Blackstart of Unbalanced Microgrids Using Grid-Forming Inverter with Voltage Balancing Capability

Abstract

This paper analyzes a blackstart operation of an unbalanced microgrid using a grid-forming inverter. The inverter sets the magnitude and frequency by generating its own frequency reference. It controls the positive- and negative-sequence voltages separately to mitigate voltage unbalance at its terminals. The current limiter of the inverter operates in the abc-frame to regulate the unbalanced current effectively. The performance of the grid-forming inverter is evaluated by blackstarting the islanded microgrid with a motor load. The inverter aims to achieve the following objectives during a blackstart process: 1) control the grid voltage at the rated value, 2) limit the inverter current during motor starting, and 3) maintain balanced voltage. The microgrid and grid-forming inverter are modeled and simulated in the PSCAD/EMTDC environment. The simulation results show that the grid-forming inverter is able to blackstart an islanded microgrid and maintain balanced voltage while supplying unbalanced loads. It is also shown that the inverter could start a motor while limiting the associated inrush current.