732
When the shunt elements in the Π circuit line model are assumed zero, it has been proved that branch flow models are equivalent to bus injection models and that the second-order cone relaxation of optimal power flow problems on a radial network is exact under certain conditions. In this note we propose a branch flow model that includes nonzero line shunts and prove that the equivalence and the exactness of relaxation continue to hold under essentially the same conditions as for zero shunt elements.https://authors.library.caltech.edu/records/rptvr-h5712Safety-Critical Control Synthesis for Network Systems With Control Barrier Functions and Assume-Guarantee Contracts
https://resolver.caltech.edu/CaltechAUTHORS:20200707-113800670
Authors: {'items': [{'id': 'Chen-Yuxiao', 'name': {'family': 'Chen', 'given': 'Yuxiao'}, 'orcid': '0000-0001-5276-7156'}, {'id': 'Anderson-James-D', 'name': {'family': 'Anderson', 'given': 'James'}, 'orcid': '0000-0002-2832-8396'}, {'id': 'Kalsi-Karan', 'name': {'family': 'Kalsi', 'given': 'Karan'}}, {'id': 'Ames-A-D', 'name': {'family': 'Ames', 'given': 'Aaron D.'}, 'orcid': '0000-0003-0848-3177'}, {'id': 'Low-S-H', 'name': {'family': 'Low', 'given': 'Steven H.'}, 'orcid': '0000-0001-6476-3048'}]}
Year: 2021
DOI: 10.1109/TCNS.2020.3029183
This article aims at the safety-critical control synthesis of network systems such that the satisfaction of the safety constraints can be guaranteed. To handle the large state dimension of such systems, an assume-guarantee contract is used to break the large synthesis problem into smaller subproblems. Parameterized signal temporal logic (pSTL) is used to formally describe the behaviors of the subsystems which we use as the template for the contract. We show that robust control invariant sets (RCIs) for the subsystems can be composed to form a robust control invariant set (RCI) for the whole network system under a valid assume-guarantee contract. An epigraph algorithm is proposed to solve for a contract that is valid, an approach that has linear complexity for sparse networks, which leads to a RCI for the whole network system. Implemented with control barrier function (CBF), the state of each subsystem is guaranteed to stay within the safe set. Furthermore, we propose a contingency tube model predictive control approach based on the RCI, which is capable of handling severe contingencies, including topology changes of the network. A power grid example is used to demonstrate the proposed method. The simulation result includes both set point control and contingency recovery, and the safety constraint is always satisfied.https://authors.library.caltech.edu/records/wdtra-wzn87Reverse and Forward Engineering of Local Voltage Control in Distribution Networks
https://resolver.caltech.edu/CaltechAUTHORS:20200514-142330504
Authors: {'items': [{'id': 'Zhou-Xinyang', 'name': {'family': 'Zhou', 'given': 'Xinyang'}, 'orcid': '0000-0001-8526-2357'}, {'id': 'Farivar-Masoud', 'name': {'family': 'Farivar', 'given': 'Masoud'}, 'orcid': '0000-0001-7298-3526'}, {'id': 'Liu-Zhiyuan', 'name': {'family': 'Liu', 'given': 'Zhiyuan'}, 'orcid': '0000-0003-4356-4183'}, {'id': 'Chen-Lijun', 'name': {'family': 'Chen', 'given': 'Lijun'}, 'orcid': '0000-0001-6694-4299'}, {'id': 'Low-S-H', 'name': {'family': 'Low', 'given': 'Steven'}, 'orcid': '0000-0001-6476-3048'}]}
Year: 2021
DOI: 10.1109/tac.2020.2994184
The increasing penetration of renewable and distributed energy resources in distribution networks calls for real-time and distributed voltage control. In this article, we investigate local Volt/VAR control with a general class of control functions, and show that the power system dynamics with nonincremental local voltage control can be seen as a distributed algorithm for solving a well-defined optimization problem (reverse engineering). The reverse engineering further reveals a fundamental limitation of the nonincremental voltage control: the convergence condition is restrictive and prevents better voltage regulation at equilibrium. This motivates us to design two incremental local voltage control schemes based on the subgradient and pseudo-gradient algorithms, respectively, for solving the same optimization problem (forward engineering). The new control schemes decouple the dynamical property from the equilibrium property, and have much less restrictive convergence conditions. This article presents another step toward developing a new foundation—network dynamics as optimization algorithms—for distributed real-time control and optimization of future power networks.https://authors.library.caltech.edu/records/njr0v-2bd65Approaching Prosumer Social Optimum via Energy Sharing With Proof of Convergence
https://resolver.caltech.edu/CaltechAUTHORS:20210113-163505633
Authors: {'items': [{'id': 'Chen-Yue', 'name': {'family': 'Chen', 'given': 'Yue'}, 'orcid': '0000-0002-7594-7587'}, {'id': 'Zhao-Changhong', 'name': {'family': 'Zhao', 'given': 'Changhong'}, 'orcid': '0000-0003-0539-8591'}, {'id': 'Low-S-H', 'name': {'family': 'Low', 'given': 'Steven H.'}, 'orcid': '0000-0001-6476-3048'}, {'id': 'Mei-Shengwei', 'name': {'family': 'Mei', 'given': 'Shengwei'}, 'orcid': '0000-0002-2757-5977'}]}
Year: 2021
DOI: 10.1109/tsg.2020.3048402
