A power sharing scheme for voltage unbalance and harmonics compensation in an islanded microgrid

In this paper, a hierarchical control structure with voltage unbalanced compensation scheme in ac islanded microgrid is proposed to improve Critical Load Bus (CLB) voltage quality and address inaccurate power sharing problems. The hierarchical scheme includes primary and secondary control levels. The primary control mainly contains the power droop controllers, voltage and current controllers, selective virtual impedance loop, and voltage unbalanced compensation. The virtual impedance loop includes virtual positive- and negative-sequence impedance loops at fundamental frequency and virtual variable harmonic impedance loop at harmonic frequencies. The secondary control is designed to restore frequency and amplitude of the CLB voltage. In order to compensate the CLB voltage, an unbalanced compensation is proposed to change the voltage reference of the distributed generation (DG) units. This strategy also employs the low bandwidth communication (LBC) technique to send the proper signals of the secondary control from the microgrid control center (MGCC) to the primary control. To evaluate the performance of the proposed control strategy, simulations are conducted on two islanded microgrid prototype. The results demonstrate the effectiveness of the proposed control structure in the unbalance and harmonic compensation of the CLB voltage and proper power sharing of reactive, unbalanced and harmonic powers among the DG units.