Droop-based co-ordination of grid-forming and grid-following inverters ensuring stability in a microgrid

This paper presents the droop-based co-ordination for active and reactive power-sharing between grid-forming (GFM) and grid-following (GFL) inverters. A 12 MVA inverter-based microgrid with two photovoltaic (PV) systems, 1 km apart is simulated to analyze the impact of different inverter control strategies on power-sharing and microgrid’s frequency response. Three simulation cases are studied for different combinations of GFM and GFL inverters, keeping the total PV headroom same (4 MVA) for regulation in all cases. Two grid-supporting GFM units (2 MVA headroom on each unit) in parallel operation results in 61% RoCoF reduction and 0.54 Hz less frequency nadir as compared to the case when one grid-supporting GFM operates in parallel with a grid-supporting GFL unit. When a GFL unit is operating together with a GFM unit, the choice of droop coefficients, inverter control mode and headroom capacity allocation plays a crucial role in overall frequency response. A grid-supporting GFM unit (4 MVA headroom capacity) in parallel with a grid-feeding GFL unit provides comparatively improved damping and frequency response with 39% reduction in RoCoF and 0.42 Hz improvement in the frequency nadir as compared to the case when both GFM and GFL units in parallel regulate the frequency.