Hierarchical Control Strategy for Fundamental and Harmonic Reactive Power Equalization in Multi-Parallel Inverters

Due to nonlinear loads in an AC microgrid, reactive power equalization needs to consider not only the fundamental reactive power but also the harmonic reactive power. To solve this problem, a hierarchical control strategy is designed, where the fast Fourier transform obtains the fundamental and harmonic voltages and currents, and the fundamental and harmonic reactive powers are calculated to obtain the fundamental and harmonic reactive powers, respectively. The two kinds of reactive powers are controlled separately. The primary control uses conventional droop control, and the secondary control is based on the consensus control algorithm to design the frequency and voltage deviation terms to realize the sharing of fundamental reactive power; the joint consensus algorithm and PI control obtain an adaptive harmonic virtual impedance, which makes the equalization of harmonic reactive power; and the voltage and current loops use multi-module quasi-proportional resonance and proportional control, respectively, to reduce the effects of harmonics. A simulation model of an islanded AC microgrid consisting of three parallel inverters is constructed in MAT-LAB/Simulink to verify that the designed control strategy can effectively realize fundamental and harmonic reactive power equalization.