Online Long-Term Voltage Stability Margin Estimation for IBR/DER Dominated Power System with Integrated VSM-Aware TSO-DSO Framework
Pith reviewed 2026-05-09 23:14 UTC · model grok-4.3
The pith
A physics-informed machine learning model produces a closed-form voltage stability margin expression that linearizes for real-time TSO optimization in IBR-dominated systems.
A machine-rendered reading of the paper's core claim, the machinery that carries it, and where it could break.
Core claim
An explicit analytical VSM expression is derived from offline T&D co-simulation data using a physics-informed ML-trained model under probabilistic loading and generation mix scenarios, while accounting for unbalanced distribution modeling. The resulting closed-form VSM representation is linearized and embedded into the TSO optimization problem, enabling real-time enforcement of minimum VSM constraints.
What carries the argument
The explicit analytical VSM expression obtained from the physics-informed ML model trained on co-simulation data, which is linearized for embedding into optimization.
If this is right
- The TSO optimization can enforce minimum VSM constraints in real time.
- VSM sensitivities allow prioritization of the most influential reactive power resources in both transmission and distribution optimizations.
- Operational efficiency is enhanced while achieving desired VSM enhancement.
- High estimation accuracy is maintained as validated through simulations on IEEE test systems.
Where Pith is reading between the lines
- This approach might extend to other stability metrics beyond voltage, such as small-signal stability, by similar ML derivations.
- If the model generalizes well, utilities could reduce reliance on conservative offline planning margins in favor of dynamic online constraints.
- Further work could explore how this framework interacts with market operations or uncertainty from renewable forecasting.
Load-bearing premise
The physics-informed ML model trained offline on probabilistic scenarios produces a closed-form VSM expression that stays accurate and generalizes under real-time operating conditions, with linearization not degrading the ability to enforce meaningful constraints.
What would settle it
Running the framework on a real power system and comparing the online VSM estimates against actual stability limits obtained from detailed dynamic simulations under varying IBR output and load conditions; significant mismatches would indicate the expression does not hold.
Figures
read the original abstract
The rapid growth of inverter-based resources (IBRs) and distributed energy resources (DERs) has fundamentally altered the long-term voltage stability characteristics of modern power systems. This article leverages the advantages of machine learning (ML) for the online estimation of long-term voltage stability margin (VSM) and enhancement of VSM through coordinated transmission system operator-distribution system operator (TSO-DSO) optimization. An explicit analytical VSM expression is derived from offline T&D co-simulation data using a physics-informed ML-trained model under probabilistic loading and generation mix scenarios, while accounting for unbalanced distribution modeling. The resulting closed-form VSM representation is linearized and embedded into the TSO optimization problem, enabling real-time enforcement of minimum VSM constraints. We further enhance operational efficiency by incorporating VSM sensitivities into both transmission and distribution optimization, allowing prioritization of the most influential reactive power resources. Simulation studies conducted on the IEEE 30-bus transmission network integrated with multiple IEEE 37-node distribution feeders validate that the proposed framework successfully achieves the desired VSM enhancement while maintaining high estimation accuracy.
Editorial analysis
A structured set of objections, weighed in public.
Referee Report
Summary. The paper claims to derive an explicit closed-form analytical expression for long-term voltage stability margin (VSM) by training a physics-informed ML model on offline T&D co-simulation data generated under probabilistic loading and generation-mix scenarios (including unbalanced distribution modeling). This expression is linearized and embedded as a constraint in a TSO optimization problem within a VSM-aware TSO-DSO coordination framework, enabling real-time enforcement of minimum VSM while using VSM sensitivities to prioritize reactive-power resources. Validation on the IEEE 30-bus transmission system integrated with multiple IEEE 37-node distribution feeders is reported to achieve the desired VSM enhancement together with high estimation accuracy.
