A Hybrid Feedforward Harmonic Mitigation Strategy for Inverters under Islanded Operation

Nonlinear loads in islanded microgrids often drive the total harmonic distortion (THD) of grid-forming inverter output voltage above 5%, exceeding IEEE Std 519-2014 limits. To address this challenge, we propose a control strategy for three-phase four-wire inverters to ensure robust power quality. We model the inverter’s output impedance to analyze harmonic behavior and develop a phase-compensated resonant controller (PCRC) to suppress harmonics at the nominal frequency. However, PCRC’s narrow bandwidth struggles with frequency deviations caused by droop control. To overcome this, we introduce a hybrid feedforward strategy, integrating wideband capacitor voltage feedforward with load current feedforward, modeled as a virtual impedance network. Compared with existing methods, our approach better sustains harmonic suppression during frequency shifts. Experimental results verify the effectiveness of the control strategy, demonstrating that the hybrid feedforward strategy significantly reduces the THD of the output voltage.