Distributed Cooperative Secondary Voltage Control in an Unbalanced Microgrid

The negative-sequence reactive power-conductance (Q-G) droop control strategy is a conventional method to compensate for the output unbalanced voltages in an islanded MG. Nevertheless, the conflict between unbalanced voltage compen-sation and negative-sequence reactive power sharing, caused by the impedance mismatching of distribution lines and distributed generators, has not been solved only by Q-G droop control. In this paper, a distributed cooperative secondary unbalanced voltage control strategy is proposed to decrease the output voltage unbalance factor (VUF) of each droop-controlled DG, as well as to further enhance the negative-sequence reactive power sharing effectiveness among DGs by properly shifting up and down the Q-G droop characteristics of each DG. An algorithm for adaptive VUF weight coefficient is proposed to better suppress VUF under severe imbalance conditions. Furthermore, a negative-sequence small-signal model of an MG under an unbalanced condition, considering the communication delay time of the proposed SUVC, is established to analyze the system's stability and transient performance under the influence of some critical parameters. Finally, the effectiveness of the proposed strategy is validated by the simulation results from a real-time emulator of StarSim HIL.