Research On The Control Strategy Of High Power NPC Converter

This paper focused its researches on the high power diode neutral-point-clamped converter with IGCTs. Theory analysis, software simulation, prototype experimentation and other methods were made to investigate the problems of the NPC three-level converter and NPC/H-bridge five-level converter, which included the PWM control strategy, subsection synchronous modulation, synchronous switching, the losses of the device and so on.The working principle and control strategy of NPC/H-bridge five-level converter were introduced in this paper. The harmonic expression of different modulation modes were calculated, which were based on the carrier phase shifted. And the characteristics of different modulation modes were analyzed. The results verified the feasibility and effectiveness of MPS (Modulation wave Phase Shift), and MPS had be applied in the prototype. On this basis, the DC voltage utilization rate can be improved 15% by the way of the third-harmonic injected.The paper studied the principle and advantage of sub-synchronous modulation, and had designed the sub-synchronous curves of NPC/H-bridge five-level converter and NPC three-level converter. The IGCT worked in the best frequency ranges, and the output waveforms were symmetry. At last the simulation results and experiment waveforms were given.The motor harmonic model was derived and the size of the switching current ripple was calculated in this paper. On this basis, current impact was analyzed from the view of flux, and angle compensation method was put forward to eliminate the current impact. The simulation and experiment results verified the simplicity and effectiveness of the method.The device simulation model and transient characteristic testing circuit based IGCT were established. According to this model, the losses of IGCTs and Snubber circuit in a period are investigated. The simulation and experiment results verified the effectiveness of the model. Finally the efficiency of NPC/H-bridge five-level inverter were analyzed, in the condition of the output frequency of 50Hz and 100Hz.