Control Scheme for Reducing Second Harmonic Current in AC–DC–AC Converter System

In the ac–dc–ac converter system, the second harmonic current (SHC) is generated by the ac–dc rectifier and the dc–ac inverter. If the SHC propagates to the intermediate dc–dc converter, the current stress of the power switches will be increased, and the conversion efficiency will be degraded. Therefore, it is necessary to suppress the SHC in the dc–dc converter. In this article, the generation and propagation mechanism of the SHC in the ac–dc–ac converter system is analyzed. Then, from a perspective of control bandwidth and dc bus port impedance, the basic ideas for reducing the SHC in the dc–dc converter with different control targets are proposed. Based on that, the virtual-impedance-based control scheme is proposed to increase both the input impedance and output impedance of the dc–dc converter. In addition, the realization and design guideline of the virtual impedance is presented. Finally, a single-phase 3-kVA ac–dc–ac converter system is fabricated and tested in the lab. The experimental results are provided to verify the effectiveness of the proposed SHC reduction control scheme.