@inproceedings{qian_xu_zhang_chakrabortty_mueller_xin_2016, title={A resilient software infrastructure for wide-area measurement systems}, DOI={10.1109/pesgm.2016.7741949}, abstractNote={To support the scalability and resilience requirements of distributed Wide-Area Measurement System (WAMS) architectures, we design and implement a software infrastructure to estimate power grid oscillation modes based on real-time data collected from Phasor Measurement Units (PMUs). This estimation algorithm can be deployed on a hierarchical structure of Phasor Data Concentrators (PDCs), which calculate local estimates and communicate with each other to calculate the global estimate. This work contributes a resilient system to WAMS with guarantees for (1) Quality of Service in network delay, (2) network failure tolerance, and (3) self-recoverability. The core component of the infrastructure is a distributed storage system. Externally, the storage system provides a cloud data lookup service with bounded response times and resilience, which decouples the data communication between PMUs, PDCs, and power-grid monitor/control applications. Internally, the storage system organizes PDCs as storage nodes and employs a real-time task scheduler to order data lookup requests so that urgent requests can be served earlier. To demonstrate the resilience of our distributed system, we deploy the system on a (1) virtual platform and (2) bare-metal machines, where we run a distributed algorithm on the basis of the Prony algorithm and the Alternating Directions Method of Multipliers (ADMM) to estimate the electro-mechanical oscillation modes. We inject different failures into the system to study their impact on the estimation algorithm. Our experiments show that temporary failures of a PDC or a network link do not affect the estimation result since the historical PMU data are cached in the storage system and PDCs can obtain the data on demand.}, booktitle={2016 ieee power and energy society general meeting (pesgm)}, author={Qian, T. and Xu, H. and Zhang, J. H. and Chakrabortty, Aranya and Mueller, F. and Xin, Y. F.}, year={2016} }
 @article{zhang_nabavi_chakrabortty_xin_2016, title={ADMM Optimization Strategies for Wide-Area Oscillation Monitoring in Power Systems Under Asynchronous Communication Delays}, volume={7}, ISSN={["1949-3061"]}, DOI={10.1109/tsg.2016.2547939}, abstractNote={In this paper, we present a suite of asynchronous distributed optimization algorithms for wide-area oscillation estimation in power systems using alternating direction method of multipliers (ADMMs). We first pose the estimation problem as a real-time, iterative, and distributed consensus problem. Thereafter, we consider a probabilistic traffic model for modeling delays in any typical wide-area communication network, and study how the delays enter the process of information exchange between distributed phasor data concentrators that are employed to execute this consensus algorithm in a coordinated fashion. Finally, we propose four different strategies by which the convergence rate and accuracy of this consensus algorithm can be made immune to the asynchrony resulting from the network traffic. We carry out extensive simulations to show possible numerical instabilities and sensitivities of the ADMM convergence on our proposed strategies. Our results exhibit a broad view of how the convergence of any distributed estimation algorithm in a generic cyber-physical system depends strongly on the uncertainties of the underlying communication models.}, number={4}, journal={IEEE TRANSACTIONS ON SMART GRID}, author={Zhang, Jianhua and Nabavi, Seyedbehzad and Chakrabortty, Aranya and Xin, Yufeng}, year={2016}, month={Jul}, pages={2123–2133} }
 @inproceedings{zhang_nabavi_chakrabortty_xin_2015, title={Convergence analysis of ADMM-based power system mode estimation under asynchronous wide-area communication delays}, DOI={10.1109/pesgm.2015.7286038}, abstractNote={In our recent paper [1], we proposed a distributed PMU-PDC architecture for estimating power system oscillation modes by integrating a Prony-based algorithm with Alternating Direction Method of Multipliers (ADMM). A critical assumption behind the proposed method was that the communication between local PDCs and the central averager is completely synchronized. In realistic wide-area networks, however, such synchronous communication may not always be possible. In this paper we address this issue of asynchronous communication, and its impact on the convergence of the distributed estimation. We first impose a probability model for the communication delays between the central PDC and the local PDCs, and then implement two strategies of averaging at the central PDC based on a chosen delay threshold. We carry out simulations to show possible instabilities and sensitivities of the ADMM convergence on delay distribution parameters under these two averaging strategies. Our results exhibit a broad view of how the convergence of distributed estimation algorithms in physical processes depends strongly on the uncertainties in the underlying communications in a generic cyber-physical system.}, booktitle={2015 ieee power & energy society general meeting}, author={Zhang, J. H. and Nabavi, S. and Chakrabortty, Aranya and Xin, Y. F.}, year={2015} }