With the advent of prosumers, the traditional centralized operation may become impracticable due to computational burden, privacy concerns, and conflicting interests. In this article, an energy sharing mechanism is proposed to accommodate prosumers' strategic decision-making on their self-production and demand in the presence of capacity constraints. Under this setting, prosumers play a generalized Nash game. We prove main properties of the game: an equilibrium exists and is partially unique; no prosumer is worse off by energy sharing and the price-of-anarchy is 1−O(1/I) where I is the number of prosumers. In particular, the PoA tends to 1 with a growing number of prosumers, meaning that the resulting total cost under the proposed energy sharing approaches social optimum. We prove that the corresponding prosumers' strategies converge to the social optimal solution as well. Finally we propose a bidding process and prove that it converges to the energy sharing equilibrium under mild conditions. Illustrative examples are provided to validate the results.https://authors.library.caltech.edu/records/8gdt7-0wv71The Optimal Power Flow Operator: Theory and Computation
https://resolver.caltech.edu/CaltechAUTHORS:20200707-114446351
Authors: {'items': [{'id': 'Zhou-Fengyu', 'name': {'family': 'Zhou', 'given': 'Fengyu'}, 'orcid': '0000-0002-2639-6491'}, {'id': 'Anderson-James-D', 'name': {'family': 'Anderson', 'given': 'James'}, 'orcid': '0000-0002-2832-8396'}, {'id': 'Low-S-H', 'name': {'family': 'Low', 'given': 'Steven H.'}, 'orcid': '0000-0001-6476-3048'}]}
Year: 2021
DOI: 10.1109/TCNS.2020.3044258
Optimal power flow (OPF) problems are mathematical programs to determine how to distribute power over networks subject to power flow and operational constraints. In this article, we treat an OPF problem as an operator that maps user demand to generated power, and allow the problem parameters to take values in some admissible set. We formalize this operator theoretic approach, define and characterize restricted parameter sets under which the mapping has a singleton output, independent binding constraints, and is differentiable. We show that for any power network, these analytical properties hold under almost all operating conditions and can thus be relied upon in applications. We further provide a closed-form expression for the Jacobian matrix of the OPF operator and describe how various derivatives can be computed using a recently proposed scheme based on homogenous self-dual embedding. In contrast to related work in the optimization literature, our results have a clear physical interpretation.https://authors.library.caltech.edu/records/qhb3h-1ry70Information Aggregation for Constrained Online Control
https://resolver.caltech.edu/CaltechAUTHORS:20210604-111535691
Authors: {'items': [{'id': 'Li-Tongxin', 'name': {'family': 'Li', 'given': 'Tongxin'}, 'orcid': '0000-0002-9806-8964'}, {'id': 'Chen-Yue', 'name': {'family': 'Chen', 'given': 'Yue'}, 'orcid': '0000-0002-7594-7587'}, {'id': 'Sun-Bo', 'name': {'family': 'Sun', 'given': 'Bo'}, 'orcid': '0000-0003-3172-7811'}, {'id': 'Wierman-A', 'name': {'family': 'Wierman', 'given': 'Adam'}}, {'id': 'Low-S-H', 'name': {'family': 'Low', 'given': 'Steven'}, 'orcid': '0000-0001-6476-3048'}]}
Year: 2021
DOI: 10.1145/3460085
This paper considers an online control problem involving two controllers. A central controller chooses an action from a feasible set that is determined by time-varying and coupling constraints, which depend on all past actions and states. The central controller's goal is to minimize the cumulative cost; however, the controller has access to neither the feasible set nor the dynamics directly, which are determined by a remote local controller. Instead, the central controller receives only an aggregate summary of the feasibility information from the local controller, which does not know the system costs. We show that it is possible for an online algorithm using feasibility information to nearly match the dynamic regret of an online algorithm using perfect information whenever the feasible sets satisfy a causal invariance criterion and there is a sufficiently large prediction window size. To do so, we use a form of feasibility aggregation based on entropic maximization in combination with a novel online algorithm, named Penalized Predictive Control (PPC) and demonstrate that aggregated information can be efficiently learned using reinforcement learning algorithms. The effectiveness of our approach for closed-loop coordination between central and local controllers is validated via an electric vehicle charging application in power systems.https://authors.library.caltech.edu/records/nha5m-d8388On the Implementation of OPF-Based Setpoints for Active Distribution Networks
https://resolver.caltech.edu/CaltechAUTHORS:20210129-120019482
Authors: {'items': [{'id': 'Liu-Michael-Ziguang', 'name': {'family': 'Liu', 'given': 'Michael Z.'}, 'orcid': '0000-0002-9609-4544'}, {'id': 'Ochoa-Luis-F', 'name': {'family': 'Ochoa', 'given': 'Luis F.'}, 'orcid': '0000-0002-7853-4286'}, {'id': 'Low-S-H', 'name': {'family': 'Low', 'given': 'Steven H.'}, 'orcid': '0000-0001-6476-3048'}]}
Year: 2021
DOI: 10.1109/tsg.2021.3054387
In the context of active distribution networks, AC Optimal Power Flow (OPF) has shown great potential to calculate setpoints for controllable devices. Although considerable literature exists, temporal aspects that may affect the actual execution of these setpoints are rarely investigated. Due to the diverse operating characteristics of controllable devices (i.e., delays, ramp rates and deadbands), when these setpoints are executed by multiple devices without adequate considerations, the resulting outcome can differ drastically from what is expected; leading to violations of network constraints and excessive control actions. Therefore, this work proposes a series of necessary adaptations within the controllers of existing devices as well as in the OPF formulation to cater for the diversity in operating characteristics, ensuring that calculated setpoints are adequately implemented by controllable devices. This involves the direct control of conventional devices and enforcing a new ramping behavior for inverter-interfaced devices. Furthermore, a linear, mixed-integer formulation is proposed to handle discrete devices and improve scalability in large networks. Co-simulation results (using a U.K. test network with the objective of maximizing renewable energy production and considering 1s time-step) demonstrate that, by catering for the operating characteristics of controllable devices, the expected outcome from OPF-based setpoints can be achieved.https://authors.library.caltech.edu/records/xmfyf-whv73Exactness of OPF Relaxation on Three-Phase Radial Networks With Delta Connections
