Sub-Transmission Ground Fault Overvoltage Management Under IBR Reverse Flow

The rise of inverter-based distributed energy resources (IBRs) is reshaping global power system planning and operation. Unlike traditional rotating machines, IBRs lack physical characteristics and rely on control algorithms to support the grid during abnormal conditions. This is especially evident in mass-adopted distributed sources like rooftop residential-scale Distributed Energy Resources (DER), which are controlled to behave as constant current sources. Numerous jurisdictions already experience steady-state reverse flows at substations during periods of minimum load and maximum generation. However, the aggregation of IBRs exacerbates ground fault overvoltages (GFOV) at the transmission level, as they appear as open circuits in the zero and/or negative sequence networks. This paper analyzes the issue through electromagnetic transient (EMT) simulations, yielding chart-based screening guidelines. These guidelines can assist utilities in evaluating necessary retrofits for substations as more IBRs are adopted, particularly for small-scale IBRs that are interconnected with little to no rigorous analysis.