 @article{nabavi_zhang_chakrabortty_2015, title={Distributed Optimization Algorithms for Wide-Area Oscillation Monitoring in Power Systems Using Interregional PMU-PDC Architectures}, volume={6}, ISSN={["1949-3061"]}, DOI={10.1109/tsg.2015.2406578}, abstractNote={In this paper, we present a set of distributed algorithms for estimating the electro-mechanical oscillation modes of large power system networks using synchrophasors. With the number of phasor measurement units (PMUs) in the North American grid scaling up to the thousands, system operators are gradually inclining toward distributed cyber-physical architectures for executing wide-area monitoring and control operations. Traditional centralized approaches, in fact, are anticipated to become untenable soon due to various factors such as data volume, security, communication overhead, and failure to adhere to real-time deadlines. To address this challenge, we propose three different communication and computational architectures by which estimators located at the control centers of various utility companies can run local optimization algorithms using local PMU data, and thereafter communicate with other estimators to reach a global solution. Both synchronous and asynchronous communications are considered. Each architecture integrates a centralized Prony-based algorithm with several variants of alternating direction method of multipliers (ADMM). We discuss the relative advantages and bottlenecks of each architecture using simulations of IEEE 68-bus and IEEE 145-bus power system, as well as an Exo-GENI-based software defined network.}, number={5}, journal={IEEE TRANSACTIONS ON SMART GRID}, author={Nabavi, Seyedbehzad and Zhang, Jianhua and Chakrabortty, Aranya}, year={2015}, month={Sep}, pages={2529–2538} }
 @inproceedings{zhang_jaipuria_chakrabortty_hussain_2014, title={A distributed optimization algorithm for attack-resilient wide-area monitoring of power systems: Theoretical and experimental methods}, volume={8840}, DOI={10.1007/978-3-319-12601-2_21}, abstractNote={In this paper we present a real-time distributed optimization algorithm based on Alternating Directions Method of Multipliers (ADMM) for resilient monitoring of power flow oscillation patterns in large power system networks. We pose the problem as a least squares (LS) estimation problem for the coefficients of the characteristic polynomial of the transfer function, and combine a centralized Prony algorithm with ADMM to execute this estimation via distributed consensus. We consider the network topology to be divided into multiple clusters, with each cluster equipped with a local estimator at the local control center. At any iteration, the local estimators receive Synchrophasor measurements from within their own respective areas, run a local consensus algorithm, and communicate their estimates to a central estimator. The central estimator averages all estimates, and broadcasts the average back to each local estimator as the consensus variable for their next iteration. By imposing a redundancy strategy between the local and the global estimators via mutual coordination, we show that the distributed algorithm is more resilient to communication failures as compared to alternative centralized methods. We illustrate our results using a hardware-in-loop power system testbed at NC State federated with a networking and cyber-security testbed at USC/ISI.}, booktitle={Decision and game theory for security, gamesec 2014}, author={Zhang, J. H. and Jaipuria, P. and Chakrabortty, Aranya and Hussain, A.}, year={2014}, pages={350–359} }
 @article{zhang_chakrabortty_xin_2014, title={Distributed Implementation of Wide-Area Monitoring Algorithms for Power Systems Using a US-Wide ExoGENI-WAMS Testbed (Invited Paper)}, ISSN={["1530-0889"]}, DOI={10.1109/dsn.2014.79}, abstractNote={In this paper we address the problem of implementing wide-area oscillation monitoring algorithms for large power system networks using distributed processing of Synchrophasor measurements. We consider two computational approaches, namely decentralized least squares (DLS) and its recursive implementation (RLS). Both algorithms are executed using multiple phasor data concentrators (PDC), deployed as virtual computing machines communicating over a fiber-optic communication network. Results are demonstrated using the US-Wide ExoGENI communication network connected to a PMU test bed at NC State University, and analyze the end-to-end computational and communication delays for both algorithms.}, journal={2014 44TH ANNUAL IEEE/IFIP INTERNATIONAL CONFERENCE ON DEPENDABLE SYSTEMS AND NETWORKS (DSN)}, author={Zhang, Jianhua and Chakrabortty, Aranya and Xin, Yufeng}, year={2014}, pages={762–767} }