Improved secondary control for optimal unbalance compensation in islanded microgrids with parallel DGs

This paper proposes a two-layer hierarchical control structure to realize compensation of voltage unbalances optimally in different buses of islanded microgrids. The primary layer controls the microgrid voltage and frequency. The secondary layer is used to realize the unbalance compensation of the sensitive load bus (SLB). Improvement of the voltage unbalance factor (VUF) at the SLB may lead to an increase in VUF at local buses and/or DG terminals. A complementary part is designed and added to the secondary control in order to tune the compensation portion of each DG source while limitations of VUF at DG terminals and local buses are considered too. This method realizes multi-power-quality-level control through a simple yet effective solution. Simulation results are given to demonstrate the advantages of the proposed control scheme.