Research On Control Strategy For High-frequency DC Link Of AC/DC Power Electronic Transformer

Power Electronics transformers(PETs)are widely used in the fields of intelligent power supply for rail transportation due to its advantages such as flexible energy regulation,high power density,reactive power compensation and harmonic control.In PETs,the Dual Active Bridge(DAB)topology is usually employed as the high-frequency DC link,which generate backflow power by phase-shift control.In order to suppress the backflow power,control strategies such as extended-phase-shift(EPS)control and double-phase-shift(DPS)control are usually employed in DAB.However,the second-order ripple power introduced by the PWM rectifier on the AC grid side of PET makes the input voltage of DAB has low-frequency ripple components.This phenomenon will weaken the effect of the phase-shift control on the backflow power suppression.In this paper,in terms of second-order ripple and backflow power,the characteristics of backflow power and the control strategy of DAB are analyzed.The research contents and innovations of this paper are as follows:Firstly,the topology and working principle of DAB are introduced.Then,the working principle and working mode of single-phase-shift control and extended-phase-shift control are also introduced.Based on the mathematical model,the current formula and transmission power characteristic formula of single-phase-shift control and extended-phase-shift control are deduced,and the backflow power characteristics of the two control strategies are finally analyzed.Secondly,to address the problem of low-frequency ripple compoments in the DAB input voltage,the reason for the low-frequency pulsation and its effect on the single phase-shift(SPS)control and EPS control are firstly analyzed.To reduce the influence of ripple voltage on the suppression of backflow power,the three-phase AC/DC PET based on power channel,namely PC-PET(power channel based PET)is adopted.and the control strategy based on power decoupling and EPS cooperative control is proposed.Whatsmore,in view of the overcurrent problem existing in the high-frequency DC link of PET at the start-up time,the working principle of soft-start is analyzed theoretically,and the soft-start control method based on peak current control is proposed.Finally,the feasibility of the control strategy is preliminarily verified by the simulation model.Finally,the experimental platform of PC-PET is built,and the design methods of the platform are introduced respectively.