Analysis of battery-based virtual inertia & primary frequency response on improving frequency dynamics in an island hydro-diesel-PV ac-grid

The displacement of conventional generation in an island power system by inertia-less generation such as PV, reduces the adequacy of the frequency response countermeasures provided by the remaining generation units. A chemical battery energy storage system (BESS) providing compensation techniques such as virtual inertia response (VIR) and droop-based primary frequency response (PFR) can supply the corrective power needed to remedy the increased frequency volatility. This study explores the frequency volatility in a 300 MW hydro-diesel ac-island grid for increasing levels of PV up to a target of 100 MW, and the corrective impact of BESS-based VIR and PFR. The analysis conducted in MATLAB-Simulink® showed that the combined governor response of both hydro and diesel units where fast enough to compensate the aggregated PV plant ramps, and therefore no BESS support would be required for such power imbalances. Analysis of the frequency volatility to the event of losing a large hydro unit, showed that the rate of change of frequency (ROCOF) would change approximately 0.043 Hz/s for every unit change in the systems rotational inertia, increasing from 0.48 Hz/s and exceeding a target ROCOF of 1 Hz/s. The final ROCOF was 1.24 Hz/s at the 100 MW PV level. In this case, 6.52 MW BESS-based VIR and PFR power was needed to correct the frequency response to a desired target ROCOF of 1 Hz/s and nadir of above the under-frequency load shed (UFLS) limit of 58.6 Hz. In comparison, running 21 MW additional diesel units reduced the ROCOF to under 1 Hz/s, but could not maintain the nadir above the UFLS.