DC capacitor voltage balancing in multi-level converters

Abstract

Multi-level converters that provide more than two levels of voltage to achieve an output waveform closer to sinusoidal waveform with less distortion are very attractive to power applications. This thesis investigates several multi-level converter topologies and different modulation strategies such as pulse-width modulation and space vector modulation. Attention is paid in particular to SVM strategy. Although SVM strategy is applicable for N-level converter, this thesis only focuses on five-level and three-level diode clamped converter (DCC). Despite their appealing harmonic spectrum and low losses, multi-level converters are known to suffer from inherent voltage imbalance on their dc side. The thesis presents a method in order to balance the dc side capacitor voltages for an N-level converter. The presented balancing method is based on minimizing a cost function which is related to voltage divergence of the dc capacitors. This method is used for a three-level SVM to overcome the voltage drifting problem.