Research On Control Strategy Of Power Electronic Transformer

In the increasingly severe energy crisis,environmental protection problems continue to appear today,the development of various new energy to improve the structure of power generation has been the general trend.In the process of development and utilization of all kinds of new energy,the problem of distributed energy grid connection cannot be avoided.Due to the difference of voltage grade and generation form,a large number of AC and DC interfaces with different voltage grade are required when distributed energy is connected to the grid,which poses a challenge to the traditional power grid structure.Traditional power transformers have been unable to meet the requirements of smart grid.A new type of transformer,power electronic transformer PET,which combines power electronics technology,emerges at the historical moment.On the basis of traditional power transformers,PET has added such functions as unit factor operation,fault isolation,and flexible AC/DC port.It has broad application prospects in smart power grid,locomotive traction system,new energy grid connection and other aspects.In order to realize the above functions well,not only complex topological structure is needed for power electronic transformers,but also appropriate control strategy is crucial.Firstly,this paper introduces the background and significance of the research,expounds the development history of power electronic transformers and the evolution process of the topology structure,analyzes the advantages and disadvantages of various power electronic transformers,and introduces the topological structure of modular cascade power electronic transformers that this paper focuses on.Secondly,the working principle and PWM modulation mode of the input stage of the power electronic transformer are introduced,and the synovial variable control idea is also introduced.An input stage control strategy based on the synovial variable control is proposed,and its effectiveness is verified by simulation.Then,the harmonic analysis of the hybrid PWM technology is carried out,and the switching function rotation method is adopted to combine the hybrid PWM technology with synovial variable control.The simulation results show that the hybrid PWM technology can achieve voltage balance between modules well.Finally this paper introduces the isolation level of single phase and double phase shift control,analysis of the two transmission power and power flow,put forward a kind of dual phase shifting control strategy based on minimum reflux power,through the power electronic transformer input stage and the isolation level of the whole simulation,validate the control strategy on power balance control and reduce reflux power is effective.