Criteria-Aware EMT-Based Short-Term Voltage Performance Index for Dynamic Assessment of Inverter-Dominated Power Systems

Mohammad Almomani, Venkataramana Ajjarapu

The increasing penetration of inverter-based resources (IBRs) into bulk power systems has fundamentally altered short-term voltage dynamics following disturbances. Conventional short-circuit capacity (SCC) metrics provide a useful screening indicator of grid strength but are unable to fully capture post-disturbance voltage behavior at buses with dynamic loads, converter controls, or protection interactions. A bus with high SCC may still experience deep voltage dips, delayed recovery, or transient overvoltage that violates operating criteria. This paper proposes the Short-Term Voltage Performance Index (STVPI), an electromagnetic-transient (EMT)-based, criteria-aware metric that quantifies the quality of the post-disturbance voltage waveform relative to user-defined performance limits. STVPI processes voltage signals at the half-cycle level by computing a weighted log-amplitude ratio between the actual waveform and an ideal half-sine reference. Monotonic recovery envelopes on the overvoltage and undervoltage sides are compared against half-normal reference distributions using Kullback--Leibler (KL) divergence, normalized by the KL divergence of the critical voltage envelope, yielding two directional indices -- $\mathrm{STVPI}^{+}$ and $\mathrm{STVPI}^{-}$ -- whose combination produces a baseline-corrected scalar severity score. Bus-level and event-level aggregation derive BSTVPI and ESTVPI, enabling simultaneous identification of dynamically weak buses and critical fault contingencies. The framework is validated on the IEEE 9-bus and 39-bus test systems with IBR integration.