Harmonic and current ripple analysis of flying capacitor multilevel inverters with active control algorithms.
This thesis is focused on enhancing flying capacitor multilevel inverters by determining the most optimal pulsewidth modulation control strategy to reduce current ripple and subsequently filter requirements and improve balancing mechanics of the flying capacitors. The two specific control methods of interest within the research work are the phase shifted and phase disposition pulsewidth modulation. There stands a cloudy consensus on the optimal of the two techniques, some researchers cite phase shifted a more suitable technique for its natural balancing mechanics, while others cite phase disposition as superior due to the improved harmonic performance. The purpose of this research is to identify and describe the aptness of each technique in terms of capacitor balancing, harmonic performance and efficiency; to clarify when either technique should be chosen over the other. Analysis of the current ripple is achieved by well-known methods for calculating current ripple in two-level inverters combined with analytical models of representing flying capacitor multilevel inverters. Using this analysis it is shown that because of the greater effective switching frequency of the phase shifted strategy it has reduced output current ripples and harmonic power losses over the phase disposition strategy, which is heavily dictated by the number of levels of the inverter. The research work additionally proposes a closed-loop modified phase shifted control strategy focused primarily on restoring voltage balance. The algorithm selects switching states bounded by times where no capacitor charging/discharging occurs. Through this strategy, it is shown that phase shifted and phase disposition can achieve similar balancing characteristics when operated optimally. Additionally, it is shown that the phase shifted strategy has a fraction of the inductor filter requirements. The critical question of which methodology is superior was found to not have a simple answer; however the circumstances where one method may be more beneficial than another are highlighted and discussed.