  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.
