The Study Of Bidirectional Isolated DC/DC Converter In The Solid State Transformer

The research content in this dissertation is an important part of “Research on Nonlinear Control Method based Solid State Transformer”,which is Fund Program for the Scientific Activities of Selected Returned Overseas Professionals in Shanxi Province in 2014.With the addition of distributed energy and energy storage devices to the grid,solid state transformers(SST)are rising rapidly to meet the needs of smart grid.Solid State Transformer can not only realize the traditional functions of electric energy transmission and voltage conversion,but also realize the bidirectional energy flow and electrical isolation.It can flexibly control the amplitude,phase and frequency of the input and output voltage and current,and provide an interface for the direct connection of renewable energy and energy storage batteries.The key part of the three level solid state transformer to complete these functions is the bidirectional DC/DC converter(BDC).The traditional full bridge topology is difficult to make the bidirectional DC/DC converter with small circulation,low power consumption and large range of voltage gain.At the same time,the traditional phase shift control method is also difficult to make the bidirectional DC/DC with high steady state precision,fast dynamic response and strong robustness.In view of the above problems,this paper studies and analyzes the topology of the bidirectional DC/DC converter,establishes a mathematical model,designs the frequency conversion control method and the bidirectional energy flow method,and verifies its effectiveness and accuracy by simulations and experiments.The specific research contents are as follows:The research background and development status of solid-state transformers are described.The topology,existing control strategies and applications of bidirectional DC/DC converters are described.For traditional bidirectional DC/DC converters,it is difficult to combine thecharacteristics of small current,small power consumption and large voltage gain range with the traditional topology.The CLLC resonant topology is selected,and the operating principle of the converter is analyzed in detail.The relationship between switch frequency and resonant frequency,the resonant characteristics of the converter and the input impedance of the converter are analyzed.The small-signal modeling method and its advantages and disadvantages of the existing bidirectional DC/DC converter are described.The working principle of the extended function description(EFD)is introduced in detail.The system control variables are selected and bidirectional DC/DC is established.The small signal model of the converter analyzes the stability of the open-loop system based on the output-control transfer function.According to the technical index of the system,the power circuit of the bidirectional DC/DC converter in the solid state transformer is designed,including the design of the high frequency transformer,the selection of the power switch tube,the construction of the resonant unit,the calculation of the high and low voltage DC capacitors,the design of the driving control circuit and so on.According to the technical specifications of the system,the power circuit of the bidirectional DC/DC converter in the solid state transformer is designed,which mainly includes the design of the high frequency transformer,the selection of the power switch tube,the design of the resonant unit,the design of the high and low voltage DC capacitors,and the sampling circuit.The design and selection of power supply modules and drive circuits.The conventional control method has better control effect on the bidirectional full-bridge DC/DC converter,but it is not suitable for the problem of the resonant DC/DC converter.A method of variable frequency control of CLLC resonant DC/DC converter is proposed.This method can realize voltage to frequency conversion and maintain stable output voltage through voltage controlled oscillator.The transfer function of the sample conditioning circuit is calculated.Based on the operating characteristics of the voltage-controlledoscillator,the transfer function is obtained,the compensation controller is designed,and the stability of the system's closed-loop operation is analyzed.Aiming at the problem of large error caused by the current-negative commutation control method when detecting smaller currents,a commutation control strategy based on current positive and negative and capacitor voltage hysteresis is proposed to complete the bidirectional flow of system energy.control.Based on the theoretical study of bi-directional DC/DC converter in solid transformer,a bi-directional CLLC resonant DC/DC converter experiment platform based on semi-physical simulation equipment dSPACE is built.The control system is introduced,the related system parameters and control parameters are listed,the system simulations and experiments are carried out,and the results show that:(1)The system adopts the CLLC topology,and when the system operates at the resonant frequency,resonance occurs in the system,ZVS is realized in the rectifier stage switch,and the switching loss is reduced;(2)The system adopts soft-start control method to suppress The start-up current is impacted,and the bi-directional DC/DC converter starts smoothly;(3)The system adopts the frequency conversion control method to realize the stable control of the DC voltage,which improves the steady-state reading and dynamic response speed of the system,and it also has better performance.Robust performance.(4)The system adopts the commutation control method based on current polarity and capacitive voltage hysteresis loop,which realizes the bidirectional flow of energy,fast switching speed and small error.