Small-signal Modeling And Stability Analysis Of Single-phase Power Electronic Transformers

Power electronic transformer is a new type of power transformer,which combines power electronic components and high frequency transformer.It can not only realize the function of traditional power transformers,but also control power two-way flow,improve power quality and suppress harmonics,so it has the ability to solve many new problems faced by modern power system.When the power electronic transformer is unstable,the stability of power system will be affected.In order to verify the stability of power electronic transformer,a stability analysis method based on Middlebrook Criterion is proposed.The research contents of this paper mainly include:Firstly,the topology,working principle and modal switching of single-phase PWM rectifier bridge,isolated DC-DC converter with high frequency transformer,and single phase PMM inverter are analyzed in detail.Then,the control strategies of three-stage power electronic transformer are studied.At the input stage,by constructing the virtual circuit,the double-closed-loop control of the voltage current in the PWM rectifier bridge based on single-phase d-q transform is used,so that the current can track the instruction current accurately and quickly,and realize the voltage-current with phase-to-phase.At the isolation stage,a dual phase shift control of constant low voltage side DC voltage is proposed to realize voltage stability and power balance at low voltage side.At the output stage,a double-closed-loop decoupling control of voltage current is used to improve the response speed and sinusoidal waveform of the control system.Finally,combined with these control strategies and topological structures,a small signal model is built for input,isolation and output stages of the power electronic transformer respectively.The transfer function is derived from the block diagram.Matlab /Simulink simulation is constructed to verify the feasibility and validity of corresponding control strategy for the various small signals models.The transfer function can be used to judge the stability of each subsystem.Based on the Middlebrook Criterion,a method is put forward to verify stability among the subsystems,thus guaranteeing the overall stability for the power electronic transformer.The validity of the proposed stability criterions is verified by simulating stability and instability.