Assessing the Impact of Increasing Non-Synchronous Generation on Frequency Stability in the Indian Power System using Measurement-Based Inertia Estimation

This paper presents a measurement-based estimation of system inertia for the Indian power system using real disturbance data recorded between January 2022 and September 2024. Highresolution PMU frequency measurements (40 ms) from electrically strong and geographically dispersed buses are combined to form a Centre of Inertia (COI) frequency, representing bulk system dynamics. Disturbances involving generation or load loss exceeding 1000 MW or 0.1

Hz are analysed, with data obtained from NLDC and RLDC SCADA systems. Inertia is estimated using the swing equation, assuming dominance of inertial response immediately after the event, constant mechanical power, and negligible governor and load-damping effects over the short analysis window. The Rate of Change of Frequency (RoCoF), a key input to inertia estimation, is computed using two methodologies:

(i) Rolling window techniques with window lengths of 80 ms, 120 ms, 200 ms, 480 ms and 520 ms, and

(ii) a fifth-order polynomial curve-fitting method, from which RoCoF is obtained by analytical differentiation.

These approaches are applied over the interval from the onset of frequency deviation to the nadir (or zenith), capturing the dominant inertial response. Across all methods, the results show a clear and consistent decline in system inertia from 2022 to 2024, coinciding with increasing renewable energy penetration. Using the 80 ms window, average inertia reduced from 3.85 s to 1.96 s, while the 200 ms window showed a decline from 5.31 s to 3.03 s. Longer windows (480– 520 ms) and polynomial fitting produced higher absolute inertia values but preserved the same downward trend, confirming that the reduction is physical and not a methodological artefact.

Inertia was found to be systematically lower during solar hours than non-solar hours due to reduced synchronous generation.

The paper also analyses frequency nadir and time-to-nadir. A near-linear relationship is observed between disturbance size and maximum frequency deviation, yielding a power number of approximately 10,000 MW/Hz for the Indian grid. Larger disturbances take longer to reach the nadir, reflecting the combined influence of inertia, load damping and primary response.

Overall, the study demonstrates that India’s grid is entering a structurally low-inertia regime, particularly during high-solar periods. The results underline the need for standardized RoCoF methodologies, real-time inertia monitoring, fast-frequency response resources, and minimum inertia requirements to ensure secure operation of a high-renewable power system.