https://resolver.caltech.edu/CaltechAUTHORS:20210412-095431267
Authors: {'items': [{'id': 'Zhou-Fengyu', 'name': {'family': 'Zhou', 'given': 'Fengyu'}, 'orcid': '0000-0002-2639-6491'}, {'id': 'Zamzam-Ahmed-S', 'name': {'family': 'Zamzam', 'given': 'Ahmed S.'}, 'orcid': '0000-0002-1175-2801'}, {'id': 'Low-S-H', 'name': {'family': 'Low', 'given': 'Steven H.'}, 'orcid': '0000-0001-6476-3048'}, {'id': 'Sidiropoulos-Nicholas-D', 'name': {'family': 'Sidiropoulos', 'given': 'Nicholas D.'}, 'orcid': '0000-0002-3385-7911'}]}
Year: 2021
DOI: 10.1109/TSG.2021.3066530
Simulations have shown that while semi-definite relaxations of AC optimal power flow (AC-OPF) on three-phase radial networks with only wye connections tend to be exact, the presence of delta connections seem to render them inexact. This article shows that such inexactness originates from the non-uniqueness of relaxation solutions and numerical errors amplified by the non-uniqueness. This finding motivates two algorithms to recover the exact solution of AC-OPF in unbalanced distribution networks featuring both wye and delta connections. In simulations using IEEE 13, 37 and 123-bus systems, the proposed algorithms provide exact optimal solutions up to numerical precision.https://authors.library.caltech.edu/records/0trtw-nqr14Combating Bufferbloat in Multi-Bottleneck Networks: Theory and Algorithms
https://resolver.caltech.edu/CaltechAUTHORS:20210412-072637490
Authors: {'items': [{'id': 'Ye-Jiancheng', 'name': {'family': 'Ye', 'given': 'Jiancheng'}, 'orcid': '0000-0002-1296-171X'}, {'id': 'Leung-Ka-Cheong', 'name': {'family': 'Leung', 'given': 'Ka-Cheong'}, 'orcid': '0000-0001-7999-2572'}, {'id': 'Low-S-H', 'name': {'family': 'Low', 'given': 'Steven H.'}, 'orcid': '0000-0001-6476-3048'}]}
Year: 2021
DOI: 10.1109/TNET.2021.3066505
Bufferbloat is a phenomenon in computer networks where large router buffers are frequently filled up, resulting in high queueing delay and delay variation. More and more delay-sensitive applications on the Internet have made this phenomenon a pressing issue. Interacting with the Transmission Control Protocol (TCP), active queue management (AQM) algorithms run on routers play an important role in combating bufferbloat. However, AQM algorithms have not been widely deployed due to complicated manual parameter tuning. Moreover, they are often designed and analyzed based on network models with a single bottleneck link, rendering their performance and stability unclear in multi-bottleneck networks. In this paper, we propose a general framework to combat bufferbloat in multi-bottleneck networks. We first present an equilibrium analysis for a general multi-bottleneck TCP/AQM system and provide sufficient conditions for the uniqueness of an equilibrium point in the system. We then decompose the system into single-bottleneck subsystems and derive sufficient conditions for the local asymptotic stability of the subsystems. Using our framework, we develop an algorithm to compute the equilibrium point of the system. We further present a case study to analyze the stability of the recently proposed Controlled Delay (CoDel) in multi-bottleneck networks and devise Self-Tuning CoDel to improve the system stability. Extensive numerical and packet-level simulation results not only verify our theoretical studies but also show that our proposed Self-Tuning CoDel significantly stabilizes queueing delay in multi-bottleneck networks, thereby mitigating bufferbloat.https://authors.library.caltech.edu/records/pk98z-sbx87Line Failure Localization of Power Networks Part II: Cut Set Outages
https://resolver.caltech.edu/CaltechAUTHORS:20200707-095927831
Authors: {'items': [{'id': 'Guo-Linqi', 'name': {'family': 'Guo', 'given': 'Linqi'}, 'orcid': '0000-0001-5771-2752'}, {'id': 'Liang-Chen', 'name': {'family': 'Liang', 'given': 'Chen'}, 'orcid': '0000-0002-0015-7206'}, {'id': 'Zocca-Alessandro', 'name': {'family': 'Zocca', 'given': 'Alessandro'}, 'orcid': '0000-0001-6585-4785'}, {'id': 'Low-S-H', 'name': {'family': 'Low', 'given': 'Steven H.'}, 'orcid': '0000-0001-6476-3048'}, {'id': 'Wierman-A', 'name': {'family': 'Wierman', 'given': 'Adam'}}]}
Year: 2021
DOI: 10.1109/TPWRS.2021.3068048
Transmission line failure in power systems prop-agate non-locally, making the control of the resulting outages extremely difficult. In Part II of this paper, we continue the study of line failure localizability in transmission networks and characterize the impact of cut set outages. We establish a Simple Path Criterion, showing that the propagation pattern due to bridge outages, a special case of cut set failures, are fully determined by the positions in the network of the buses that participate in load balancing. We then extend our results to general cut set outages. In contrast to non-cut outages discussed in Part I whose subsequent line failures are contained within the original blocks, cut set outages typically impact the whole network, affecting the power flows on all remaining lines. We corroborate our analytical results in both parts using the IEEE 118-bus test system, in which the failure propagation patterns exhibit a clear block-diagonal structure predicted by our theory, even when using full AC power flow equations.https://authors.library.caltech.edu/records/5epx3-x6t87Line Failure Localization of Power Networks Part I: Non-Cut Outages
https://resolver.caltech.edu/CaltechAUTHORS:20200707-101019648
