Natural balancing of the neutral-point-clamped converter

[摘要] ENGLISH ABSTRACT:The three-level neutral-point-clamped(NPC) converter, being a widely used multilevelinverter, received a lot of attention recently due to problems associated with de-linkcapacitor voltage balancing. There are mainly two problems associated with the neutralpointvoltage of the NPC inverter:1. At high modulation indices a low frequency ripple occurs on the neutral-point voltage.2. Steady-state unbalance in the neutral-point voltage may arise due to a variety offactors including component imperfections, transients and other non-idealities andimbalances.In this thesis we study the balancing problem with focus on the steady-state imbalance.This is achieved by a systematic and mathematically rigorous study of the natural balancingmechanisms of the three-level three-phase NPC inverter. Orthogonality of two sets ofswitching spectra in the frequency domain would imply that the DC-bus voltages balancein the steady state. This is done through mathematical analysis using Carrara's PWMstrategy of alternative phase opposition disposition(APOD), phase opposition disposition(POD) and phase disposition(PD); and Bennet's geometric model for double Fourierseries adapted for use with power converter systems by Bowes. The theory is verifiedthrough simulation.

[发布机构] Stellenbosch University