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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, ISSN: 0378-7796
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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 Oscillations

  , IEEE Transactions on Power Systems

  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,
  , 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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• Journal article
  OMalley M, 2026,
  , Nature Energy, Vol: 11, Pages: 14-15
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• Journal article
  Gao J, Chaudhuri B, Astolfi A, 2026,
  , IEEE Transactions on Control Systems Technology, Vol: 34, Pages: 112-122, ISSN: 1063-6536

  We propose an explicit analytical direct method for the transient stability analysis of multimachine power systems with nonzero transfer conductances (TCs). The proposed method addresses two issues. In the first issue, we study the transient stabilization of the entire power system through excitation control design. To this end, a globally well-defined Lyapunov function is constructed, and a locally well-defined dynamic passivity-based control law is proposed. The closed-loop equilibrium is therefore guaranteed to be locally asymptotically stable. In the second issue, we study the transient stability property of post-fault initial states. To this end, an optimization-based approach to calculate the critical level set of the proposed Lyapunov function is proposed. This allows to estimate an explicit region of attraction of the closed-loop equilibrium. Therefore, the transient stability property of a post-fault initial state can be directly assessed. A case study on the IEEE 10-machine 39-bus power system, to demonstrate the performance and effectiveness of the proposed direct method, is presented.

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• Journal article
  Gu Y, Chaudhuri B, Green T, Xiaoyao Z, Ramachandran Jet al., 2026,
  , IEEE Power and Energy Magazine, Vol: 24, Pages: 39-48, ISSN: 1540-7977

  Increasing shares of inverter-based resources (IBRs) in power grids are triggering complex dynamic interactions and new stability challenges. A particular challenge for system operators is poorly damped sub-synchronous oscillations (SSO) induced by adverse interaction among IBRs through the network. These oscillations are difficult to foresee, threaten system security and often force grid operators to limit the instantaneous share of IBRs. The Control and Power research group at 911½ñÈÕºÚÁÏ are working with the Network Operability team in National Energy System Operator (NESO) in the UK to develop a multi-layered defense strategy to identify and mitigate the risk of poorly damped SSO. Starting from advanced IBR control design which is the genesis of the SSO problem, these layers are: 1) an enhanced IBR connection compliance process capture the risk of SSO more comprehensively, 2) new system strength metrices to identify parts of the grid vulnerable to SSO, 3) characterize operating point dependency of IBRs to detect incipient SSO near real-time and 4) post-event root-cause analysis for targeted and effective mitigation of SSO. Success of this research will enable secure grid operation with high fractions of renewables to facilitate net zero transition.

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• Journal article
  Suski A, Spyrou E, Green R, 2025,
  , IEEE Transactions on Energy Markets, Policy and Regulation, ISSN: 2771-9626

  The ability of deeply decarbonized power systems to ensure adequacy may increasingly depend on long-duration energy storage (LDES). A central challenge is whether capacity markets (CMs), originally designed around thermal generation, can provide efficient investment signals when storage becomes a central participant. While recent studies have advanced methods for accrediting variable renewables and short-duration storage, the effectiveness of these methods in CMs with substantial LDES penetration remains largely unexplored. To address this gap, we extend a two-stage stochastic equilibrium investment model by endogenizing continuous, duration-based capacity accreditation for storage and apply it to a Great Britain-based case using 40 years of weather-driven demand and renewable profiles under varying emission limits. Results show that well-calibrated CMs can sustain near-efficient investment and mitigate revenue volatility, but their effectiveness diminishes in deeply decarbonized systems, underscoring both their potential and the regulatory challenges of supporting large-scale LDES.

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• Conference paper
  Javaid MS, Chaudhuri B, Teng F, Akhtar Zet al., 2025,

  Impact of inner control in GFM-induced sub-synchronous oscillations

  , 2025 IEEE PES General Meeting, Publisher: IEEE

  n power systems dominated by grid-forming inverters (GFMs), strong grid conditions can lead to sub-synchronous oscillations (SSOs) due to the reduced time-scale separation between GFM control loops and network dynamics, potentially causing instability. Modal analysis reveals that states associated with inner control (comprising current and voltage control loops) and network dynamics are the primary contributors to these oscillatory modes. This paper provides an analytical explanation of adverse interactions between GFM inner control and network dynamics that lead to SSOs. Through damping ratio sensitivity analysis, we establish that increasing the current control closed-loop bandwidth and the voltage control proportional gaincan effectively mitigate these oscillations. The proposed control adjustments also prove effective in larger systems with 100% inverter-based resources penetration, comprising of a mix of both grid-following and grid-forming inverters.

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