Grid-Forming Inverters in a Microgrid: Maintaining Power During an Outage and Restoring Connection to the Utility Grid Without Communication

This article presents an autonomous control architecture for grid-interactive inverters, focusing on the inverters providing power in a microgrid during utility outages. In scenarios where the microgrid becomes isolated from the utility grid, achieving load sharing among inverters, typically via droop controls, and restoring frequency and voltage amplitude become essential tasks. When the microgrid is ready to reconnect with the utility grid, a synchrocheck relay verifies synchronization, assuming the inverters have already restored voltage and frequency to the nominal levels. Before reconnection, an automatic tap-changer transformer can correct minor voltage discrepancies on both sides of the relay (circuit-breaker). This article introduces a novel method that enables inverters to autonomously detect utility grid reconnection without relying on communication, transferring from grid-forming (GFM) to grid-following (GFL) mode. The control architecture employs a direct controller for inverters, implementing a control-sync technique featuring two parallel control paths with minimal control loops, facilitating seamless transitions between GFM and GFL modes. To validate the effectiveness of this control architecture, experiments were conducted using a 30-kVA grid emulator and three 5-10 kVA, 208-V three-phase inverters. The results offer valuable insights into the practical implementation and demonstrate performance of the proposed controller.