The Impact of Three-phase Impedances on the Stability of DER systems

In this work we explore impedance-based interactions that arise when inverter-connected distributed energy resources (DERs) inject real and reactive power to regulate voltage and power flows on three-phase unbalanced distribution grids. We consider two inverter control frameworks that compute power setpoints: a mix of volt/var and volt/watt control, and phasor-based control. On a two-bus network we isolate how line length, R/X ratio, and mutual impedances each affects the stability of the DER system. We validate our analysis through simulation of the two-bus network, and validate that the effects found extend to the IEEE 123-node feeder. Furthermore, we find that the impedance properties make it more challenging to design a stable DER system for certain placements of DER on this feeder.