Two-level centralized and local voltage control in distribution systems mitigating effects of highly intermittent renewable generation

Abstract

This paper describes a method for voltage control towards optimal power flow (OPF) operation of radial distribution systems through reactive power (Q) control of distributed energy resources (DERs). A two-level control system consisting of an upper-level centralized controller and lower-level local controllers is proposed. First, a stable local control scheme and a criteria for its voltage stability via a discrete-time state-space model are presented. Second, the local control scheme is combined with a centralized AC OPF optimization to minimize delivery losses in the distribution system while maintaining bus voltages at optimal levels. The control system is modeled and simulated in Matlab/OpenDSS considering real world input data. In a simulation study of the IEEE 123-bus test system with historical load and solar generation data for a 24-h period, the proposed method is compared to other Q modes of DERs, specifically the volt-var droop control mode and a centralized OPF control mode. Furthermore, the control system is used in a scenario with an on-load tap changer and in unbalanced loading conditions. Overall, the proposed control method enhances existing local controllers with faster convergence and reduced voltage variations while ensuring stability, particularly in cases with highly intermittent renewable generation. In addition, it can reduce system voltage unbalance and transformer tap changer operations.