| Medium voltage AC drive systems have bright prospect in high power pumps and fans, electric propulsion systems, rolling mill and so on, where using adjustable speed scheme can not only save energy greatly but also improve the dynamic performance of the motor. With the development of high-voltage and large-current device, multilevel inverter becomes the most popular topology in medium-high adjustable speed scheme. In this dissertation a three-level six-phase SM(synchronous motor) drive system was designed, and some key technologies, such as PWM methods of multilevel inverter, control techniques of six-phase SM and system integration, were researched after the summarization of the power devices, the topologies and control methods of the medium voltage inverter and so on.. The main contents are shown as follow.First, the space vector PWM (SVPWM) of multilevel and the relative technologies are studied in this dissertation. A scheme to achieve three-level SVPWM using DSP is proposed, in which three-level SVPWM is discomposed into two two-level carried-based PWMs. Then the relationship between multilevel SVPWM and multilevel carried-based PWM is analyzed, and a new multilevel PWM scheme based on three-level SVPWM is proposed which is not only much easier to realize than that of SVPWM, but also can get better output voltage than that of carried-based PWM. Moreover, several methods of neutral-point control based on SVPWM in NPC three-level inverters are discussed, and a new hysteresis-band control method of neutral-point balance is proposed, which is based on BANG-BANG control scheme and suitable for the PWM with unstable period. It is verified by simulation and experiment of three-level inverter with SHEPWM control scheme.Second, the air gap flux-oriented vector control of six-phase SM is researched in this dissertation. The six-phase SM model is built by modifying the three-phase SM component in SIMULINK/MATLAB, and the vector control of six-phase SM is simulated based on the new model. Also, an air gap flux observation of six-phase SM using voltage and current mixed model is presented, and a simple adaptive control scheme based on voltage and current of SM stator is proposed to compensate the drift of SM parameters. What's more, the fully digital vector control scheme of six-phase SM is researched. To meet the requirement of high speed response of the inner current loop, a full-order observation is proposed to compensate the delay of current sample. Also, to compensate the torque disturbance caused by load variations, a torque disturbance feed-forward is designed in motor speed loop.Last, an integrated approach for multi-phase and multilevel inverter is researched in this dissertation. After studying the main circuit of multilevel inverter, one leg of NPC multilevelinverter is designed as a basic power module which is composed of power devices, drive circuits and cooling system. These modules can be used to build single phase, three-phase or multi-phase inverter. The control system of the inverter is analyzed and a four-layer control system is developed which includes hardware manager, topology manager, application manager and system manager. According to the response time of each layer, the communications between layers are also designed, among which the key attention is paid to the communication between topology and application layers, and an AC signal communication method based on rotating coordinate frame is proposed to compensate the phase delay in the process of the AC signal communication.A 690V/200kW three-level inverter of six-phase SM is developed, and experimental results verify the techniques presented in the dissertation.
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