Analysis Of Control Method Of Solid State Transformer Cascade System Based On Energy-power Control

The single-phase three-stage(rectifier stage,dual active bridge(DAB)stage and inverter stage)solid state transformer(SST)is studied in this paper.In order to improve the performance of steady-state and dynamic-state,especially the response speed and steady-state error of the current and the recovery time and ripple voltage of the voltage,control strategies are studied based on the analysis of constant power load of cascade system.Focusing on the application of predictive control and energy-power control in the single-phase SST,synergetic control for the whole single-phase solid state transformer is studied to optimize the performance of steady-state and dynamic state,form inner current loop to outer voltage loop,from single converter to multastage converter.First,the switching model,average model and small signal model of the single SST are developed.The SST controller is designed based on the small singal model bode plot.The average and swtiching mode simulation are implemented in Matlab/Simulink to verify the effectiveness of the controllers and the features of single SST.Second,dq frame integral predictive current control(IPCC)is proposed to achieve faster dynamic response and zero steady-state error of the current of rectifier stage.The PI controller has only small signal stability.Predictive current control can quickly track the reference current signal.However,the influence of inductance parameters and control delay can result in increasing current harmonics and steady-state error.The proposed IPCC deduced the incremental model of the rectifier stage,defined the weithted current error to embed the integral into the IPCC.The proposed IPCC can eliminate the steady-state errors the current control system and improve its closed-loop performance.It is much easier to perform analysis and design controllers for rectifier stage become all time-varying state variables of rectifier stage becom DC time-invariant.Third,An energy-power control strategy of DAB stage based on power feedforward control is presented to improve the response of the output voltage under the constant powe load and uncertainty of DC load.A DC power estimation method based on sliding mode observer is proposed,which improve the dynamic response of DAB stage under step change of DC load.Active power feedforward and instantaneous power feedforward of inverter stage are recommended to improve the dynamic response of DAB stage under step change of the load of inverter stage.Fourth,An energy-power control strategy of rectifier stage based on power feedforward control is presented to improve the response of the output voltage under step change of the power of DAB stage.The estimitated DC power of DAB stage feedforward and the active power of the inverter stage are included in the power control of rectifier stage to improve the dynamic response of rectifier stage under step change of DC load of DAB stage and AC load of inverter stage.A power ripple control based on instaneous powe feedforward and power synchronization method is designed to minimize the DC bus voltage ripple.Finally,the single-phase SST hardware prototype is designed and implemented to verify the proposed strageties above.