Field Verification of Grid-Forming Inverter-Based Power Plant Performance for Transmission System Reliability

This report describes some examples of the operation of two grid-forming (GFM) inverter-based photovoltaic-battery hybrid power plants connected to the electric system of the island of Kauai in Hawaii. Both plants are photovoltaic power plants with battery energy storage and are connected to the networked transmission system operated by Kauai Island Utility Cooperative (KIUC), along with several other power plants including an oil-fired plant, diesel generators, small hydroelectric plants, a biofuel plant, and several other photovoltaic plants. While many studies have examined the potential benefits of GFM operation to enhance grid stability in regions with high levels of inverter-based resources (IBRs), nearly all grid-connected IBRs have historically been operated using grid-following (GFL) controls. At the time of the events described in this report, the two GFM plants in Kauai were the only known IBR plants in the U.S. that operated in grid-forming mode while in parallel with a transmission system. A primary goal of this report is to demonstrate the feasibility, potential benefits, and challenges of operating IBR plants in GFM mode. In particular, this report describes two major grid stability events that, along with related studies by the authors and others, indicate significant benefits of GFM operation. This report also compares the performance of the two GFM plants to a specification for GFM IBRs developed by the UNIFI Consortium. The benefits described here are enabled in part because the two plants in question include both PV and battery energy storage (BESS). For two of the PV-BESS hybrid facilities described in this report, the solar inverters operated in GFL mode, while the battery inverters operated in GFM mode, known as AC-coupling. The other hybrid facilities use DC-coupled inverters that deliver both PV and BESS energy to the grid through the same power electronic device. Battery inverters that operate in GFM mode while grid-connected are now available from several vendors for utility-scale plants. In addition, some vendors offer GFM inverters that accept DC-coupled PV and battery inputs simultaneously. However, while GFM operation of PV inverters is feasible, GFM-capable inverters for utility-scale PV-only applications are not widely available currently.