Energization of Transformers in Medium Voltage Island Microgrids by Leveraging Grid Forming Inverter Control

Microgrids with high penetration of photovoltaic (PV) resources are becoming more common in the energy infrastructure landscape including islanded microgrids occurring in rural areas where establishing new electric services are prohibitively expensive. In the case of islanded microgrids, the lack of cost effective, long duration storage options requires the integration of conventional fossil fuel based generation with these renewable resources for operation when sufficient irradiance is not available. This creates operational challenges as was the case for a midstream gas compressing, medium voltage, islanded microgrid recently installed and commissioned in the southwestern United States. Energization of 10MVA of PV plant and battery energy storage system (BESS) step up transformers created the risk of nuisance tripping of synchronous generator protection systems. As a result, this paper proposes to operate the BESS inverter in grid forming mode to utilize a soft-starting and synchronization procedure to avoid this scenario. An investigation is carried out to demonstrate modeling of the BESS and test the proposed solution during energization of the transformers and subsequent synchronization to a synchronous generator utilizing RTDS and controller-hardware-in-the loop testing of generating set controller hardware. The results of the investigation illustrate that strategic placement of the BESS and use of grid forming controls enable the energization of the PV plants while eliminating inrush currents and voltage sags characteristic of using synchronous generators. Furthermore, the proposed solution is demonstrated to be robust to the selection of inverter control parameters and transformer saturation characteristics.