A grid-connected cascaded H-bridge multilevel converter with quasi seven-level Selective Harmonic Elimination

Abstract

In this paper, a high power grid-connected cascaded H-bridge multilevel inverter (CHB-MLI) operating with Selective Harmonic Elimination (SHE) is presented complying with the low switching frequency operation limitation. In order to alleviate the complexity of employing SHE at high level order, Quasi seven level SHE (Q7L-SHE) approach is employed in this paper. Q7L-SHE is based on cancelling the fifth and seventh order harmonics irrelevant of the number of employed levels. Compared with the conventional SHE approach, the Q7L-SHE method requires solving simple equation groups, especially if a large number of switching angles is required, or if each voltage level requires multiple switching angles. The grid-connected CHB-MLI is controlled in the synchronous rotating reference frame which is used to control the modulation index and the phase angle. A 5MW medium voltage 17-level grid-connected CHB-MLI is considered to validate the proposed concept. Simulation results using the MATLAB/SIMULINK platform have been presented to elucidate the proposed concept.