Authors: {'items': [{'id': 'Guo-Linqi', 'name': {'family': 'Guo', 'given': 'Linqi'}, 'orcid': '0000-0001-5771-2752'}, {'id': 'Liang-Chen', 'name': {'family': 'Liang', 'given': 'Chen'}, 'orcid': '0000-0002-0015-7206'}, {'id': 'Zocca-Alessandro', 'name': {'family': 'Zocca', 'given': 'Alessandro'}, 'orcid': '0000-0001-6585-4785'}, {'id': 'Low-S-H', 'name': {'family': 'Low', 'given': 'Steven H.'}, 'orcid': '0000-0001-6476-3048'}, {'id': 'Wierman-A', 'name': {'family': 'Wierman', 'given': 'Adam'}}]}
Year: 2021
DOI: 10.1109/tpwrs.2021.3066336
Transmission line failures in power systems propagate non-locally, making the control of the resulting outages extremely difficult. In this work, we establish a mathematical theory that characterizes the patterns of line failure propagation and localization in terms of network graph structure. It provides a novel perspective on distribution factors that precisely captures Kirchhoff's Law in terms of topological structures. Our results show that the distribution of specific collections of subtrees of the transmission network plays a critical role on the patterns of power redistribution, and motivates the block decomposition of the transmission network as a structure to understand long-distance propagation of disturbances. In Part I of this paper, we present the case when the post-contingency network remains connected after an initial set of lines are disconnected simultaneously. In Part II, we present the case when an outage separates the network into multiple islands.https://authors.library.caltech.edu/records/xtnd5-5ye52Conditions for Exact Convex Relaxation and No Spurious Local Optima
https://resolver.caltech.edu/CaltechAUTHORS:20210510-075841014
Authors: {'items': [{'id': 'Zhou-Fengyu', 'name': {'family': 'Zhou', 'given': 'Fengyu'}, 'orcid': '0000-0002-2639-6491'}, {'id': 'Low-S-H', 'name': {'family': 'Low', 'given': 'Steven H.'}, 'orcid': '0000-0001-6476-3048'}]}
Year: 2021
DOI: 10.1109/TCNS.2021.3112758
Non-convex optimization problems can be approximately solved via relaxation or local algorithms. For many practical problems such as optimal power flow (OPF) problems, both approaches tend to succeed in the sense that relaxation is usually exact and local algorithms usually converge to a global optimum. In this paper, we study conditions which are sufficient or necessary for such non-convex problems to simultaneously have exact relaxation and no spurious local optima. Those conditions help us explain the widespread empirical experience that local algorithms for OPF problems often work extremely well.https://authors.library.caltech.edu/records/4kk8d-6v016Adaptive Charging Networks: A Framework for Smart Electric Vehicle Charging
https://resolver.caltech.edu/CaltechAUTHORS:20210503-115705210
Authors: {'items': [{'id': 'Lee-Zachary-J', 'name': {'family': 'Lee', 'given': 'Zachary J.'}, 'orcid': '0000-0002-5358-2388'}, {'id': 'Lee-George-S', 'name': {'family': 'Lee', 'given': 'George'}}, {'id': 'Lee-Ted', 'name': {'family': 'Lee', 'given': 'Ted'}, 'orcid': '0000-0002-5793-7457'}, {'id': 'Jin-Cheng-S', 'name': {'family': 'Jin', 'given': 'Cheng'}}, {'id': 'Lee-Rand', 'name': {'family': 'Lee', 'given': 'Rand'}}, {'id': 'Low-Zhi-H', 'name': {'family': 'Low', 'given': 'Zhi'}, 'orcid': '0000-0003-4962-7771'}, {'id': 'Chang-Daniel', 'name': {'family': 'Chang', 'given': 'Daniel'}}, {'id': 'Ortega-Christine', 'name': {'family': 'Ortega', 'given': 'Christine'}}, {'id': 'Low-S-H', 'name': {'family': 'Low', 'given': 'Steven H.'}, 'orcid': '0000-0001-6476-3048'}]}
Year: 2021
DOI: 10.1109/TSG.2021.3074437
We describe the architecture and algorithms of the Adaptive Charging Network (ACN), which was first deployed on the Caltech campus in early 2016 and is currently operating at over 100 other sites in the United States. The architecture enables real-time monitoring and control and supports electric vehicle (EV) charging at scale. The ACN adopts a flexible Adaptive Scheduling Algorithm based on convex optimization and model predictive control and allows for significant over-subscription of electrical infrastructure. We describe some of the practical challenges in real-world charging systems, including unbalanced three-phase infrastructure, non-ideal battery charging behavior, and quantized control signals. We demonstrate how the Adaptive Scheduling Algorithm handles these challenges, and compare its performance against baseline algorithms from the deadline scheduling literature using real workloads recorded from the Caltech ACN and accurate system models. We find that in these realistic settings, our scheduling algorithm can improve operator profit by 3.4 times over uncontrolled charging and consistently outperforms baseline algorithms when delivering energy in highly congested systems.https://authors.library.caltech.edu/records/f00wy-s1473Learning-based Predictive Control via Real-time Aggregate Flexibility
https://resolver.caltech.edu/CaltechAUTHORS:20210510-084600512
Authors: {'items': [{'id': 'Li-Tongxin', 'name': {'family': 'Li', 'given': 'Tongxin'}, 'orcid': '0000-0002-9806-8964'}, {'id': 'Sun-Bo', 'name': {'family': 'Sun', 'given': 'Bo'}, 'orcid': '0000-0003-3172-7811'}, {'id': 'Chen-Yue', 'name': {'family': 'Chen', 'given': 'Yue'}, 'orcid': '0000-0002-7594-7587'}, {'id': 'Ye-Zixin', 'name': {'family': 'Ye', 'given': 'Zixin'}}, {'id': 'Low-S-H', 'name': {'family': 'Low', 'given': 'Steven H.'}, 'orcid': '0000-0001-6476-3048'}, {'id': 'Wierman-A', 'name': {'family': 'Wierman', 'given': 'Adam'}}]}
Year: 2021
DOI: 10.1109/TSG.2021.3094719
