Simultaneous Overvoltage and Overcurrent Mitigation of Grid-Forming Inverters under A Single-Line-Ground Fault

Single-line-ground fault, which happens at the delta terminal of a $\mathrm{Y}g\Delta$ transformer or the ungrounded wye terminal of a $\mathrm{Y}g\mathrm{Y}$ transformer in a grid-forming inverter system will cause severe overcurrent and overvoltage simultaneously. However, they are rarely investigated together and mitigated through a control strategy at the same time. In this paper, phase voltages at the point of common coupling (PCC) and inverter output currents during the fault are firstly calculated based on the sequence network of the system. Subsequently, to ride through the fault, the hybrid mitigation strategy based on the virtual negative-sequence and positive-sequence impedance is proposed. The virtual negative-sequence impedance, realized through current feedback control, can not only reduce the overvoltage at healthy phases slightly and equalize them but also reduce inverter fault currents significantly. Besides, its weak overvoltage and strong overcurrent limiting abilities are also analyzed with varying grid short-circuit ratios and fault impedances. To limit the overvoltage, the virtual positive-sequence impedance can be increased during the fault in each control time step until the maximum phase voltage at the PCC is lower than the fault ride-through requirement. Consequently, the proposed mitigation strategy is verified by real-time simulations.