Voltage Quality Improvement Method for 400Hz LCL-type Inverter

400Hz inverters are usually connected to work in parallel to provide larger power. Considering the suppression of circulating current, LCL filter is a desirable selection and widely applied in parallel inverter systems. Whereas, when connecting with nonlinear loads, the harmonic currents through the load-side inductor will lead to a severe distortion on the output voltage of the inverter. After a detailed analysis of the inverter impedance, it reveals that although the output voltage feedback control can reduce harmonics, the suppression of circulating current also weakens. To improve the output voltage quality, an impedance reshaping method realized by multiple proportional and resonant(MPR) control and load current feed-forward is proposed, with which the inverter impedance appears very small at low-order harmonic frequencies and much larger at high-order harmonic frequencies. Applying the method, the suppression of both harmonic voltages and circulating current can be obtained. Finally, a prototype is built and tested, and the experimental results validated the effectiveness of the proposed control method.