Aggregators have emerged as crucial tools for the coordination of distributed, controllable loads. To be used effectively, an aggregator must be able to communicate the available flexibility of the loads they control, as known as the aggregate flexibility to a system operator. However, most of existing aggregate flexibility measures often are slow-timescale estimations and much less attention has been paid to real-time coordination between an aggregator and an operator. In this paper, we consider solving an online optimization in a closed-loop system and present a design of real-time aggregate flexibility feedback, termed the maximum entropy feedback (MEF). In addition to deriving analytic properties of the MEF, combining learning and control, we show that it can be approximated using reinforcement learning and used as a penalty term in a novel control algorithm – the penalized predictive control (PPC), which modifies vanilla model predictive control (MPC). The benefits of our scheme are (1). Efficient Communication . An operator running PPC does not need to know the exact states and constraints of the loads, but only the MEF. (2). Fast Computation . The PPC often has much less number of variables than an MPC formulation. (3). Lower Costs We show that under certain regularity assumptions, the PPC is optimal. We illustrate the efficacy of the PPC using a dataset from an adaptive electric vehicle charging network and show that PPC outperforms classical MPC.https://authors.library.caltech.edu/records/j0s8j-1aa33ACN-Sim: An Open-Source Simulator for Data-Driven Electric Vehicle Charging Research
https://resolver.caltech.edu/CaltechAUTHORS:20210510-085201964
Authors: {'items': [{'id': 'Lee-Zachary-J', 'name': {'family': 'Lee', 'given': 'Zachary J.'}, 'orcid': '0000-0002-5358-2388'}, {'id': 'Sharma-Sunash', 'name': {'family': 'Sharma', 'given': 'Sunash'}, 'orcid': '0000-0002-8763-5927'}, {'id': 'Johansson-Daniel', 'name': {'family': 'Johansson', 'given': 'Daniel'}}, {'id': 'Low-S-H', 'name': {'family': 'Low', 'given': 'Steven H.'}, 'orcid': '0000-0001-6476-3048'}]}
Year: 2021
DOI: 10.1109/TSG.2021.3103156
ACN-Sim is a data-driven, open-source simulation environment designed to accelerate research in the field of smart electric vehicle (EV) charging. It fills the need in this community for a widely available, realistic simulation environment in which researchers can evaluate algorithms and test assumptions. ACN-Sim provides a modular, extensible architecture, which models the complexity of real charging systems, including battery charging behavior and unbalanced three-phase infrastructure. It also integrates with a broader ecosystem of research tools. These include ACN-Data, an open dataset of EV charging sessions, which provides realistic simulation scenarios and ACN-Live, a framework for field-testing charging algorithms. It also integrates with grid simulators like MATPOWER, PandaPower and OpenDSS, and OpenAI Gym for training reinforcement learning agents.https://authors.library.caltech.edu/records/s94ac-pr864DeepOPF-V: Solving AC-OPF Problems Efficiently
https://resolver.caltech.edu/CaltechAUTHORS:20210510-075001060
Authors: {'items': [{'id': 'Huang-Wanjun', 'name': {'family': 'Huang', 'given': 'Wanjun'}, 'orcid': '0000-0002-6851-3705'}, {'id': 'Pan-Xiang', 'name': {'family': 'Pan', 'given': 'Xiang'}, 'orcid': '0000-0002-6565-2339'}, {'id': 'Chen-Minghua', 'name': {'family': 'Chen', 'given': 'Minghua'}, 'orcid': '0000-0003-4763-0037'}, {'id': 'Low-S-H', 'name': {'family': 'Low', 'given': 'Steven H.'}, 'orcid': '0000-0001-6476-3048'}]}
Year: 2022
DOI: 10.1109/TPWRS.2021.3114092
AC optimal power flow (AC-OPF) problems need to be solved more frequently in the future to maintain stable and economic power system operation. To tackle this challenge, a deep neural network-based voltage-constrained approach (DeepOPF-V) is proposed to solve AC-OPF problems with high computational efficiency. Its unique design predicts voltages of all buses and then uses them to reconstruct the remaining variables without solving non-linear AC power flow equations. A fast post-processing process is also developed to enforce the box constraints. The effectiveness of DeepOPF-V is validated by simulations on IEEE 118/300-bus systems and a 2000-bus test system. Compared with existing studies, DeepOPF-V achieves decent computation speedup up to four orders of magnitude and comparable performance in optimality gap, while preserving feasibility of the solution.https://authors.library.caltech.edu/records/5tnxk-4aj52Robustness and Consistency in Linear Quadratic Control with Untrusted Predictions
https://resolver.caltech.edu/CaltechAUTHORS:20210716-225846876
Authors: {'items': [{'id': 'Li-Tongxin', 'name': {'family': 'Li', 'given': 'Tongxin'}, 'orcid': '0000-0002-9806-8964'}, {'id': 'Yang-Ruixiao', 'name': {'family': 'Yang', 'given': 'Ruixiao'}}, {'id': 'Qu-Guannan', 'name': {'family': 'Qu', 'given': 'Guannan'}, 'orcid': '0000-0002-5466-3550'}, {'id': 'Shi-Guanya', 'name': {'family': 'Shi', 'given': 'Guanya'}, 'orcid': '0000-0002-9075-3705'}, {'id': 'Yu-Chenkai', 'name': {'family': 'Yu', 'given': 'Chenkai'}, 'orcid': '0000-0001-8683-7773'}, {'id': 'Wierman-A', 'name': {'family': 'Wierman', 'given': 'Adam'}, 'orcid': '0000-0002-5923-0199'}, {'id': 'Low-S-H', 'name': {'family': 'Low', 'given': 'Steven'}, 'orcid': '0000-0001-6476-3048'}]}
Year: 2022
DOI: 10.1145/3508038
We study the problem of learning-augmented predictive linear quadratic control. Our goal is to design a controller that balances consistency, which measures the competitive ratio when predictions are accurate, and robustness, which bounds the competitive ratio when predictions are inaccurate.https://authors.library.caltech.edu/records/p10m3-kqj37Online Station Assignment for Electric Vehicle Battery Swapping
https://resolver.caltech.edu/CaltechAUTHORS:20201209-153308228
Authors: {'items': [{'id': 'You-Pengcheng', 'name': {'family': 'You', 'given': 'Pengcheng'}, 'orcid': '0000-0003-1532-8773'}, {'id': 'Pang-John-Z-F', 'name': {'family': 'Pang', 'given': 'John Z. F.'}, 'orcid': '0000-0002-6485-7922'}, {'id': 'Low-S-H', 'name': {'family': 'Low', 'given': 'Steven H.'}, 'orcid': '0000-0001-6476-3048'}]}
