Research On Control Strategy Of Modular Multilevel Power Electronic Transforme

With the introduction of the smart grid concept,many new changes have occurred in the power system,and the traditional power transformer with a single function can no longer meet the requirements of the new power system.Power Electronic Transformer(PET)is a combination of high frequency transformer and power electronic converter,which not only has the function of traditional power transformer,but also can realize flexible regulation of power quality and provide grid connection interface for distributed energy.Modular Multilevel Converter(MMC)is highly modular and has a high-voltage DC bus.The power electronic transformer with MMC as input stage can provide various AC and DC ports,which is important for the development of smart grid.In this paper,the control strategy of MMC-PET at all levels is studied,and the main research contents are summarized as follows:(1)According to the three-stage structure of MMC-PET,the working principles of input stage,isolation stage and output stage are analyzed respectively.The input stage is the MMC,and based on its topology and the operating states of sub-modules,the equivalent models of AC side and DC side are established respectively,and the appropriate modulation methods are selected according to the characteristics of commonly used modulation strategies.The isolation stage is a bidirectional active bridge(ISOP-DAB)with input in series and output in parallel.The working principle of single phase shift control is analyzed based on the topology of DAB,and the topology and basic principle of ISOP-DAB are briefly introduced.The output stage is a three-phase voltage-source inverter,and the operating principle is analyzed based on its topology,and the SVPWM modulation method applicable to this inverter is introduced.(2)Based on the voltage and current double closed-loop control,the control strategy of the input stage MMC is studied in terms of capacitor voltage equalization control and loop current suppression respectively.The conventional equalization control strategy using carrier phase shift is introduced,and aiming at the problems of large voltage fluctuation of DC bus and poor capacitor voltage consistency in the traditional strategy,an improved equalization control strategy based on feedforward control of the capacitor voltage average is proposed,which reduces the DC bus voltage fluctuation and also improves the consistency of the capacitor voltage.The components of the AC component in the circulating current are analyzed,and a mathematical model of the negative-sequence two-fold circulating current is established,based on which a negative-sequence two-fold circulating current suppression controller is designed.Based on the above study,the overall control strategy of MMC is finally derived.(3)The control strategies of the isolation stage and the output stage are studied separately.In order to realize the power balancing control of ISOP-DAB,the voltage dual-loop control with input equalization is proposed based on the common shift compared control.In order to realize the control of the three-phase voltage-source inverter under unbalanced load,the voltage single-closed-loop control and voltagecurrent double-closed-loop control applicable to the balanced load are studied first,then aiming at the problem of unbalanced output voltage under unbalanced load in traditional strategy,an improved double-loop control strategy based on delay cancellation technology was proposed,which can maintain the balanced three-phase output voltage under unbalanced load.Finally,simulation models of each level are built according to the control strategy of each level,and the effectiveness of the control strategy used in each level is verified by comparative simulation analysis,and the overall simulation of the MMC-PET is carried out to verify the correctness of the models of each level.