Studies On Control Strategies Of Cascaded Multilevel Converter Based Power Electronic Transformer

With energy crisis and environmental pollution are increasingly seriously, the distributed generation system develops rapidly. Cause the differences of voltage grade and electricity form, makes the distributed generation system hard to connect to the power grid, and it has been the key problem. It depends on the construction of smart grid to solve this problem. As the core equipment of the smart grid, power electronic transformer (PET) can provide distributed generation system with AC and DC interconnection interfaces. PET is new type of transformer, it not only has the functions of traditional transformer, it can also realize flexible controls of current on grid side、voltage on user side and power transformation direction, unity power factor operation, isolation between grid side and user side, power reverse transmission by advanced power electronic transformation technology.Firstly, the paper introduces the development of PET, analyzes several kinds of typical topologies and their advantages and disadvantages, according to design needs, the topology studied in this thesis is determined to be cascaded multilevel converter based PET.In this paper, by analyzing the average models of grid-side converter and middle stage, the small signal models are established, and.the transfer function are deduced. Based on the control objective, grid-side converter should realize equilibrium output DC voltage of each module, so voltage balance control is proposed based on single-phase D-Q vector common duty ratio control. Middle stage should realize power balance among each module, so power balance control is added based on common duty ratio control; and the closed-loop system design of input stage and middle stage are introduced in detail in this paper.The sensorless power balance control strategy is introduced in detail in this paper. In addition, in order to realize the function of energy bidirectional transmission, control strategy is presented when energy is transferred reverse, grid-side converter adopts power control strategy and middle stage uses voltage control which are different from the control strategies when the power is forward transferred, besides working principle of power control and the implement method of phase shift are given in detail.Furthermore, in order to verify the proposed control strategies, a cascaded multilevel converter based PET experiment platform is designed based on DSP and FPGA. Finally, the validity of the proposed control strategies in the thesis are confirmed by the results of PSIM simulation and experiment.