Year: 2022
DOI: 10.1109/tits.2020.3033731
This paper investigates the online station assignment for (commercial) electric vehicles (EVs) that request battery swapping from a central operator, i.e., in the absence of future information a battery swapping service station has to be assigned instantly to each EV upon its request. Based on EVs' locations, the availability of fully-charged batteries at service stations in the system, as well as traffic conditions, the assignment aims to minimize cost to EVs and congestion at service stations. Inspired by a polynomial-time offline solution via a bipartite matching approach, we develop an efficient and implementable online station assignment algorithm that provably achieves the tight (optimal) competitive ratio under mild conditions. Monte Carlo experiments on a real transportation network by Baidu Maps show that our algorithm performs reasonably well on realistic inputs, even with a certain amount of estimation error in parameters.https://authors.library.caltech.edu/records/wnzah-8xz59Smoothed Least-Laxity-First Algorithm for Electric Vehicle Charging: Online Decision and Performance Analysis with Resource Augmentation
https://resolver.caltech.edu/CaltechAUTHORS:20220127-230706500
Authors: {'items': [{'id': 'Chen-Niangjun', 'name': {'family': 'Chen', 'given': 'Niangjun'}, 'orcid': '0000-0002-2289-9737'}, {'id': 'Kurniawan-Christian', 'name': {'family': 'Kurniawan', 'given': 'Christian'}, 'orcid': '0000-0002-3178-4042'}, {'id': 'Nakahira-Yorie', 'name': {'family': 'Nakahira', 'given': 'Yorie'}, 'orcid': '0000-0003-3324-4602'}, {'id': 'Chen-Lijun', 'name': {'family': 'Chen', 'given': 'Lijun'}, 'orcid': '0000-0001-6694-4299'}, {'id': 'Low-S-H', 'name': {'family': 'Low', 'given': 'Steven H.'}, 'orcid': '0000-0001-6476-3048'}]}
Year: 2022
DOI: 10.1109/tsg.2021.3138615
Adaptive charging can charge electric vehicles (EVs) at scale cost effectively, despite of the uncertainty in EV arrivals. We formulate adaptive EV charging as a feasibility problem that meets all EVs' energy demands before their deadlines while satisfying constraints in charging rate and total charging power. We propose an online algorithm, smoothed least-laxity-first (sLLF), that decides the current charging rates without the knowledge of future arrivals and demands. We characterize the performance of the sLLF algorithm analytically and numerically. Numerical experiments with real-world data show that it has a significantly higher rate of feasible EV charging than several other existing EV charging algorithms. Resource augmentation framework is employed to assess the feasibility condition of the algorithm. The assessment shows that the sLLF algorithm achieves perfect feasibility with only a 7% increase in the maximal power supply of the charging station.https://authors.library.caltech.edu/records/61zpj-ex175Reinforcement Learning for Selective Key Applications in Power Systems: Recent Advances and Future Challenges
https://resolver.caltech.edu/CaltechAUTHORS:20220307-188369000
Authors: {'items': [{'id': 'Chen-Xin', 'name': {'family': 'Chen', 'given': 'Xin'}, 'orcid': '0000-0002-0952-0008'}, {'id': 'Qu-Guannan', 'name': {'family': 'Qu', 'given': 'Guannan'}, 'orcid': '0000-0002-5466-3550'}, {'id': 'Tang-Yujie', 'name': {'family': 'Tang', 'given': 'Yujie'}, 'orcid': '0000-0002-4921-8372'}, {'id': 'Low-S-H', 'name': {'family': 'Low', 'given': 'Steven'}, 'orcid': '0000-0001-6476-3048'}, {'id': 'Li-Na', 'name': {'family': 'Li', 'given': 'Na'}, 'orcid': '0000-0001-9545-3050'}]}
Year: 2022
DOI: 10.1109/tsg.2022.3154718
With large-scale integration of renewable generation and distributed energy resources, modern power systems are confronted with new operational challenges, such as growing complexity, increasing uncertainty, and aggravating volatility. Meanwhile, more and more data are becoming available owing to the widespread deployment of smart meters, smart sensors, and upgraded communication networks. As a result, data-driven control techniques, especially reinforcement learning (RL), have attracted surging attention in recent years. This paper provides a comprehensive review of various RL techniques and how they can be applied to decision-making and control in power systems. In particular, we select three key applications, i.e., frequency regulation, voltage control, and energy management, as examples to illustrate RL-based models and solutions. We then present the critical issues in the application of RL, i.e., safety, robustness, scalability, and data. Several potential future directions are discussed as well.https://authors.library.caltech.edu/records/1mttm-4w433Running Primal-Dual Gradient Method for Time-Varying Nonconvex Problems
https://resolver.caltech.edu/CaltechAUTHORS:20220811-235221000
Authors: {'items': [{'id': 'Tang-Yujie', 'name': {'family': 'Tang', 'given': 'Yujie'}, 'orcid': '0000-0002-4921-8372'}, {'id': "Dall'Anese-Emiliano", 'name': {'family': "Dall'Anese", 'given': 'Emiliano'}}, {'id': 'Bernstein-Andrey', 'name': {'family': 'Bernstein', 'given': 'Andrey'}}, {'id': 'Low-S-H', 'name': {'family': 'Low', 'given': 'Steven'}, 'orcid': '0000-0001-6476-3048'}]}
Year: 2022
DOI: 10.1137/20m1371063
