Natural balancing of neutral-point-clamped converters under POD pulsewidth modulation

The three-level neutral-point-clamped (NPC) converter, being a widely used multilevel inverter, received a lot of attention due to problems associated with DC-link capacitor voltage balancing. There are mainly two problems associated with the neutral-point voltage of the NPC inverter: (1) at high modulation indices a low frequency ripple occurs on the neutral-point voltage; and (2) steady-state unbalance in the neutral-point voltage may arise due to a variety of factors including component imperfections, transients and other nonidealities and imbalances. In this paper we study the balancing problem with focus on the steady-state unbalance. This is achieved by a systematic and mathematically rigorous study of the natural balancing mechanisms of the three-level three-phase NPC inverter. Orthogonality of two sets of switching spectra in the frequency domain would imply that the DC-bus voltages balance in the steady state. This is done through mathematical analysis using Carrara´s PWM strategy of phase opposition disposition (POD) and Bennet´s geometric model for double Fourier series adapted for use with power converter systems by Bowes. The theory is verified through simulation.