Significance. If the ML-derived closed-form VSM expression generalizes reliably beyond the offline training distribution and its linearization preserves meaningful constraint enforcement, the approach could support practical online VSM monitoring and coordinated control in IBR/DER-heavy grids. The explicit incorporation of unbalanced distribution modeling and the use of sensitivities for resource prioritization are constructive elements that could improve operational relevance over purely transmission-focused methods.
major comments (3)
- [Abstract / Simulation studies] Abstract and simulation studies section: the central claim of 'high estimation accuracy' and successful real-time VSM enforcement is not supported by any reported quantitative metrics (e.g., out-of-sample MAE, RMSE, or R² on unseen operating points), training-set size, hyper-parameter details, or linearization-error bounds; without these the generalization and fidelity assumptions remain unverified.
- [VSM expression derivation and optimization embedding] The derivation of the closed-form VSM expression via fitting a physics-informed ML model to offline probabilistic co-simulation data, followed by linearization for the TSO problem, creates a potential circularity: enforcement of the VSM constraint largely reproduces the fitted surface rather than an independent physical prediction. A post-optimization comparison of the linearized VSM value against the full nonlinear model (or time-domain simulation) on the IEEE test cases is required to quantify any degradation.
- [Validation / IEEE 30-bus + 37-node studies] No sensitivity analysis or additional test cases are provided for operating points outside the probabilistic training scenarios (e.g., extreme IBR/DER configurations or contingency conditions), which directly challenges the weakest assumption that the closed-form expression remains accurate under real-time conditions.
minor comments (1)
- [Optimization formulation] Notation for the linearized VSM constraint and the sensitivity coefficients should be introduced with explicit definitions and units to improve readability.
Simulated Author's Rebuttal
We thank the referee for the constructive and detailed review of our manuscript. We address each major comment point by point below, providing clarifications and indicating revisions that will be incorporated to improve the rigor and completeness of the work.
read point-by-point responses
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Referee: [Abstract / Simulation studies] Abstract and simulation studies section: the central claim of 'high estimation accuracy' and successful real-time VSM enforcement is not supported by any reported quantitative metrics (e.g., out-of-sample MAE, RMSE, or R² on unseen operating points), training-set size, hyper-parameter details, or linearization-error bounds; without these the generalization and fidelity assumptions remain unverified.
Authors: We agree that explicit quantitative metrics strengthen the claims. In the revised manuscript we will update the abstract and expand the simulation studies section to report out-of-sample MAE, RMSE, and R² values for the physics-informed ML VSM estimator evaluated on unseen operating points drawn from the probabilistic T&D co-simulation dataset. We will also include the training-set size, hyper-parameter configuration of the ML model, and explicit bounds on the linearization error. These additions will directly support the reported high estimation accuracy and real-time enforcement performance. revision: yes
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Referee: [VSM expression derivation and optimization embedding] The derivation of the closed-form VSM expression via fitting a physics-informed ML model to offline probabilistic co-simulation data, followed by linearization for the TSO problem, creates a potential circularity: enforcement of the VSM constraint largely reproduces the fitted surface rather than an independent physical prediction. A post-optimization comparison of the linearized VSM value against the full nonlinear model (or time-domain simulation) on the IEEE test cases is required to quantify any degradation.
Authors: The physics-informed ML model is trained directly on data generated by the full nonlinear T&D co-simulations, so the resulting closed-form expression is an explicit approximation of the underlying physics rather than an arbitrary fit. To address the concern about degradation, the revised manuscript will include a dedicated post-optimization validation subsection. For the IEEE 30-bus + 37-node test cases we will evaluate the linearized VSM constraint value at the optimized operating point against the full nonlinear model (and time-domain simulation where feasible) and report the observed deviation, thereby quantifying the fidelity of the embedded constraint. revision: yes
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Referee: [Validation / IEEE 30-bus + 37-node studies] No sensitivity analysis or additional test cases are provided for operating points outside the probabilistic training scenarios (e.g., extreme IBR/DER configurations or contingency conditions), which directly challenges the weakest assumption that the closed-form expression remains accurate under real-time conditions.
