Active Power Quality Improvement Strategy for Grid-Connected Microgrid Based on Hierarchical Control

When connected to a distorted grid utility, droop-controlled grid-connected microgrids (DCGC-MGs) exhibit low equivalent impedance. The harmonic and unbalanced voltage at the point of common coupling (PCC) deteriorates the power quality of the grid-connected current (GCC) of DCGC-MG. This paper proposes an active, unbalanced, and harmonic GCC suppression strategy based on hierarchical theory. The voltage error between the bus of the DCGC-MG and the grid's PCC was transformed to the dq frame. On the basis of the grid, an additional compensator, which consists of multiple resonant voltage regulators, was then added to the original secondary control to generate the negative fundamental and unbalanced harmonic voltage reference. Proportional integral and multiple resonant controllers were adopted as voltage controller at the original primary level to improve the voltage tracking performance of the inverter. Consequently, the voltage difference between the PCC and the system bus decreased. In addition, we established a system model for parameter margin and stability analyses. Finally, the simulation and experiment results from a scaled-down laboratory prototype were presented to verify the validity of the proposed control strategy.