@article{nabavi_chakrabortty_2016, title={A Graph-Theoretic Condition for Global Identifiability of Weighted Consensus Networks}, volume={61}, ISSN={["1558-2523"]}, DOI={10.1109/tac.2015.2438425}, abstractNote={In this technical note, we present a sufficient condition that guarantees identifiability for a class of linear network dynamic systems exhibiting continuous-time weighted consensus protocols. Each edge of the underlying network graph G of the system is defined by a constant parameter, referred to as the weight of the edge, while each node is defined by a scalar state whose dynamics evolve as the weighted linear combination of its difference with the states of its neighboring nodes. Following the classical definition of output distinguishability, we first derive a condition that ensures the identifiability of the edge-weights of G in terms of the associate transfer function. Using this characterization, we propose a sensor placement algorithm that guarantees identifiability of the edge-weights. We describe our results using illustrative examples.}, number={2}, journal={IEEE TRANSACTIONS ON AUTOMATIC CONTROL}, author={Nabavi, Seyedbehzad and Chakrabortty, Aranya}, year={2016}, month={Feb}, pages={497–502} }
 @article{nudell_nabavi_chakrabortty_2015, title={A Real-Time Attack Localization Algorithm for Large Power System Networks Using Graph-Theoretic Techniques}, volume={6}, ISSN={["1949-3061"]}, DOI={10.1109/tsg.2015.2406571}, abstractNote={We develop a graph-theoretic algorithm for localizing the physical manifestation of attacks or disturbances in large power system networks using real-time synchrophasor measurements. We assume the attack enters through the electro-mechanical swing dynamics of the synchronous generators in the grid as an unknown additive disturbance. Considering the grid to be divided into coherent areas, we pose the problem as to localize which area the attack may have entered using relevant information extracted from the phasor measurement data. Our approach to solve this problem consists of three main steps. We first run a phasor-based model reduction algorithm by which a dynamic equivalent of the clustered network can be identified in real-time. Second, in parallel, we run a system identification in each area to identify a transfer matrix model for the full-order power system. Thereafter, we exploit the underlying graph-theoretic properties of the identified reduced-order topology, create a set of localization keys, and compare these keys with a selected set of transfer function residues. We validate our results using a detailed case study of the two-area Kundur model and the IEEE 39-bus power system.}, number={5}, journal={IEEE TRANSACTIONS ON SMART GRID}, author={Nudell, Thomas R. and Nabavi, Seyedbehzad and Chakrabortty, Aranya}, year={2015}, month={Sep}, pages={2551–2559} }
 @inproceedings{nabavi_chakrabortty_khargonekar_2015, title={A global identifiability condition for consensus networks on tree graphs}, DOI={10.1109/acc.2015.7170999}, abstractNote={In this paper we present a sufficient condition that guarantees identifiability of linear network dynamic systems exhibiting continuous-time weighted consensus protocols with acyclic structure. Each edge of the underlying network graph G is defined by a constant parameter, referred to as the weight of the edge, while each node is defined by a scalar state whose dynamics evolve as the weighted linear combination of its difference with the states of its neighboring nodes. Following the classical definitions of identifiability and indistinguishability, we first derive a condition that ensures the identifiability of the edge weights of G in terms of the associated transfer function. Using this characterization, we propose a sensor placement algorithm that guarantees identifiability of the edge weights. We describe our results using illustrative examples.}, booktitle={2015 american control conference (acc)}, author={Nabavi, S. and Chakrabortty, Aranya and Khargonekar, P. P.}, year={2015}, pages={1830–1835} }
 @inproceedings{nabavi_chakrabortty_2015, title={An intrusion-resilient distributed optimization algorithm for modal estimation in power systems}, DOI={10.1109/cdc.2015.7402084}, abstractNote={In this paper we present an intrusion-resilient distributed algorithmic approach to estimate the electro-mechanical oscillation modes of a large power system using Synchrophasor measurements. For this, we first show how to distribute the centralized Prony method over a network consisting of several computational areas using a distributed variant of alternating direction method of multipliers (D-ADMM). We then add a cross-verification step to show the resiliency of this algorithm against the cyber-attacks that may happen in the form of data manipulation. We illustrate the robustness of our method in face of intrusion for a case study on IEEE 68-bus power system.}, booktitle={2015 54th IEEE conference on decision and control (CDC)}, author={Nabavi, S. and Chakrabortty, Aranya}, year={2015}, pages={39–44} }