This paper focuses on a time-varying constrained nonconvex optimization problem, and considers the synthesis and analysis of online regularized primal-dual gradient methods to track a Karush--Kuhn--Tucker (KKT) trajectory. The proposed regularized primal-dual gradient method is implemented in a running fashion, in the sense that the underlying optimization problem changes during the execution of the algorithms. In order to study its performance, we first derive its continuous-time limit as a system of differential inclusions. We then study sufficient conditions for tracking a KKT trajectory, and also derive asymptotic bounds for the tracking error (as a function of the time-variability of a KKT trajectory). Further, we provide a set of sufficient conditions for the KKT trajectories not to bifurcate or merge, and also investigate the optimal choice of the parameters of the algorithm. Illustrative numerical results for a time-varying nonconvex problem are provided.https://authors.library.caltech.edu/records/02jhx-97p42DeepOPF: A Feasibility-Optimized Deep Neural Network Approach for AC Optimal Power Flow Problems
https://resolver.caltech.edu/CaltechAUTHORS:20221010-454096500.22
Authors: {'items': [{'id': 'Pan-Xiang', 'name': {'family': 'Pan', 'given': 'Xiang'}, 'orcid': '0000-0002-6565-2339'}, {'id': 'Chen-Minghua', 'name': {'family': 'Chen', 'given': 'Minghua'}, 'orcid': '0000-0003-4763-0037'}, {'id': 'Zhao-Tianyu', 'name': {'family': 'Zhao', 'given': 'Tianyu'}, 'orcid': '0000-0002-9541-0197'}, {'id': 'Low-S-H', 'name': {'family': 'Low', 'given': 'Steven H.'}, 'orcid': '0000-0001-6476-3048'}]}
Year: 2022
DOI: 10.1109/jsyst.2022.3201041
To cope with increasing uncertainty from renewable generation and flexible load, grid operators need to solve alternative current optimal power flow (AC-OPF) problems more frequently for efficient and reliable operation. In this article, we develop a deep neural network (DNN) approach, called DeepOPF, for solving AC-OPF problems in a fraction of the time used by conventional iterative solvers. A key difficulty for applying machine learning techniques for solving AC-OPF problems lies in ensuring that the obtained solutions respect the equality and inequality physical and operational constraints. Generalized a prediction-and-reconstruction procedure in our previous studies, DeepOPF first trains a DNN model to predict a set of independent operating variables and then directly compute the remaining ones by solving the power flow equations. Such an approach not only preserves the power-flow balance equality constraints but also reduces the number of variables to be predicted by the DNN, cutting down the number of neurons and training data needed. DeepOPF then employs a penalty approach with a zero-order gradient estimation technique in the training process toward guaranteeing the inequality constraints. We also drive a condition for tuning the DNN size according to the desired approximation accuracy, which measures its generalization capability. It provides theoretical justification for using DNN to solve AC-OPF problems. Simulation results for IEEE 30/118/300-bus and a synthetic 2000-bus test cases demonstrate the effectiveness of the penalty approach. They also show that DeepOPF speeds up the computing time by up to two orders of magnitude as compared to a state-of-the-art iterative solver, at the expense of < 0.2% cost difference.https://authors.library.caltech.edu/records/pvbnj-1nm65Towards balanced three-phase charging: Phase optimization in adaptive charging networks
https://resolver.caltech.edu/CaltechAUTHORS:20221010-449776900.1
Authors: {'items': [{'id': 'Ye-Zixin', 'name': {'family': 'Ye', 'given': 'Zixin'}, 'orcid': '0000-0002-4107-3842'}, {'id': 'Li-Tongxin', 'name': {'family': 'Li', 'given': 'Tongxin'}, 'orcid': '0000-0002-9806-8964'}, {'id': 'Low-S-H', 'name': {'family': 'Low', 'given': 'Steven'}, 'orcid': '0000-0001-6476-3048'}]}
Year: 2022
DOI: 10.1016/j.epsr.2022.108322
We study the problem of phase optimization for electric-vehicle (EV) charging. We formulate our problem as a non-convex mixed-integer programming problem whose objective is to minimize the charging loss. Despite the hardness of directly solving this non-convex problem, we solve a relaxation of the original problem by proposing the PXA algorithm where "P", "X", and "A" stand for three variable matrices in the formed phase optimization problems. We show that under certain conditions, the solution is given by the PXA precisely converges to the global optimum. In addition, using the idea of model predictive control (MPC), we design the PXA-MPC, which is an online implementation of the PXA. Compared to other empirical phase balancing strategies, the PXA algorithm significantly improves the charging performance by maximizing energy delivery, minimizing charging price, and assisting future energy planning. The efficacy of our algorithm is demonstrated using data collected from a real-world adaptive EV charging network (ACN).https://authors.library.caltech.edu/records/8k3x6-7ec55Dispatch-aware planning for feasible power system operation
https://resolver.caltech.edu/CaltechAUTHORS:20221011-128968500.6
Authors: {'items': [{'id': 'Christianson-Nicolas-H', 'name': {'family': 'Christianson', 'given': 'Nicolas'}, 'orcid': '0000-0001-8330-8964'}, {'id': 'Werner-Lucien-D', 'name': {'family': 'Werner', 'given': 'Lucien'}}, {'id': 'Wierman-A', 'name': {'family': 'Wierman', 'given': 'Adam'}, 'orcid': '0000-0002-5923-0199'}, {'id': 'Low-S-H', 'name': {'family': 'Low', 'given': 'Steven H.'}, 'orcid': '0000-0001-6476-3048'}]}
Year: 2022
DOI: 10.1016/j.epsr.2022.108597
Maintaining stable energy production with increasing penetration of variable renewable energy requires sufficient flexible generation resources and dispatch algorithms that accommodate renewables' uncertainty. In this work, we study the feasibility properties of real-time economic dispatch (RTED) algorithms and establish fundamental limits on their performance. We propose a joint methodology for resource procurement and online economic dispatch with guaranteed feasibility. Our algorithm, Feasible Fixed Horizon Control (FFHC) is a regularized form of Receding Horizon Control (RHC) that balances exploitation of good near-term demand predictions with feasibility requirements. Empirical evaluation of FFHC in comparison to the standard RHC on realistic load profiles highlights that FFHC achieves near-optimal performance while ensuring feasibility in high-ramp scenarios where RHC becomes infeasible.https://authors.library.caltech.edu/records/tmnrt-9y586Inverse Power Flow Problem
