Research On Voltage Balancing Strategy For Input Stage Of Modular Cascaded Power Electronic Transformer

Power Electronic Transformer(PET)is a Smart Transformer(ST)which realizes power transformation through electromagnetic induction principle and power electronic technology.PET not only realizes the function of energy transmission and voltage level transformation of traditional power transformer,but also has characteristics of reactive power compensation,new energy grid connection,small size,light weight and so on.consequently,PET plays an significant role in the new energy smart grid.Since PET runs in a high power,high voltage environment,its input stage is mostly Modular Multilevel Converter(MMC).In this thesis,the operation principle of PET is analyzed based on topology in detail.Three states and six modes of PET input stage MMC sub modules are introduced.The implementation principle,advantages and disadvantages of Nearest Level Modulation(NLM)and Carrier Phase Shift SPWM(CPS-SPWM)are expounded.In order to illustrate the feasibility and stability of PET input stage MMC system,a double closed loop PI control system is built in Matlab,and the simulation analysis of reactive step response and power flow reversal response is carried out.When NLM is adopted in PET input stage MMC,the unbalanced voltages of sub modules cause phase-to-phase circulating currents,which increase device loss and switching capacity.Thus it is crucial to control capacitor voltage balance of MMC system sub modules.Based on switching functions of upper and lower bridge arms of the system,this thesis detailedly analyses the causes of sub modules capacitor voltage fluctuations and circulating currents.In order to suppress harmonic currents,a universal circulating current suppressor in MMC is added to the simulation system,and its rationality is verified by simulation.In practical application,there are a large number of per bridge arm sub modules in PET input stage MMC.To solve the problems of conventional voltage balancing sorting algorithm,such as high switching frequency,large amount of calculation and switching loss,an optimization method combining quick sorting algorithm and reordering factor is proposed.The simulation results show this optimization method can greatly reduce the sorting time complexity and switching frequency of sub modules.The sub modules switching frequency of PET input stage MMC affect system stability.In the past,the sub modules voltage balancing method only focused on reducing calculation and switching frequency of per sampling period of the system,but it could not accurately control the switching frequency of sub modules.The traditional PID control principle is simple and easy to understand,but three-phase currents are time variables,the fixed PID parameters are difficult to achieve a satisfactory control effect.Aiming at the above problems,this thesis puts forward voltage balancing method for the precise control of the average switching the frequency of sub modules.The fuzzy PID controller is used to adjust the impact factors of sub modules voltages.Then the sub modules voltages are sorted,so as to reduce the system calculation and device switching frequency.Meanwhile,the switching frequency can be accurately controlled.Finally,the sub modules voltage balancing control is realized.The simulation models based on traditional PID and fuzzy PID are built respectively,whose results demonstrate their effectiveness and superiority.