Research On The Soft-switching And Parallel Techniques Of T-NPC Three-level Inverter

ABSTRCT:With the scale of new source energy power generation industry continues to expand, the performance of the inverter is more and more important. Based on the T type-Neutral point clamped (T-NPC) three level topology, this dissertation focuses on the research of soft-switching and parallel techniques, involves the Zero Current Transition (ZCT) soft-switching technique, the reduction of the zero sequence circulating current and the neutral point balance control in the parallel system.Firstly, the ZCT soft-switching techniques of the T-NPC are discussed. According to the analysis of the T-NPC inverter commutation characteristics and the existing ZCT soft switching techniques, a new T-NPC three level ZCT soft-switching techniques is proposed. The principle of the resonant process is given. According to the state plane diagram, the dual relationship with the shift of the resonant center is also revealed. Furthermore, a summary about the family of the T-NPC three level ZCT soft-switching techniques is given to make a better choice, and the feasibility of the proposed techniques is proved by the simulation and experiment results.Secondly, because of the non ideal parameters in the real applications, the influences on the process of the ZCT soft-switching are analyzed in detail. Some problems are pointed out to pay more attentions. At last, the experiment results validate the correctness of the theoretical analysis and the losses compassion of the hard-switching and ZCT soft-switching is given to show the advantages of ZCT soft-switching technique. Thirdly, the high frequency zero sequence circulating current in the T-NPC parallel system is discussed in depth. According to the switching function, the mathematical model of the high frequency circulating current is built. The main reason causing the high frequency circulating current are the differences of carriers angle. In a switching period, the actions of the devices are analyzed to explain the peak current changing rule of the high frequency circulating current and the extra current stress of the power device. It is helpful to the hardware design of the inverter parallel system.Fourthly, based on the zero sequence inject SPWM modulation and the model of average duty cycle, the low frequency zero sequence circulating current in the T-NPC parallel system are discussed. A Fourier analysis on the low frequency circulating current’s harmonic components is provided to describe the effects of the zero sequence voltage quantificationally. Based on the low frequency circulating current characteristics, this dissertation proposes to use the PIR controller to make zero sequence circulating current suppression more targeted and effective. Finally, the simulation and experiment results are given to verify the theoretical analysis and the proposed control method.Fifthly, the relationship between zero sequence circulating current and neutral point voltage is discussed. The mathematical expressions show that they have a "strong coupling" relationship. A new control strategy which restrains circulating current by zero sequence voltage injection and regulates the neutral point balance by modulation waveforms decomposition is proposed to realize the two control objectives together. Then, the neutral point control abilities of the strategy are given separately, considering the zero sequence circulating current suppression. Finally, simulations and experiments are made to verify the theoretical analysis about the relationship between zero sequence circulating current and neutral point voltage, and also to prove the effectiveness and feasibility of the proposed control strategy.