Research On Topology And Control Of Two-stage PET Based On Push-pull Isolation Transform

With the gradual reduction of fossil energy and the increasing environmental pollution,the development of renewable energy such as solar energy and wind energy has attracted more and more attention.However,due to the dispersibility of renewable energy,traditional power equipment and power grid structures have gradually shown their defects in accepting distributed renewable sources.In recent years,DC grid technology has become the priority choice to solve the problem of renewable energy operated in grid-connected mode.Power Electronic Transformer(PET),as a power conversion interface,is the key equipment in DC grid.This thesis introduces the advantages and development of DC distribution system and PET.After analyzing the characteristics of PET using in the DC distribution network,a twostage structure of PET is proposed,which can be used to connect AC network and DC distribution network.In order to solve the problems of large number of switches and the limited of soft switching in traditional DAB,this thesis uses isolated transformation based on push-pull high frequency transformer.In the open-loop control mode,the leakage inductance of the high-frequency transformer and the resonant capacitor form a series resonant circuit,and isolation stage is easier to implement the soft-switching.Under the background of energy transmission from AC network to DC distribution network,the working process and modulation principle of cascaded H bridge in AC/DC stage are introduced in detail.The operate mode of isolation stage circuit and the realization process of soft-switching are analyzed as well.The commonly used PET port control technology is introduced,and the average mathematical models of AC and DC sides of PET are established.An unified control strategy of AC/DC and isolation stages is adopted to simplify the complexity of traditional PET control.According to the average model,the parameters of the controller are analyzed and designed,and the simulation process is given for the theoretical analysis.In order to make PET better used in actual distribution networks,this thesis presents a fault location scheme,which can quickly locate the fault location when the distribution line containing PET.The exit scheme of PET is given when the post-stage circuit of PET fails.In addition,a soft start scheme of PET is analyzed and verified by simulation.The experimental prototype of the two-stage PET was built,and the advantage of carrier phase shift modulation in the AC/DC stage was verified.The soft-switching implementation process of all the switches in the isolation stage was proved as well.