The DC Voltage Balance And Power Control For PET Based On Cascaded H-Bridge

With the high permeability of random loads connected to the power grid,the power generation forms and voltage levels are different.The future distribution system aims to provide plug-and-play interfaces for distributed renewable energy and distributed energy storage devices.So power electronic transformer(PET)as a power switch and energy router is expected to develop.The three-stage PET can not only realize the comprehensive functions such as voltage conversion,dynamic reactive power compensation,adjustment of power factor and renewable energy access,but also realize electrical isolation by the middle DC link circuit.This paper analyzes the circuit model and mathematical model of a three-stage PET.The high-voltage stage of PET is a cascade H-bridge(CHB)structure an isolation stage of PET is dual active bridge structure(DAB).Factors such as PWM signal,switching loss,and system parameter differences among all the cells can lead to the DC output voltage unbalanced of CHB and the output power unbalanced of DAB module.To solve the DC voltage imbalance problem of CHB,this paper uses the duty cycle vector reconstruction technology based on ?? stationary coordinate control by comparing with the voltage balance strategy in dq coordinates.This method takes into account the control of voltage and reactive power and does not introduce a coupling effect to the double closed loop.In order to complete the design of the system control,the stability analysis of the high-voltage stage is also carried out.In order to study the power imbalance problem of the DAB,this paper analyzes the working state and mathematical model of the DAB,and adopts the coordinated control method between high-voltage stage and isolation stage.The method does not need to sample the isolation stage current,but introduces duty cycles of the high voltag stage based on the common duty cycle as feedback amounts to adjust the current to complete the power balance control.Therefore,the hardware cost can be reduced.In order to verify the feasibility of the control strategy used,this paper shows the related simulation experiments for the high-voltage stage and the isolation stage.Finally,based on the high-voltage stage experimental platform controlled by TMS320F28377D,transient and steady-state experiments and verification experiment of voltage balance controller are carried out.