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• Journal article
  Van Caelenberg G, Stratigakos A, Spyrou E, 2027,

  Constructing deployment scenarios for reserve deliverability via adaptive robust optimization

  , Electric Power Systems Research, Vol: 263, ISSN: 0378-7796

  Network congestion often hinders the deployment of reserves needed to balance forecast errors during real-time operations. A pertinent idea to tackle this challenge involves adding deployment scenarios of spatial distributions of forecast errors as contingencies to the day-ahead problem. However, current approaches disregard the effect of grid characteristics and the day-ahead schedule on the induced congestion and, consequently, reserve deliverability. In this work, we formulate a two-stage adaptive robust optimization problem to jointly consider interactions between day-ahead and real-time operations and forecast errors. Using a column-and-constraint algorithm, we iteratively construct deployment scenarios by finding the worst-case forecast error for reserve deliverability. Simulations on the RTS-GMLC system show that adding these scenarios to the day-ahead problem significantly reduces the frequency of congestion-driven reserve undeliverability. Notably, the choice and number of scenarios dynamically adapts to the day-ahead schedule.

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• Journal article
  Gallegos P, Spyrou E, Sauma E, 2027,

  Comparative analysis of reserve formulations: leveraging flexibility from energy-limited resources

  , Electric Power Systems Research, Vol: 263, ISSN: 0378-7796

  As power systems integrate higher shares of variable renewable energy, balancing supply and demand across different timescales becomes challenging. Historically, reserves procured per time interval have helped power system operators maintain that balance. Per-interval procurement can compromise reliability when reserves are sourced from energy-limited resources. This article compares, for the first time, two solutions: (a) a set of constraints, termed envelopes, which uses a per-interval estimate of reserve energy intensity and calculates the cumulative impact of reserve activation on stored energy as the sum of per-interval impacts and (b) a product existing in the literature, called energy reserves, which procures reserves for periods, with durations ranging from a single interval to the model horizon. For the IEEE RTS-GMLC 2019 system, the latter approach, at an increased computational burden, produces more cost-effective day-ahead schedules by allowing resources, including storage, to determine the energy intensity of the reserves provided.

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• Conference paper
  Gupta A, Chaudhuri B, O'Malley M, 2026,

  Equivalent circuit modeling of grid-forming inverters in (sub)-transient time-frame

  , Power Systems Computation Conference (PSCC), Publisher: Elsevier, Pages: 1-9, ISSN: 0378-7796

  The widely accepted definition of grid-forming (GFM) inverter states that it should behave as a (nearly) constant voltage source behind an impedance by maintaining a (nearly) constant internal voltage phasor in the sub-transient to transient time frame. Some system operators further mandate permissible ranges for this effective impedance. However, these specifications do not clearly define the location of the internal voltage source, and no systematic method exists to quantify its effective impedance for a black-box GFM model. To address this, we first compare the transient responses of an ideal voltage source and a GFM to show that an idealistic GFM maintains a (nearly) constant voltage across the filter capacitor, rather than at the inverter switches. Then we propose a systematic method to quantify the effective impedance of a GFM from its black-box model using frequency-domain admittance plots. Using standard PSCAD GFM models developed by NLR (formerly NREL), we demonstrate that the GFM’s equivalent impedance model captures the sub-transient response and static voltage stability limit accurately. Further, replacing the GFM with the proposed equivalent circuit model in the modified IEEE-39 bus system is shown to reproduce the small-signal stability characteristics with reasonable accuracy.

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• Journal article
  Ochoa Abett De La Torre T, Chaudhuri B, O'Malley M, 2026,

  Optimal control for robust dynamic performance in inverter-dominated power systems

  , IEEE Transactions on Power Systems, Pages: 1-14, ISSN: 0885-8950

  This paper develops a novel optimization framework for synthesizing inverter-based resource (IBR) controllers to enhance robust large-signal dynamic performance in inverter-dominated power systems (IDPSs) via coordinated selection of control architectures and parameter tuning. Despite extensive research on grid-forming and grid-following control architectures, systematic control design methods that explicitly account for large-signal dynamics and robustness across a range of operating conditions and disturbances remain limited. This paper addresses this gap by formulating a robust optimal control problem that integrates control design, dispatch, disturbance, and dynamic models within a unified min-max optimization framework. To solve the resulting problem, a solution method combining direct collocation, mesh refinement, local reduction, nonlinear programming, and electromagnetic-transient simulations is proposed, fundamentally differing from conventional power system optimization approaches. Application to the IEEE 9-bus system demonstrates that the method captures key large-signal effects with high fidelity, including nonlinear transients arising from current saturation. Moreover, results show improved robust dynamic performance relative to conventional control design practices and reveal the strong sensitivity of system behavior to control architecture allocation and parameter tuning, underscoring control design optimization as an effective mechanism for enhancing power system security, resilience, and stability.

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• Journal article
  Yuan H, Chaudhuri B, Tan SC, Hui SYet al., 2026,

  A robust synchronization method for enhancing stability of IBR-dominated grids

  , IEEE Transactions on Power Systems, ISSN: 0885-8950

  Inverter-based resources (IBRs) operating in grid following (GFL) mode are particularly vulnerable to instabilities in weak grid conditions. This is largely due to the inherent limitations of synchronization with a phase locked loop (PLL). This paper presents a novel voltage magnitude-based synchronization (VMSync) method that ensures stable and well damped response even in very weak grid conditions, without compromising dynamic performance or adding any stabilization control components. Through comprehensive frequency- and time-domain analysis, VMSync is shown to outperform conventional synchronous reference frame PLL (SRF-PLL) for both small- and large-signal disturbances. Systems with multiple GFL inverters employing VMSync demonstrate significantly improved damping and stability. Furthermore, mixed deployment of VMSync alongside legacy SRF-PLL-based GFL inverters exhibits superior dynamic performance compared to deployment of solely SRF-PLL-based GFL inverters. The results highlight the potential of VMSync as a robust synchronization strategy to mitigate PLL-driven instabilities in power grids with high IBR penetration.

