Switching Voltage Modeling And PWM Control For Multilevel Inverters Under DC Voltage Unbalance

Abstract

In this paper, a novel switching voltage modeling and offset-based Pulse Width Modulation (PWM) scheme for multilevel inverters with unbalanced dc sources is presented. The switching voltage model under DC-voltage imbalance will be formulated in general form for both Neutral point Clamped and cascaded multilevel inverter topologies. An analysis of the reference switching voltages from its active and non-active switching voltage components in coordinates can enable the voltage implementing for the unbalanced dc source condition. The offset voltage is introduced as an indispensable variable in the switching voltage model for multilevel voltage source inverters. The PWM performance is controlled through the design of two offset components in a subsequence. One main offset may refer to the common mode voltage, while the second offset restricts its effect on the quality of PWM control in related dc levels. An improving of the PWM quality as reduction of the switching loss in Discontinuous PWM mode can be obtained by setting the local offset related to the load currents. The validity of the proposed algorithm was verified by experimental results.