Research On Control Strategy Of Cascaded H-bridge Power Electronic Transformer

Facing to the severe environment situation,the application of new energy has become an urgent problem.Power electronic transformer(PET)comes into being by the introduction of power electronic conversion technology on the basis of traditional power transformer.The PET has some advantages: it can flexibly control the power flow,isolate fault and harmonic,also improve the power quality.PET has broad application prospects in solving the reliability of grid connection of distributed generation such as wind energy and hydroenergy.Therefore,in order to show the function of PET better,the study of control strategy is of great significance.Cascaded H-bridge(CHB)PET is a widely used PET,consisting of rectifier stage,isolation stage and inverter stage.Most of the research on the grid connection of the rectifier stage is based on the ideal grid,while the voltage of the distribution network will be unbalanced due to load imbalance,short-circuit fault and other reasons during the actual operation.To solve this problem,based on the traditional virtual synchronous generator(VSG)control strategy,an improved VSG technology when the grid voltage is unbalanced is adopted to achieve multi-objective control.When the output side of the inverter stage of PET has unbalanced load,it will cause serious unbalance of its output voltage.It has a great impact on the main network.Therefore,in order to solve this problem,a VSG control strategy of inverter stage with carrying unbalanced load is adopted.The specific control strategies for rectifier stage,isolation stage and inverter stage are as follows.For rectifier stage,an improved VSG control strategy is mainly applied.Firstly,under dq coordinate system,VSG balance current control is employed to obtain reference positive-sequence current commands.Secondly,according to grid voltage unbalanced parameters,positive and negative sequence current commands of grid-connected inverter are produced for different control objectives.Thirdly,positive and negative sequence current commands are tracked separately to get positive and negative sequence voltage modulation signals.Also,both carrier phase-shifted SPWM(CPS-SPWM)and the voltage-sharing of each submodule and phase are applied.For inverter stage,since the problem that the power contains double frequency component under the unbalanced load,based on traditional VSG control strategy,the improved VSG control strategy with the voltage loop based on proportional integral(PI)and proportion resonant(PR)is applied.The PI controller is used to realize the zero-error-control of the DC component in the d-q axis voltage component.The PR controller is used to realize the zero-error-control of the second harmonic AC component.Besides,for isolation stage,it is controlled in open loop control.Square waves with 50% duty cycle are on both sides of high and low voltage.In the Matlab/Simulink simulation environment,the simulation models of rectifier stage,isolation stage,inverter stage and the whole system are built.Simulation results are described below.The improved VSG control strategy under the unbalance power grid produces balanced three-phase current output and mitigates active or reactive power fluctuations separately.The open-loop control strategy can realize the constant output of low-voltage DC voltage and it is good at power transmission.The improved VSG control strategy with carrying unbalanced load can maintain the stability of frequency and voltage,eliminate the fluctuation of second harmonic,and output the stable three-phase voltage after the sudden increase of load in inverter stage.Its effect is promoted than traditional VSG control strategy.The simulation results verify correctness and effectiveness of the established control strategy.