Harmonic Analysis and Mitigation of Low-Frequency Switching Voltage Source Inverter With Auxiliary VSI

The output currents of high-power Voltage Source Inverters (VSIs) are distorted by the switching harmonics and the background harmonics in the grid voltage. This paper presents an active harmonic filtering scheme for high-power, low-frequency switching VSIs with an additional auxiliary VSI. In the approach, the auxiliary low-power, high-frequency switching VSI is used to compensate both the switching harmonics of the high-power VSI and the low-order harmonics introduced by the distorted grid voltage. Impedance modeling and analysis are implemented for the hybrid VSI system, where the impacts of the grid impedance are analyzed. Different control targets are then formulated based on the analysis, and the corresponding control strategies are proposed. Lastly, simulations and experimental results are provided to validate the theoretical analysis and the performance of the proposed control strategy.