IGCT－based Multilevel Converters For Medium Voltage Drive Systems

The switching devices with high performance, application of multilevel topology and advanced control strategies became the effective approach and development tendency to achieve high power converters for many applications, such as adjustable speed drive, FACTs and so on. The three-level Neutral Point Clamped (NPC) inverter based on IGCTs is selected to be the main study focus in this thesis. The investigation includes the operation characteristics of IGCTs, the basic cell of multilevel topology, the analysis of instantaneous energy variation in the converter, optimizing design of the converter, and the control strategies. The power electronic device is the base of power electronics and its application equipments. A functional model of IGCTs is investigated in this thesis, which is based on the main physical characteristics of IGCTs, especially the anode current and the terminal voltage. The model is of the key features such as certain accuracy, high simulation speed and easy parameter solution. The equivalent circuit, structure, and parameters determination of the model for a specific type of IGCT are described in details. The simulation accuracy of the model is proved through experiments with the IGCTs test circuit, which is also designed in this thesis.Based on the analysis of converters with Waveform High Frequency Synthesis, the basic cells of multilevel topology with ideal switches and IGCTs are both proposed. According to the characteristics of the basic cell, the design rules and optimized snubbers for IGCT-based inverters are presented. Based on the models, basic cell with IGCTs and the relevant strategy, the simulation and computer aided-design software package for the IGCT-based three-level NPC converters is described, which is the efficient tool for analysis and optimization of the converter. And a novel solution method, in which the transients are classified according to the time frame, is proposed for the analysis of instantaneous energy variation in the converter. Then, the different simulation schematic diagram is selected for different transient in this package. The simulation, analysis and experiment results for a 5MVA converter are given in this thesis.Vector Control and Direct Torque Control, which are both widely used in the adjustable speed drive systems, are discussed. Based on the theoretical analysis of both strategies, many comparison experiments are accomplished with an advanced inverter-fed induction motor test platform. To control DC link voltage for the three level NPC converters with the rating power of 55kW, the energy-feedback equipment is implemented, and the improved scheme for high power converters is explained.