OPTIMAL PULSE WIDTH MODULATION OF MULTILEVEL INVERTERS FOR MEDIUM VOLTAGE DRIVES

Abstract

Low device switching frequency modulation of multilevel inverters (MLI) for medium voltage drives is essential to increase semiconductor device utilization, minimize cooling requirements and achieve higher overall system efficiency. However, there exists a trade-off between device switching frequency and harmonic distortion of machine stator currents. Therefore, classical pulse width modulation (PWM) techniques such as sinusoidal PWM and space vector modulation techniques utilize higher device switching frequency (around 1 kHz) to generate good quality of machine stator currents. Synchronous optimal pulsewidth modulation (SOP) is an effective and emerging modulation technique to achieve low device switching frequency operation, while minimizing the harmonic distortion of machine stator currents. Until now, implementation of SOP technique has been limited to classical three-level and five-level MLI topologies.

One of the main goal of the thesis is to investigate the state-of-the-art SOP technique and develop systematic analysis to implement SOP for MLIs with any number of voltage levels. After detailed investigation, two different methods have been developed to implement SOP for n-level MLIs. Next, the thesis has explored the feasibility of implementing SOP technique for emerging topologies such as modular multilevel converters (MMC). One of the main challenges for modulation of MMC is to maintain floating capacitor voltages around their nominal value. Based on the study, a systematic algorithm has been developed to implement SOP for MMCs. Finally, this thesis has investigated the issue of common-mode currents for the dual inverter based MLI fed open-end winding induction motor drives. An enhanced SOP technique has been proposed to eliminate the common-mode currents in the open-end stator windings of induction motor drives. Also, the thesis has proposed new dual inverter MLI topology based on MMC topology, which inherits features of modularity and scalability. All of the above research proposals have been verified experimentally using low power prototypes of MLI feeding an induction motor drive.