Modeling, analysis and mitigation of load neutral point voltage for Three-phase four-leg inverter

The Three-phase four-leg inverter is very suitable to be used in the high power UPS, and the load neutral point voltage should be low to meet the requirement of the UPS. In this paper, the load neutral point voltage is modeled. Two factors influencing the load neutral point voltage are revealed, which are the switching states and the ratio of neutral inductance to phase inductance. Then, the load neutral voltage is analyzed. Sixteen switching states can be grouped into four classes, and each class is with the same load neutral point voltage. Switching states of the first and the second class come to the minimum valve of zero with the special ratio of k. Thirdly, three methods to mitigate the load neutral point voltage are proposed, and the benefit and the limitation of each method are analyzed. Finally, the proposed methods are verified by simulation and experiment.