Space vector pulse width modulation techniques for multilevel inverter with fault tolerant capability

Abstract

ABSTRACT: Multilevel inverters (MLI) play a very dominant role in the field of medium voltage and high-power applications over the classical two-level inverters. The most widely used common MLIs topologies are neutral point clamped or diode clamped (NPC), flying capacitor (FC) and cascaded H-bridge (CHB) inverter. Among these three topologies, CHB is preferred due to its modularity structure and cell redundancy. The well known modulation techniques for inverter switching are carrier-based pulse width modulation (PWM) and the space vector PWM (SVPWM). SVPWM has high DC bus utilization and lower harmonic distortion; in addition it provides a number of redundant switching states, which is useful to accomplish the particular objective in the application. This thesis presents the simplification of SVPWM techniques for the CHB MLIs. The main concept of SVPWM is to realize the reference vector which is rotating in space, from a number of available stationary space vectors (SVs) of the inverter switching states. Identification of the nearest three stationary vectors to the rotating reference vector are not easy. As the inverter level increases, then the problem of finding nearest three vectors (NTVs) are also increases. In this work, initially three techniques have been proposed to simplify the finding of NTVs to the reference vector. The first technique, called modified space vector PWM (MSVPWM) technique, is based on disintegrating the MLI space vector diagram (SVD) into suitable two-level hexagons. This fallout in to generalization of the multilevel SVPWM difficulty into a conventional two-level SVPWM problem. The second technique, called further modified space vector modulation (FMSVPWM) technique, is also based on the disintegrating the multilevel SVD into suitable two-level hexagons, but the number of two-level hexagons considered are significantly less than the MSVPWM technique. The third technique, called the r-s coordinate transformation method, based on the use of 60º spaced r-s coordinate system to achieve SVW