Current THD comparison of different offset injections in the linear modulation range for cascaded H-bridge converters

Abstract

The equivalence of space vector pulse width modulation (PWM) and carrier-based PWM through the injection of a common-mode voltage is already known for twolevel (first min-max injection) and multilevel (double min-max injection) three-phase converters. However, under regular sampled phase voltage references, these injections do not produce the lowest total harmonic distortion (THD) of the load phase current throughout the linear modulation index range. This paper proposes the use of a similar common-mode injection, called the second min-max injection that achieves the lowest current THD in regions of the modulation index range where the double min-max injection fails to do so. A cascaded H-bridge (CHB) topology is analyzed, and comparison is made between different multicarrier modulation strategies, using simulation results, under different injections for the whole modulation index range. The level-shifted in-phase disposition modulation method is found to produce the lowest load phase current THD with second min-max offset voltage injection for modulation indices below unity. For modulation indices greater than one, the double min-max injection is still considered as optimal.