Simultaneous Overvoltage and Overcurrent Mitigation Strategy of Grid-Forming Inverters Under a Single-Line-To-Ground Fault

A single-line-to-ground (SLG) fault occurs at the ungrounded side of a transformer (e.g., wye-grounded-delta, wye-grounded-wye, or wye-grounded-wye-grounded through a large impedance transformer) in a grid-forming inverter will cause severe physical overvoltage and inverter output overcurrent simultaneously. Utility distribution grid has been facing increasing challenges related to this in recent years. However, this issue has not been well investigated and addressed by state-of-the-art control schemes. To fill this important gap, this article first proposes a negative-sequence voltage compensation method to equalize magnitudes of healthy phase voltages under SLG faults, which helps limit the healthy phase voltages concurrently. Subsequently, a virtual impedance with a simultaneous overvoltage and overcurrent limiting capability is proposed to atomically mitigate the simultaneous overvoltage and overcurrent. Additionally, the well-known current limiting factor in the inner current control loop with a high bandwidth is adopted to limit the transient overcurrent. Simulation and experimental results reveal that the simultaneous overvoltage and overcurrent issues under SLG faults can be addressed by the proposed control strategies. Moreover, the effectiveness of the current limiting under other fault types with the proposed strategy is also confirmed.