A Study of DER Volt and Var Droop Aggregation for Reactive Power Support to Transmission System

Rapid growth of distributed energy resources (DERs) is shifting the center of gravity of power supply towards the distribution system. DERs can provide real and reactive power support to the transmission system through aggregation as virtual power plant (VPP). The challenge is how to control the individual DERs to realize the desired volt and var droop response at the grid supply point (GSP). This paper analyzes the aggregation behavior of droop-controlled DERs on a study system designed to capture the essential attributes of the problem being studied. The simulation result shows that the aggregate droop curve deviates significantly from the ideal droop reference by more than 25% on average and by 230% in the worst case. The deviation tends to deteriorate in heavy loading conditions, and the load model effect on the deviation is found to be less than 1% on average. Notably, the study demonstrates that a given target piece-wise linear droop curve can best be approximated by tuning the individual piece-wise linear droop curves. These conclusions are meaningful in defining GSP performance indices and control targets in the context of DER-dominated VPP.