| Multilevel converters offer benefits of increased power ratings, low switching frequency, reduced output harmonics, fast dynamic response and good electromagnetic compatibility (EMC), which will make it possible to enlarge the power rating and improve the output performance simultaneously. With the development of superconducting magnetic energy storage technologies (SMES) and the semiconductor devices with high blocking Voltage (such as MCT and IGCT), the efficiency of the conventional CSI's will be overcome and hence their use is becoming a more attractive proposition, particularly for higher power applications. The topology construction, PWM strategy, self current-balancing characteristics and current-balancing control methods of multilevel CSI's are systematically researched, also a high power-factor three-phase multilevel PWM current-source rectifier is presented in this dissertation.After reading a lot of relevant published papers, the topology evolutions of multilevel CSI's, together with their operational principles have been reviewed and summarized. Then, the modulation performances of the multilevel PWM strategies in common use and their applications in multilevel CSI's are introduced. Based on reviewing the most recent research achievements of CSI's in high power application fields, the application prospect and research value for multilevel CSI's are discussed.Several kinds of multilevel CSI's topologies are constructed in the thesis, which provides steady basis for the whole research job. By using dual transform, a kind of generalized single-phase 5-level CSI topology is deduced, with proper simplification, a single-phase 6-switch 5-level CSI topology is proposed. However, how to construct three-phase multilevel CSI's topology is a more difficult topic comparing to build single-phase multilevel CSI's topology. In order to apply dual theory, the ichnography of the three-phase multilevel VSI topology is obtained with complanation, then three-phase 5-level CSI topology 1 is obtained by using dual transform. By modifying the input power supply's connection mode and making some further improvements, three-phase 5-level CSI topology 2 is built from AC-AC matrix converter. According to the generation mechanism for multilevel current and with the idea of connecting the inductors in parallel to share the total DC-link current, three-phase 5-1evel CSI topology 3 is deduced. Taking the single-phase 6-switch 5-level CSI as the basic cell, three-phase 5-level CSI topology 4 and topology 5 are proposed respectively by appropriate combinations. The PWM control strategies of multilevel CSI's are researched in the thesis. The multilevel PWM techniques which are commonly used in VSI's are extended and applied to the single-phase multilevel CSI's and three-phase CSI's of class A, the output harmonic characteristics is analyzed in theory, and the performance is as same as that of the multilevel VSI's. As for three-phase CSI's of class B, the three-phase multilevel PWM method is implemented by shifting three modulation signals 120 degree respectively on the basis of the single-phase 6-switch 5-level CSI. Due to the coupling relation among three phases, 3-logic PWM technique is researched in the three-phase CSI's of class C. Because the 3-logic carrier phase-shifted SPWM technique has the disadvantage of fundmental magnitude loss in the linear modulation region, in order to improve the utility of the DC-link current, the output harmonic characteristics of 3-logic carrier phase-shifted SPWM method in the overmodulation region is analyzed in detail, the quantitative relationship between magnitude ratio and fundmental magnitude, harmonic characteristics of output current is obtained, which will contribute to select a moderate magnitude ratio in the overmodulation region.The self-current balancing performances of different kinds of multilevel CSI's are researched in the thesis. By setting up mathematic expressions of inductor voltage, the self-current balancing performances of multilevel CSI's of classâ… andâ…¡are analyzed in detail, and the relevant results are obtained, which will provide theoretic basis for the implementation of current-balancing control. By using Double Fourier transform, the harmonic performances of 3-logic carrier phase-shifted SPWM and multi-carrier disposition PWM are analyzed respectively when the intermediate DC-link current is unbalanced, and the results illustrate that current-unbalance will influnce the output performances of the multilevel CSI's with different PWM strategy.According to the PWM strategies of multilevel CSI's of classâ… andâ…¡, the current-balancing control methods of these two kinds of multilevel CSI's are proposed. For the multilevel CSI's of classâ… ,with the carrier-rotation control method, the current-balancing performance will be affected by load characteristics variation and circuit parameters asymmetry, in order to overcome these disadvantages, a new current-balancing method based on DC-link current feedback is introduced in the thesis. To solve the current-unbalance problem for the multilevel CSI's of classâ…¡, on the basis of carrier phase-shifted SPWM technique, a current-balancing method based on automatically changing the redundant switching combinations is developed. Because it is hard to be implemented in high output-level converters, another current-balancing method which is based on modifying pulse width is presented in the thesis, because the control mode at different modules is relatively independent, this method will not be restricted by the numbers of ouput-level and thus can be applied to the converters with high output-level.According to the reversibility of multilevel SPWM CSI's, by utilizing the carrier phase-shifted SPWM rectification technique and module parallel connection technique, a kind of three-phase multilevel PWM current-source rectifier is researched in the thesis. The operational principle of unit power-factor PWM rectification is analyzed by setting up the average model of this multilevel current-source rectifier. To control the current-source PWM rectifier properly, a simple digital implementation method for 3-logic SPWM is presented in the thesis. An experimental prototype of 7-level PWM rectifier has been set up to verify the proposition. Comparing with the multi-pulse phase-controlled rectifier, the three-phase multilevel PWM current-source rectifier has no use for the complicated phase-shifting transformer, and has the dominant advantage in the input harmonic performance and the line power factor.Finally, the dissertation makes a summary for the whole work, and gives an expectation for future study direction about multilevel current-source inverters.
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