Switching Voltage Modeling and PWM Control in Multilevel Neutral-Point-Clamped Inverter under DC Voltage Imbalance

Abstract

This paper presents a novel switching voltage model and an offset-based pulse width modulation (PWM) scheme for multilevel inverters with unbalanced DC sources. The switching voltage model under a DC voltage imbalance will be formulated in general form for multilevel neutral-point-clamped topologies. Analysis of the reference switching voltages from active and non-active switching voltage components in abc coordinates can enable voltage implementation for an unbalanced DC-source condition. 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, and the other offset restricts its effect on the quality of PWM control in related DC levels. The PWM quality can be improved as the switching loss is reduced in a discontinuous PWM mode by setting the local offset, which is related to the load currents. The validity of the proposed algorithm is verified by experimental results.

An Improved SPWM Strategy to Reduce Switching in Cascaded Multilevel Inverters

“…These imbalanced voltages, which are represented by ripples in DC links, have a negative effect on output quality because they cause distorted output waveforms with low-frequency harmonics. This phenomenon can lead to the degradation of load performance [15], [16], [17]. When CPS-SPWM is employed in the study of the CPS angles of cascaded multi-level inverters, each unit adopts double frequency SPWM such that a PWM pulse generator becomes a requirement of each switching device.…”