Research On Dual Active Bridge DC-DC Converter And Its Power Distribution For DC Microgrid

DC microgrid has the advantage of efficiently accepting distributed power sources.With the rise of distributed generation technology,it has been widely used.Due to the intermittent and volatile nature of distributed power sources,energy storage units have been widely introduced into DC microgrid,which plays an important role in suppressing distributed power source power fluctuations,stabilizing bus voltage,improving system power supply reliability and power quality.This paper focuses on the research of converter control strategies and power distribution strategies when the energy storage cells are connected in parallel for an energy storage unit composed of energy storage batteries and dual active bridge DC-DC converters.Firstly,the basic operating principles of single phase shift control and extended phase shift control for dual active bridge DC-DC converters were analyzed,and the transmission power characteristics,current stress characteristics,and soft switching characteristics under the two control methods were derived.The two control methods were compared.Aiming at the disadvantages of slow dynamic response when using PI control for dual active bridge DC-DC converters and high current stress when using single phase shift control,a linear self disturbance rejection control current stress optimization control strategy is proposed under extended phase shift control.The Lagrange multiplier method is used to obtain the global optimal solution of the current stress,and the linear active disturbance rejection controller is introduced into the voltage loop,which is combined with the direct power control.The per unit value of the transmission power is output to the current stress optimization algorithm,and the corresponding phase shift angle is calculated to optimize the dynamic response and current stress simultaneously.Secondly,considering the output power allocation problem when multiple energy storage units operate in parallel in a DC microgrid,an improved droop control method based on SOC equalization is proposed.The SOC value of the energy storage unit is introduced into the constructed SOC function,and each energy storage unit allocates input or output power according to its own SOC value to achieve the power allocation of the energy storage unit.A stability analysis is conducted to verify the stability of the strategy.In addition,to compensate for the bus voltage offset caused by SOC droop control,a secondary control of the bus voltage is used to achieve power distribution while ensuring that the bus voltage is stable at the reference value.A DC microgrid simulation model was built in Matlab/Simulink,and the converter control strategy and energy storage unit power allocation strategy were simulated and analyzed,verifying the effectiveness of the proposed strategy.Finally,a low-power DC microgrid experimental platform composed of two 100 W dual active bridge DC-DC converters was designed,including component selection,transformer design,auxiliary circuit design,and software design.The experimental verification of the optimized control strategy for DAB converters proposed in Chapter 3was carried out,and the results showed that the proposed strategy can simultaneously improve the converter response speed and reduce current stress.The experimental verification of the power allocation strategy proposed in Chapter 4 shows that the two energy storage units can reasonably allocate input and output power according to the SOC value,proving the correctness of the proposed strategy.