Authors: We acknowledge that generalization beyond the training distribution is critical for online applicability. The current probabilistic scenarios already span a broad range of loading, generation-mix, and IBR/DER penetration levels. In the revision we will add a sensitivity analysis subsection together with additional test cases that include extreme IBR/DER configurations and selected N-1 contingencies. For these out-of-distribution points we will report the VSM estimation accuracy of the closed-form expression, thereby providing direct evidence of robustness under conditions closer to real-time operation. revision: yes
Circularity Check
No significant circularity; surrogate modeling chain is independent of target result
full rationale
The derivation proceeds from independent offline T&D co-simulations (physics-based) to training a physics-informed ML model, extraction of a closed-form VSM expression, linearization, and embedding as a constraint. This is standard surrogate-based optimization and does not reduce any claimed prediction or enforcement to the fitted inputs by construction. No self-citations, uniqueness theorems, or self-definitional steps are present in the abstract or described chain. Validation on IEEE test cases provides an external check separate from the training data.
Axiom & Free-Parameter Ledger
free parameters (1)
- ML model parameters
axioms (1)
- domain assumption Detailed T&D co-simulation models accurately capture long-term voltage stability behavior under unbalanced conditions and probabilistic scenarios.
Reference graph
Works this paper leans on
-
[1]
EIA, “Annual Energy Outlook 2020,” Jan. 2020. [Online]. Available: https://www.eia.gov/outlooks/aeo/pdf/aeo2020%20full%20report.pdf
work page 2020
-
[2]
Global Market Outlook For Solar Power 2023– 2027,
SolarPower Europe, “Global Market Outlook For Solar Power 2023– 2027,” Jun. 13, 2023. [Online]. Available: https://www.solarpowereurope. org/insights/outlooks/global-market-outlook-for-solar-power-2023-2027/ detail
work page 2023
-
[3]
Australian Energy Market Operator (AEMO), “Quarterly Energy,” Jan. 2024
work page 2024
-
[4]
NERC, “V oltage and Reactive Control.” [Online]. Available: https://www. nerc.com/pa/Stand/Reliability%20Standards/V AR-001-6.pdf
-
[5]
FERC, “FERC Order No. 2222 Explainer: Facilitating Participation in Electricity Markets by Distributed Energy Resources,” 2020
work page 2020
-
[6]
IEEE Standard for Interconnection and Interoperability of Dis- tributed,
IEEE, “IEEE Standard for Interconnection and Interoperability of Dis- tributed,” IEEE Std 1547–2018, 2018
work page 2018
-
[7]
A. K. Bharati and V . Ajjarapu, “Investigation of Relevant Distribution System Representation With DG for V oltage Stability Margin Assess- ment,”IEEE Transactions on Power Systems, vol. 35, no. 3, pp. 2072– 2081, 2020
work page 2072
-
[8]
S. M. H. Rizvi and A. K. Srivastava, “Integrated T&D V oltage Stability Assessment Considering Impact of DERs and Distribution Network Topology,”IEEE Access, vol. 11, pp. 14702–14714, 2023
work page 2023
-
[9]
The continuation power flow: a tool for steady state voltage stability analysis,
V . Ajjarapu and C. Christy, “The continuation power flow: a tool for steady state voltage stability analysis,”IEEE Transactions on Power Systems, vol. 7, no. 1, pp. 416–423, 1992
work page 1992
-
[10]
Long Term V oltage Stability Assessment of an Integrated Transmission Distribution System,
A. Singhal and V . Ajjarapu, “Long Term V oltage Stability Assessment of an Integrated Transmission Distribution System,” inProc. 49th North American Power Symposium, Morgantown, WV , 2017
work page 2017
-
[11]
A. R. R. Matavalam, A. Singhal, and V . Ajjarapu, “Monitoring Long Term V oltage Instability Due to Distribution and Transmission Interaction Using UnbalancedµPMU and PMU Measurements,”IEEE Transactions on Smart Grid, vol. 11, no. 1, pp. 873–883, 2020
