Effects of imbalances and nonlinear loads on the voltage balance of a neutral-point-clamped inverter

The effects of linear imbalances and nonlinear loads on the voltage balance of the neutral-point-clamped converter are described in this paper. The study reveals that a negative sequence of output currents (linear imbalance) may produce additional low-frequency oscillations to the neutral-point voltage. Similar consequences are produced by odd-order current harmonics from a nonlinear load, while even-order harmonics can cause the neutral-point voltage to shift. Furthermore, the second, fourth and eighth output current harmonics might produce instability to the neutral-point voltage. The second and fourth harmonics are the worst components. The maximum amplitudes of these harmonics superposed to the current fundamentals that the system can tolerate are described. Simulated and experimental examples are presented.