Short-circuit overvoltage induced by gridfollowing inverters and its mitigation via optimal neutral ground impedance selection for inverter connection transformers

As more high-capacity distributed energy resources are added to the grid, their contributions to shortcircuit current (SCC) and short-circuit overvoltage (SCOV) can no longer be ignored due to their significant impact on fault levels and post-fault SCOV transients. Therefore, the aim of this study is to present a method to mitigate increases in SCC and SCOV. To this end, the study first introduces a method for calculating SCOV in an unbalanced grid containing an unbalanced power electronic-based distributed energy resource (PDR), commonly referred to as grid-forming and grid-following inverters. Specifically, this study decomposes the unbalanced network into sequence networks corresponding to four typical fault types, defines the unbalanced current injection matrix, and calculates the SCOV. Next, the study investigates the effect of PDRs on variations in SCC and SCOV. The proposed method is then used to determine the optimal NGI value that mitigates the increase in SCOV and the decrease in SCC caused by high-capacity unbalanced PDRs following a ground fault. As a result, this study recommends grounding the three-phase transformer with an optimally selected NGI to effectively mitigate SCOV.