 @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{nabavi_chakrabortty_2014, title={A real-time distributed prony-based algorithm for modal estimation of power system oscillations}, DOI={10.1109/acc.2014.6859381}, abstractNote={In this paper, we present a set of distributed algorithms to estimate the oscillatory electro-mechanical eigenvalues of large power system networks using real-time measurements of phase angles and frequencies. We consider the underlying network to exhibit a strong coherency structure resulting in a two-time-scale behavior of the states. Our goal is to develop a distributed strategy by which only the slow eigenvalues, or the inter-area oscillation modes, can be estimated. Assuming a reliable bound for the slow frequencies, we first pass the actual measurements from the full-order network through a bandpass filter, and use the filter outputs as the effective inputs to our distributed estimation routine. We integrate the centralized Prony method for modal extraction with two commonly-used distributed optimization algorithms, namely, distributed subgradient method (DSM), and alternating direction method of multipliers (ADMM), and show how the filtered outputs can be used in each case to asymptotically estimate the slow eigenvalues of interest. We illustrate our results using a IEEE 39-bus power system, and show the robustness of our methods in face of communication failures.}, booktitle={2014 american control conference (acc)}, author={Nabavi, S. and Chakrabortty, Aranya}, year={2014}, pages={729–734} }
 @inproceedings{nabavi_chakrabortty_2014, title={Distributed estimation of inter-area oscillation modes in large power systems using alternating direction multiplier method}, DOI={10.1109/pesgm.2014.6939475}, abstractNote={In this paper we present a distributed algorithmic approach to estimate the electro-mechanical oscillation modes of a large power system network using real-time Synchrophasor measurements of phase angles and frequencies. Considering the underlying network to exhibit strong coherency, we are particularly interested in estimating the inter-area oscillation frequencies and their damping factors in a distributed fashion. For this we first integrate the centralized Prony method with a commonly used distributed optimization algorithm, namely, alternating direction method of multipliers (ADMM), and show how the resulting approach can utilize PMU data to identify the modes of interest asymptotically. We illustrate our results using IEEE 39-bus, and 68-bus power systems, and also show the robustness of our method in face of communication failures.}, booktitle={2014 ieee pes general meeting - conference & exposition}, author={Nabavi, S. and Chakrabortty, Aranya}, year={2014} }
 @inproceedings{nabavi_chakrabortty_2013, title={Topology identification for dynamic equivalent models of large power system networks}, DOI={10.1109/acc.2013.6579989}, abstractNote={In this paper we propose two algorithms to identify the equivalent topology of reduced-order models of power systems using measurements of phase angles and frequencies available from Phasor Measurement Units (PMUs). We first show that the topology identification problem can be posed as a parameter estimation problem. Thereafter, we extract the slow oscillatory component of the PMU measurements using subspace identification methods and use them for identifying the topological parameters via conventional ℓ2 minimization approach. Next, we consider the problem of identifying a sparse equivalent network topology using a ℓ1/ℓ2 minimization technique. We illustrate our results using a IEEE 39-bus model.}, booktitle={2013 american control conference (acc)}, author={Nabavi, S. and Chakrabortty, Aranya}, year={2013}, pages={1138–1143} }
 @inproceedings{nabavi_williams_2012, title={A novel cost function for parameters estimation in oscillatory biochemical systems}, DOI={10.1109/secon.2012.6196978}, abstractNote={Oscillatory pathways are among the most important classes of biochemical systems with examples ranging from circadian rhythms and cell cycle maintenance. Mathematical modeling of these highly interconnected biochemical networks is needed to meet numerous objectives such as investigating, predicting and controlling the dynamics of these systems. Identifying the kinetic rate parameters is essential for fully modeling these and other biological processes. These kinetic parameters, however, cannot be measured directly and most of them have to be estimated using parameter fitting techniques. One of the issues with estimating kinetic parameters in oscillatory systems is the irregularities in the Least Square (LS) cost function surface used to estimate these parameters, which is caused by the periodicity of the measurements. These irregularities result in numerous local minima, which limit the performance of even some of the most robust global optimization algorithms. We proposed a cost function to address these issues by integrating temporal information with periodic information embedded in the measurements. This new cost function has better surface properties leading to fewer local minima and better performance of global optimization algorithms. We verified for two oscillatory biochemical systems that our method results in an increased ability to estimate accurate kinetic parameters as compared to the traditional LS cost function. This will eventually lead to biochemical models that are more precise, predictable and controllable.}, booktitle={2012 Proceedings of IEEE Southeastcon}, author={Nabavi, S. and Williams, Cranos}, year={2012} }