Secondary Control for Compensation of Voltage Harmonics and Unbalance in Microgrids

In this paper, a hierarchical control scheme is proposed for enhancement of Sensitive Load Bus (SLB) voltage quality in microgrids. The control structure consists of primary and secondary levels. The primary control level comprises Distributed Generators (DGs) local controllers. Each of these controllers includes a selective virtual impedance loop which is considered to improve sharing of fundamental and harmonic components of load current among the DG units. The sharing improvement is provided at the expense of increasing voltage unbalance and harmonic distortion. Thus, the secondary control level is applied to manage the compensation of SLB voltage unbalance and harmonics by sending proper control signals to the primary level. DGs compensation efforts are controlled locally at the primary level. The system design procedure for selecting proper control parameters is discussed. Simulation results are provided in order to demonstrate the effectiveness of the proposed control scheme.