Sub-Transmission Ground Fault Overvoltage due to Distribution-Connected IBR Reverse Flow

Abstract-The proliferation of inverter-based distributed energy resources has been driving paradigm changes in the planning and operation of power systems around the globe. Lacking the physical characteristics of traditional rotating machines, IBRs rely on their control algorithms to support the grid when abnormal conditions, such as a system fault, occur. This is especially true in the case of mass adopted distributed sources such as in the case of aggregations of rooftop residential scale DER. These are not manufactured with sophisticated algorithms and behave as constant current sources. Many jurisdictions are experiencing steady-state reverse flows at substations during sunlight hours, where minimum load and maximum generation coincide. This aggregation of IBRs cannot provide zero or negative sequence currents to ground faults and aggravate ground fault overvoltages experienced at the transmission level for most substation configurations. This paper provides analysis and insight on the problem using EMT simulations. Thousands of EMT simulations are used to create chart-based screening guidelines that demonstrate the impact of these IBRs and drive recommendations for arrester selection and voltage-based protective relays. With this contribution, utilities can use the screening charts to assess the extent of retrofits required in their substations as more IBRs are adopted by its customers utilizing available system parameters. This is particularly useful for cases of small-scale IBR which interconnect without extensive analysis.