https://resolver.caltech.edu/CaltechAUTHORS:20230502-984953000.1
Authors: {'items': [{'id': 'Yuan-Ye', 'name': {'family': 'Yuan', 'given': 'Ye'}, 'orcid': '0000-0001-7858-0437'}, {'id': 'Low-S-H', 'name': {'family': 'Low', 'given': 'Steven H.'}, 'orcid': '0000-0001-6476-3048'}, {'id': 'Ardakanian-Omid', 'name': {'family': 'Ardakanian', 'given': 'Omid'}, 'orcid': '0000-0002-6711-5502'}, {'id': 'Tomlin-Claire-J', 'name': {'family': 'Tomlin', 'given': 'Claire J.'}, 'orcid': '0000-0003-3192-3185'}]}
Year: 2023
DOI: 10.1109/tcns.2022.3199084
We formulate an inverse power flow problem that infers a nodal admittance matrix from voltage and current phasors measured at a number of buses. We show that the admittance matrix can be uniquely identified from a sequence of measurements corresponding to different steady states when every node in the system is equipped with a measurement device, and a Kron-reduced admittance matrix can be determined even if some nodes in the system are not monitored (hidden nodes). We propose an effective algorithm to uncover the actual admittance matrix of a radial system with hidden nodes from its Kron-reduced admittance matrix.https://authors.library.caltech.edu/records/99twq-8jn27An Energy Sharing Mechanism Considering Network Constraints and Market Power Limitation
https://resolver.caltech.edu/CaltechAUTHORS:20230502-727238500.2
Authors: {'items': [{'id': 'Chen-Yue', 'name': {'family': 'Chen', 'given': 'Yue'}, 'orcid': '0000-0002-7594-7587'}, {'id': 'Zhao-Changhong', 'name': {'family': 'Zhao', 'given': 'Changhong'}, 'orcid': '0000-0003-0539-8591'}, {'id': 'Low-S-H', 'name': {'family': 'Low', 'given': 'Steven H.'}, 'orcid': '0000-0001-6476-3048'}, {'id': 'Wierman-A', 'name': {'family': 'Wierman', 'given': 'Adam'}, 'orcid': '0000-0002-5923-0199'}]}
Year: 2023
DOI: 10.1109/tsg.2022.3198721
As the number of prosumers with distributed energy resources (DERs) grows, the conventional centralized operation scheme may suffer from conflicting interests, privacy concerns, and incentive inadequacy. In this paper, we propose an energy sharing mechanism to address the above challenges. It takes into account network constraints and fairness among prosumers. In the proposed energy sharing market, all prosumers play a generalized Nash game. The market equilibrium is proved to have nice features in a large market or when it is a variational equilibrium. To deal with the possible market failure, inefficiency, or instability in general cases, we introduce a price regulation policy to avoid market power exploitation. The improved energy sharing mechanism with price regulation can guarantee the existence and uniqueness of a socially near-optimal market equilibrium. Some advantageous properties are proved, such as the prosumer's individual rationality, a sharing price structure similar to the locational marginal price, and the tendency towards social optimum with an increasing number of prosumers. For implementation, a practical bidding algorithm is developed with a convergence condition. Experimental results validate the theoretical outcomes and show the practicability of our model and method.https://authors.library.caltech.edu/records/npwrx-d8m85Stability Constrained Reinforcement Learning for Decentralized Real-Time Voltage Control
https://authors.library.caltech.edu/records/v26v8-j6q25
Authors: {'items': [{'id': 'Feng-Jie', 'name': {'family': 'Feng', 'given': 'Jie'}, 'orcid': '0000-0002-5049-9423'}, {'id': 'Shi-Yuanyuan', 'name': {'family': 'Shi', 'given': 'Yuanyuan'}, 'orcid': '0000-0002-6182-7664'}, {'id': 'Qu-Guannan', 'name': {'family': 'Qu', 'given': 'Guannan'}, 'orcid': '0000-0002-5466-3550'}, {'id': 'Low-S-H', 'name': {'family': 'Low', 'given': 'Steven H.'}, 'orcid': '0000-0001-6476-3048'}, {'id': 'Anandkumar-A', 'name': {'family': 'Anandkumar', 'given': 'Anima'}, 'orcid': '0000-0002-6974-6797'}, {'id': 'Wierman-A', 'name': {'family': 'Wierman', 'given': 'Adam'}, 'orcid': '0000-0002-5923-0199'}]}
Year: 2023
DOI: 10.1109/tcns.2023.3338240
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<div>Deep reinforcement learning has been recognized as a promising tool to address the challenges in real-time control of power systems. However, its deployment in real-world power systems has been hindered by a lack of explicit stability and safety guarantees. In this paper, we propose a stability-constrained reinforcement learning (RL) method for real-time implementation of voltage control, that guarantees system stability both during policy learning and deployment of the learned policy. The key idea underlying our approach is an explicitly constructed Lyapunov function that leads to a sufficient structural condition for stabilizing policies, i.e., monotonically decreasing policies guarantee stability. We incorporate this structural constraint with RL, by parameterizing each local voltage controller using a monotone neural network, thus ensuring the stability constraint is satisfied by design. We demonstrate the effectiveness of our approach in both single-phase and three-phase IEEE test feeders, where the proposed method can reduce the transient control cost by more than 26.7% and shorten the voltage recovery time by 23.6% on average compared to the widely used linear policy, while always achieving voltage stability. In contrast, standard RL methods often fail to achieve voltage stability.</div>
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</div>https://authors.library.caltech.edu/records/v26v8-j6q25