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• Conference paper
  Suski A, Spyrou E, Mays J, Green Ret al., 2026,

  Comparing Contract-Based Support Mechanisms for Long-Duration Energy Storage

  , European Energy Markets
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• Journal article
  Gao J, Javaid MS, Bhattacharjee D, Chen Y, Chaudhuri Bet al., 2026,

  In situ estimation of IBR models for analyzing sub-synchronous ocscillations

  , IEEE Transactions on Power Systems, ISSN: 0885-8950

  The opaqueness of vendor-specific black-box modelsof inverter-based resources (IBRs) is a barrier to the systematic analysis of IBR-induced sub-synchronous oscillations (SSO). Existing approaches via impedance scan require isolating each IBR from the power system and interfacing it with a voltage or current source. However, this procedure is not feasible when the IBR device is embedded within the rest of the system (RoS). In this paper, we propose an augmented eigensystem realizationalgorithm (ERA)-based method to estimate the IBR model in situ, i.e., while the IBR remains connected to RoS. The core idea is to disentangle the IBR’s impedance spectra from its closed-loop voltage and current responses to a pulse probing. Notably, the augmented ERA enables automated model order selection, ensuring scalability for realistic IBR-rich zones of a power grid. The estimated linearized IBR models are then combined with the known RoS model, which yields a linearized state-space model of the overall power system. For validation, the case studies demonstrate the applicability and scalability of the proposed method. The frequency-domain and time-domain responses of the estimated IBR models closely match those of the actual models. Modal analysis accurately identifies the IBR with dominant contribution to poorly damped SSO, therefore enabling targeted and effective mitigation.

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• Journal article
  Spyrou E, Suski A, Green R, 2026,

  Designing a policy mechanism for long-duration energy storage: the British experience

  , Current Sustainable/Renewable Energy Reports, Vol: 13, ISSN: 2196-3010

  Purpose of reviewThe UK parliament recently introduced a cap-and-floor mechanism for net revenues of Long-Duration Energy Storage in Great Britain (GB). The article summarizes learnings from the UK proceedings around four questions: (a) What drives the need for LDES? (b) What are the barriers to LDES deployment in GB? (c) Which options could mitigate these barriers? (d) What are the key design choices for the cap-and-floor mechanism?Recent findingsGB evidence indicates that (1) energy shifting over long timeframes and security-of-supply benefits drive LDES needs; (2) revenue uncertainty is the primary barrier; and (3) a cap-and-floor mechanism could mitigate investment risk.SummaryEvidence on LDES needs is extensive, whereas analysis of barriers, their impact on investment, and the effects of alternative policies and cap-and-floor designs remains limited. Further research could address this gap and contribute insights into the interplay of long-term contracts and short-term markets in hybrid electricity markets for deeply decarbonized power systems.

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• Conference paper
  Javaid MS, Covarrubias Maureira G, Gupta A, Bhattacharjee D, Gao J, Chaudhuri B, O'Malley Met al., 2026,

  Spatial characterization of sub-synchronous oscillations using black-box IBR models

  , IEEE Power and Energy Society General Meeting (PESGM) 2026, Publisher: IEEE

  Power systems with high penetration of inverter-basedresources (IBRs) are prone to sub-synchronous oscillations (SSO). The opaqueness of vendor-specific IBR models limits the ability to predict the severity and the spread of SSO. This paper demonstrates that black-box IBR models estimated through frequency-domain identification techniques, along with dynamic network model can replicate the actual oscillatory behavior. The estimated IBR models are validated against actual IBR models in a closed-loop multi-IBR test system through modal analysis by comparing closed-loop eigenvalues, and participation factors. Furthermore, using output-observable right eigenvectors, spatial heatmaps are developed to visualize the spread and severity of dominant SSO modes. The case studies on the 11-bus and 39-bus test systems confirm that even with the estimated IBR models, the regions susceptible to SSO can be identified in IBR-dominated power systems.

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• Conference paper
  Chen Y, Bhattacharjee D, Chaudhuri B, O'Malley M, Qin N, Expethit APet al., 2026,

  Data-driven post-event analysis with real-world oscillation data from Denmark

  , IEEE Power and Energy Society General Meeting (PESGM) 2026, Publisher: IEEE

  This paper demonstrates how Extended Dynamic Mode Decomposition (EDMD), grounded in Koopman operator theory, can effectively identify the main contributor(s) to oscillations in power grids. We use PMU data recorded from a real 0.16 Hz oscillation event in Denmark for post-event analysis. To this end, the EDMD algorithm processed only voltage and current phasors from nineteen PMUs at different voltage levels across the Danish grid. In such a blind-test setting with no supplementary system information, EDMD accurately pinpointed the location of the main contributor to the 0.16 Hz oscillation. Energinet later confirmed that this was a 145 MW solar photovoltaic (PV) park with control system issues. Notably, conventional approaches, such as the dissipating energy flow (DEF) method used in the ISO-NE OSLp tool did not correctly identify this plant. This joint validation with Energinet, reinforcing earlier studies using simulated IBR-dominated systems and real PMU data from ISO-NE, highlights the potential of EDMD-based post-event analysis for identifying major oscillation contributors and enabling targeted SSO mitigation.

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