Voltage Balancing of Grid-Forming Inverters in Unbalanced, Islanded Microgrids

In microgrids, unbalanced loads (single or multi-phase) can create voltage imbalances. In grid-connected mode, the utility grid helps balance the microgrid load voltages. However, in an islanded mode of operation of unbalanced microgrids without a synchronous machine, a three-leg grid-forming (GFM) inverter with only a positive-sequence controller cannot maintain balanced load voltages. But, the presence of certain transformer configurations can improve the voltage-balancing capability of GFMs. An in-depth analysis of sequence networks is essential to understand an unbalanced power system. This paper explains the coupling of sequence networks in an unbalanced system and estimates the sequence currents required to balance the voltage at the load terminals. Simulating simplified test systems, this paper demonstrates a grid-forming inverter with decoupled controllers and a grounding transformer together in an islanded microgrid mimic the voltage balancing capability of a grid-connected microgrid, resulting in negligible unbalanced voltage.