Research On Control Strategy Of ANPC Three Phase Three Level Inverter

An introduction of the procedure to realize space vector pulse width modulation (SVPWM) strategy is studied in this thesis firstly, results from the simulation and experiments has shown that:The SVPWM strategy has adopted an more complicated space vector diagram, and has more alternative vectors in one single area comparing with the two level inverter, thus refining the vector composing process.Because of the fact that the current flowing through the neutral point of the dc bus capacitor is not zero, neutral point clamped three level inverter has an intrinsic fluctuation problem in the bus capacitor voltage or the neutral point. In order to get an insight of the mechanism in bus capacitor voltage fluctuation, switching model, averaged model and equivalent circuit in synchronous rotating coordinate system were built in this thesis. Conceptions of equivalent parallel admittance and equivalent current source were introduced to find the mathematical expression of the bus capacitor voltage fluctuation. An explanation to the transient adjusting process, and the relationship of different bus capacitor voltage balance strategy, e.g. zero sequence injection, SVPWM with balance factor and parallel resistors, was given afterwards.Different from the DNPC structure, the two additional clamping IGBT in the active neutral point clamped (ANPC) topology extend the output current path of traditional O level to 4 types, and commutation types to 32 from 8. Because of the fact that these new commutation types were able to be chosen by control the clamping IGBTs, ANPC provides a possibility to control the distribution of power loss among the power devices. Beginning with descriptions of different current path, the effect of different commutation process on the power loss was analyzed in the ANPC three level inverter. Conception of passive commutation mode and active commutation mode was introduced to classify theses commutation process, conduction loss and switching loss model under the two types of commutation modes were built, variance was used to evaluate the unbalance degree of the power loss distribution. Results have shown that, different commutation modes will cause different loss distribution among power devices; however, they have the same effect on the total conduction loss, switch loss and output voltage waveform. Based on the model, a loss balance control strategy by comparing the loss variance was proposed.To improve the reliability of the ANPC three level inverter, the effect on the inverter output and the bus capacitor voltage fluctuation according to different types of fault was analyzed in this paper, corresponding fault tolerant solutions for each types of fault were proposed. In order to solve the bus capacitor fluctuation problem after fault reconstruction, equivalent circuit models were built; corresponding control strategies were given afterwards. Simulation and experiment were given to verify the validity.