Per-Phase Output Current Control of 4-Leg Active Power Filter Based on Adaptive Notch Filter

This paper presents control strategy of four-leg four-wire active power filter for independent phase current control. The strategy is based on digital notch filter adaptive to grid voltage frequency to maintain high quality compensation under grid voltage frequency variations inherent in microgrids with distributed generation and standalone power supply systems using fast response phase-locked loop system. Proposed control strategy requires no coordinate transformation and operates directly with load and converter currents which significantly reduces computational load and dynamic phase interaction. Mathematical modeling of the converter with LC filter and neutral wire inductor was carried out and mathematical equations were obtained to choose the parameters of the current controller (proportional and integral gains) based on the desired bandwidth of the current control loop, power stage reactive elements nominal values, and DC-side nominal voltage level. Proposed control strategy was studied using Matlab/Simulink simulation under distorted grid with variable frequency, and experimental verification using laboratory prototype of the system was implemented.