Space vector modulation and control of multilevel converters

This dissertation is the result of the research and development of a power conditioning system for superconductive magnetic energy storage systems (SMES). The dominant challenge of this research was to develop a power conditioning system that can match slowly varying dc voltages and dc currents on the superconductive magnet side with the ac voltages and ac currents on the utility side. At the same time, the power conditioning system was required to provide a bidirectional power flow to the superconductive magnet.; The focus of this dissertation is a three-level diode-clamped dc/ac converter, which is a principle part of the power conditioning system. Accordingly, this dissertation deals with the space vector modulation (SVM) of three-level converters and introduces a computationally very efficient three-level SVM algorithm that is experimentally verified.; Furthermore, the proposed SVM algorithm is successfully generalized to allow equally efficient, real-time implementation of SVM to dc/ac converters with virtually any number of levels. The most important advantage of the proposed concept is that the number of instructions required to implement the algorithm is almost independent from the number of levels in a multilevel converter.; More on the side of the control of multilevel converters, particular attention in this dissertation is paid to the problem of charge balance in the split dc-link capacitors of three-level neutral-point-clamped converters. It is a known fact that although the charge balance in the neutral point (NP) can be maintained on a line cycle level, a significant third harmonic current flows into the NP for certain loading conditions, causing the neutral-point voltage ripple. The logical consequence of that ripple is the deteriorated quality of the output voltage waveforms as well as the increased voltage stress on the switching devices.; This was the motivation to more carefully explore the loading conditions that cause the imbalance, as well as to study the fundamental limitations of dc-link capacitor charge balancing algorithms. As part of this work, a new model of the neutral-point current in the rotating coordinate frame is developed as a tool for the investigation of theoretical limitations and in order to provide some insight into the problem. Additionally, the low-frequency ripple is quantified and guidelines are offered that can help determine the correct size for the dc-link capacitors.; Because the study of the neutral-point balance identified the loading conditions that (under some possible system constraints) cause an unavoidable neutral-point voltage ripple, a feed-forward type of control method is developed next. The proposed feed-forward algorithm can effectively prevent the neutral-point voltage ripple from creating distortions in the converter output voltage under all loading conditions and without causing additional disturbance in the neutral-point voltage. The feed-forward method is developed for a sine triangle as well as for the SVM type PWM algorithm.; The simulation results that include the full dynamic model of the converter and load system validate the feed-forward approach and prove that the feed-forward algorithm can effectively compensate for the effect of the neutral-point voltage ripple. The simulation results are then experimentally verified.