On the Sizing and Benefits of Grounding Transformers with Distribution-Connected Inverters

Distribution-connected inverter-based distributed energy resources (DERs) plants can under certain circumstances contribute to transient overvoltages. System operators often require the use of grounding transformers with such plants as an overvoltage mitigation strategy. In this paper, a set of grounding transformer design relationships is derived from the sequence network representation of a distribution circuit hosting a DER with a grounding transformer. The key findings are that a) solutions to the equations only exist for certain loading conditions and transient voltage levels; and b) even then, the grounding transformer may not provide full mitigation of the overvoltage condition. Ultimately, fast overvoltage relaying is a better solution.