An analysis of midpoint balance for the neutral-point-clamped three-level VSI

The three level voltage source inverter (VSI) is a good topology for high voltage and high power applications where no semiconductor devices are available. However, it has an inherent problem of midpoint charge balance. The problem becomes critical when the load is nonlinear and/or has large phase angle and harmonics. If it exceeds the critical points, there is no way to control the midpoint voltage by itself. The midpoint balancing problem is analyzed with respect to the reactive loads with harmonic currents and a way to solve the problem is suggested