Research On Bidirectional DC-DC Converter Of Power Electronic Transformer

Compared with traditional transformers,Power Electronic Transformer(PET)can not only change the voltage level,but also has the advantages of flexible control of output voltage and high power density.It is used in DC transmission and distribution,distributed energy systems and track traction.Other fields have broad application prospects.The high-frequency isolated bidirectional DC-DC converter is a key link in the power electronic transformer,and plays the role of electrical isolation and voltage conversion.In this thesis,the dual active high frequency isolated bidirectional DC-DC converter is studied.This thesis analyzes the working principle,working mode,switching state and power transfer characteristics of the dual active full-bridge DC-DC converter.The converter is studied under Single-Phase-Shift(SPS).Working characteristics.When the voltage gain is not 1,the current stress,the current effective value,the return power become larger,the soft switching range is reduced,the loss of the converter is increased,and the efficiency is lowered.In order to reduce the on-state loss and switching loss of the converter and improve the efficiency,the phase-shift optimization control strategy of the converter is studied.The local optimization strategy is mainly based on Extend-Phase-Shift(EPS)control.Compared with SPS control,EPS control adds an internal phase shift angle to the full bridge on the transformer side to improve the basic characteristics of the converter..Compared with the SPS control,the EPS control has lower current stress and return power,but still cannot achieve soft switching.Therefore,only the control strategy optimized with current is given,and the simulation verification is carried out.In order to make up for the insufficiency of soft switching in the local optimization strategy,a global optimization strategy based on Trio-Phase-Shift(TPS)control is proposed.TPS control is added to the entire bridge on both sides of the transformer.The phase shift angle allows the converter to achieve soft switching over the full power range and reduces current stress and return current.The current and soft switching range are optimized respectively.The optimization analysis process is simplified by the graphical analysis method,and the optimal phase shift angle relationship is derived.Finally,the optimization control strategy is verified by simulation and experiment.For the calculation of the segmentation of the inductor current,the model is not uniform.Fourier analysis of the voltage and inductor current at both ends of the transformer,using the fundamental wave to calculate and give a unified mathematical model,design an engineering software to predict the voltage and current waveform of the converter,the design of the actual converter It has a reference meaning.A 1kW experimental prototype was designed and developed.The experimental results of SPS control,local optimization control and global optimization control are verified.The experimental results verify the correctness of theory,simulation and optimization control strategy.