Voltage Unbalance Quantification and Mitigation Using a PMU-based Combined Transmission and Distribution System Linear State Estimator

Phasor Measurement Units (PMUs) provide three phase information of GPS time-synchronized voltage, current and breaker status. Hence, a PMU-based linear state estimator (LSE) designed to simultaneously monitor a combined transmission and distribution (C-T&D) system can also estimate system-wide voltage unbalance. Such a framework can localize the source of unbalance to a particular distribution network (DN) and also track its propagation to neighboring DNs or the upstream transmission network (TN). In this paper, we consider this problem and discuss a simple mitigation strategy based on the real-time output of the C-T&D LSE. Using the C-T&D voltage visibility, the proposed method can coordinate among several dispersed voltage control devices to offset the voltage unbalance. Such coordination would be difficult to achieve with the traditional decoupled analysis of a TN and DNs. The case-study uses a modified IEEE 30-bus transmission network with multiple lumped loads replaced by equivalent IEEE 13-node radial distribution feeders.