work page 2020
-
[12]
Sensitivity Based Thevenin Index With Systematic Inclusion of Reactive Power Limits,
A. R. R. Matavalam and V . Ajjarapu, “Sensitivity Based Thevenin Index With Systematic Inclusion of Reactive Power Limits,”IEEE Transactions on Power Systems, vol. 33, no. 1, pp. 932–942, 2018
work page 2018
-
[13]
T. K. Chattopadhyay, S. Banerjee, and C. K. Chanda, “Impact of distributed generator on voltage stability analysis of distribution networks under critical loading conditions,” inProc. 1st Int. Conf. Non Conven- tional Energy (ICONCE), India, 2014
work page 2014
-
[14]
V oltage stability assessment in power systems using line voltage stability index,
S. Ratra, R. Tiwari, and K. R. Niazi, “V oltage stability assessment in power systems using line voltage stability index,”Comput. Electr. Eng., vol. 70, pp. 199–211, 2018
work page 2018
-
[15]
Static voltage stability analysis of distribution systems based on network-load admittance ratio,
Y . Song, D. J. Hill, and T. Liu, “Static voltage stability analysis of distribution systems based on network-load admittance ratio,”IEEE Transactions on Power Systems, vol. 34, no. 3, pp. 2270–2280, 2019
work page 2019
-
[16]
B. Leonardi, V . Ajjarapu, M. Djukanovic, and P. Zhang, “Application of multi-linear regression models and machine learning techniques for online voltage stability margin estimation,” inProc. 2010 IREP Symp. Bulk Power System Dynamics and Control - VIII (IREP), pp. 1–10, 2010
work page 2010
-
[17]
Adaptive Online Monitoring of V oltage Stability Margin via Local Regression,
S. Li, V . Ajjarapu, and M. Djukanovic, “Adaptive Online Monitoring of V oltage Stability Margin via Local Regression,”IEEE Transactions on Power Systems, vol. 33, no. 1, pp. 701–713, 2018
work page 2018
-
[18]
PMU-Based V oltage Stability Measurement Under Contingency Using ANN,
S. Kumar, B. Tyagi, V . Kumar, and S. Chohan, “PMU-Based V oltage Stability Measurement Under Contingency Using ANN,”IEEE Transac- tions on Instrumentation and Measurement, vol. 71, pp. 1–11, 2022
work page 2022
-
[19]
H.-Y . Su and C.-C. Lai, “Improving Online V oltage Stability Monitoring in Smart Grids: A Physics-Informed Guided Deep Learning Model,”IEEE Transactions on Industry Applications, vol. 61, no. 2, pp. 1–13, 2025
work page 2025
-
[20]
Online Monitoring of V oltage Stability Margin Using an Artificial Neural Network,
D. Q. Zhou, U. D. Annakkage, and A. D. Rajapakse, “Online Monitoring of V oltage Stability Margin Using an Artificial Neural Network,”IEEE Transactions on Power Systems, vol. 25, no. 3, pp. 1566–1574, 2010
work page 2010
-
[21]
M. Pandit and R. Sodhi, “Phasor Measurement Sensor-Assisted Time- to-Collapse Estimation Under Long-Term V oltage Instability of Smart Grids,”IEEE Sensors Journal, vol. 24, no. 5, pp. 6523–6531, 2024
work page 2024
-
[22]
B. Cui and X. A. Sun, “A New V oltage Stability-Constrained Optimal Power-Flow Model: Sufficient Condition, SOCP Representation, and Relaxation,”IEEE Transactions on Power Systems, vol. 33, no. 5, pp. 5092–5102, Sept. 2018, doi: 10.1109/TPWRS.2018.2801286
-
[23]
Y . Song, T. Liu, and Y . Hou, “V oltage Stability Constrained Optimal Power Flow Considering PV–PQ Bus Type Switching: Formulation and Convexification,”IEEE Transactions on Power Systems, vol. 39, no. 2, pp. 3336–3348, Mar. 2024, doi: 10.1109/TPWRS.2023.3313656
-
[24]
An approach for real time voltage stability margin control via reactive power reserve sensitivities,
B. Leonardi and V . Ajjarapu, “An approach for real time voltage stability margin control via reactive power reserve sensitivities,”IEEE Transactions on Power Systems, vol. 28, no. 2, pp. 615–625, May 2013
