Decentralized Optimal Servo Control System for Implementing Instantaneous Reactive Power Sharing in Microgrids

Active power is dispatched among distributed generation (DG) units in microgrids (MG) by means of f/P droop control loop, which controls the frequency set-point of voltage source converter (VSI). Since the frequency is a global variable, active power sharing is implemented well proportional to droop coefficients. However, the reactive power is not shared accurately, through V/Q control loop and according to the droop gains, as the voltage is a local variable. Furthermore, considering the small scale of DG units, reactive power sharing should be implemented instantaneously to prevent DG units from overcurrent or even blackout of the MG. This paper deals with reactive power sharing issue in droop control-based MGs as well as stability and dynamic performance concerns of V/Q control loop. A servo control system is designed to control power converters in MGs, by which droop-based VSIs are converted to servo VSIs (S-VSIs). A novel decentralized method is proposed to obtain the reactive power set-points of S-VSIs according to their droop coefficients, and fuzzy particle swarm optimization method is used to optimize the S-VSI's parameters, so that, in addition to securing stability of the V/Q loop, the desired (fast) response in reference tracking is achieved. The simulation results show that the proposed strategy is effective and its performance is not affected by delay or interruption of the existing low bandwidth communication link.