Voltage And Power Balance Control Method Of Cascaded Power Electronic Transformer

Since the 21 st century,the energy crisis and environmental problems have become increasingly prominent.Optimizing the energy structure and increasing the proportion of new energy sources has become the mainstream development trend in the world.Due to the different voltage levels and forms of electric energy generated by new energy sources,a series of problems will arise during grid connection.For this reason,a new type of intelligent power conversion equipment,Power Electronic Transformer(PET),which has different voltage levels of AC and DC interfaces and can provide grid-connected services for different types of new energy power generation systems has emerged.In addition to the voltage level conversion function of the traditional transformer,PET can control the transmission direction of electric energy through power electronic conversion technology,realize unit power factor operation,and electrically isolate the user from the grid side,so that the power distribution system is more intelligent.This paper first introduces the evolution of PET,analyzes the advantages and disadvantages of typical PET topology in each period,and clarifies that the module cascade PET is the core research object of this paper.The working principle and modulation mode of PET rectifier and isolation stages are analyzed.The small-signal model of H-bridge rectifier is built in single-phase dq coordinate system.The double-closed control based on single-phase dq decoupling is designed to realize the rectifier stage.The complete control,the working mode and power transmission characteristics of the isolation stage under the traditional phase shift control are analyzed.The return power characteristics of the isolation stage under different transformation ratios are analyzed,and the rectifier and isolation stages are simulated.Secondly,the influence of the DC side voltage imbalance of the rectifier stage and the unbalanced transmission power of each module of the isolation stage on the safe operation of the power electronic transformer is analyzed.The three typical power control of the isolation stage are introduced in detail.An average power control based on output current feedback is proposed.The output current of the DAB module is used as the feedback amount to adjust the shift ratio to achieve the output power balance.The simulation model is built and the effectiveness of the method is verified.Finally,a voltage equalization power control strategy based on minimum power reflow is proposed for the voltage imbalance of the rectification stage,thetransmission power imbalance of the isolation stage and the power reflow of the DAB module.For this control,the power transmitted by each DAB module is utilized.Adjusting the shift between the front and rear bridges of the DAB module as a feedback amount is achieved compared to the internal shift of the front axle.Even when the parameters of each DAB module are unbalanced,this control strategy can effectively reduce the power reflow and improve the efficiency of the converter while ensuring power balance.The feasibility and effectiveness of this control strategy are verified by simulation and experiments.