Islanding and Resynchronization Process of a Grid-Connected Microgrid with Series Transformerless H-Bridge Inverter Installed at PCC

This paper discusses the disconnection and reconnection process of a microgrid and series transformerless H-bridge inverter installed at PCC between the utility and the microgrid. When a short circuit event occurs at the grid side, the transformerless H-bridge inverter should be turned off or bypassed depending on the fault location and the microgrid should disconnect itself to work in island mode. After fault clearance, the H-bridge inverter should be inserted smoothly into the network and the microgrid should be synchronized with the main grid before closing the breaker. Therefore, a modified island control strategy incorporating synchronization controller, and proper coordination with flip flops are implemented in order to satisfy these conditions during disconnection and reconnection process. The transformerless H-bridge inverter operation during fault and its smooth reinsertion after fault clearance have also been clarified.