High and Low Frequency Control Strategy for APF DC Side Ripple Voltage under Unbalanced Load

The problem of the APF (Active Power Filter) compensation ability is reduced when the grid has an unbalanced load. Firstly, the positive and negative sequence mathematical models are established based on parallel APF with LCL. The main reason for the generation of higher harmonics on both sides of the APF AC and DC is the fundamental sequence negative component of the system. The high and low frequency mathematical expressions of the DC side voltage are obtained, and the design controller controls the negative sequence and the positive sequence of the system separately, thereby suppressing the DC side ripple voltage and the AC side harmonic current. Finally, the simulation is carried out from both steady state and dynamics, and compared with the conventional PI, the results show the superiority of the control strategy.