work page 2013
-
[25]
Coordinated Transmission and Distribution AC Optimal Power Flow,
Z. Li, Q. Guo, H. Sun and J. Wang, “Coordinated Transmission and Distribution AC Optimal Power Flow,”IEEE Transactions on Smart Grid, vol. 9, no. 2, pp. 1228–1240, Mar. 2018
work page 2018
-
[26]
A. O. Rousis, D. Tzelepis, Y . Pipelzadeh, G. Strbac, C. D. Booth and T. C. Green, “Provision of V oltage Ancillary Services Through Enhanced TSO-DSO Interaction and Aggregated Distributed Energy Resources,” IEEE Transactions on Sustainable Energy, vol. 12, no. 2, pp. 897–908, Apr. 2021
work page 2021
-
[27]
Computing Cost Curves of Active Distribution Grids Aggregated Flexibility for TSO-DSO Coordination
F. Capitanescu, “Computing Cost Curves of Active Distribution Grids Aggregated Flexibility for TSO-DSO Coordination”, in IEEE Transac- tions on Power Systems, vol. 39, no. 1, pp. 2381-2384, Jan. 2024
work page 2024
- [28]
-
[29]
Improving V oltage Stability by Reactive Power Reserve Management,
F. Dong, B. H. Chowdhury, M. L. Crow, and L. Acar, “Improving V oltage Stability by Reactive Power Reserve Management,”IEEE Trans- actions on Power Systems, vol. 20, no. 1, 2005
work page 2005
-
[30]
Online voltage stability monitoring using V Ar reserves,
L. Bao, Z. Huang, and W. Xu, “Online voltage stability monitoring using V Ar reserves,”IEEE Transactions on Power Systems, vol. 18, no. 4, 2003
work page 2003
-
[31]
B. Leonardi and V . Ajjarapu, “Investigation of various generator reactive power reserve (GRPR) definitions for online voltage stability/security assessment,” inProc. IEEE Power and Energy Society General Meeting, Pittsburgh, PA, USA, 2008
work page 2008
-
[32]
”IEEE Standard for Interconnection and Interoperability of Inverter- Based Resources (IBRs) Interconnecting with Associated Transmission Electric Power Systems,” in IEEE Std 2800-2022 , vol., no., pp.1-180, 22 April 2022
work page 2022
-
[33]
B. Leonardi and V . Ajjarapu, “An Approach for Real-Time V oltage Stability Margin Control via Reactive Power Reserve Sensitivities,”IEEE Transactions on Power Systems, vol. 28, no. 2, 2013
work page 2013
-
[34]
Deriving DERs V AR- Capability Curve at TSO-DSO Interface to Provide Grid Services,
A. Singhal, A. K. Bharati, and V . Ajjarapu, “Deriving DERs V AR- Capability Curve at TSO-DSO Interface to Provide Grid Services,”IEEE Transactions on Power Systems, vol. 38, no. 2, Mar. 2023
work page 2023
-
[35]
Identification of Repre- sentative Operating Hours for High Renewable Grid Planning,
K. K. Challa, A. K. Bharati, and V . Ajjarapu, “Identification of Repre- sentative Operating Hours for High Renewable Grid Planning,” inProc. IEEE Kansas Power and Energy Conf. (KPEC), Manhattan, KS, USA, 2025
work page 2025
-
[36]
A. K. Bharati and V . Ajjarapu, “SMTD Co-Simulation Framework With HELICS for Future-Grid Analysis and Synthetic Measurement-Data Generation,”IEEE Transactions on Industry Applications, vol. 58, no. 1, pp. 131–141, 2022
work page 2022
-
[37]
Illinois Center for a Smarter Electric Grid (ICSEG), “IEEE 30-Bus Sys- tem.” [Online]. Available: https://icseg.iti.illinois.edu/ieee-30-bus-system/
-
[38]
IEEE, “IEEE PES Test Feeder.” [Online]. Available: https://cmte.ieee. org/pes-testfeeders/resources/
-
[39]
Renewable Electricity Future Scenario Viewer
NREL, “Renewable Electricity Future Scenario Viewer.” Available on: https:https://scenarioviewer.nrel.gov/
-
[40]
Sensitivity- Aware Reactive Power Dispatch of DERs to Support Transmission Grid During Emergency,
A. Alkhonain, A. Singhal, A. K. Bharati, and V . Ajjarapu, “Sensitivity- Aware Reactive Power Dispatch of DERs to Support Transmission Grid During Emergency,” inProc. North American Power Symposium (NAPS), Asheville, NC, USA, 2023
work page 2023
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