Research On Parameter Robustness And Control Strategy Of DC Transformer For DC Distribution System

The DC microgrid is an effective way for the large-scale development and consumption of new energy power generation.The DC transformer is the core equipment of the DC microgrid.The resonant DC/DC converter based on dual active bridge circuits and high-frequency transformers has become a typical topology for DC transformers.It meets the requirements of DC transformer with electrical isolation,bidirectional power flow,high power density,high efficiency and high voltage conversion ratio.Based on the topology of bidirectional CLLC resonant isolated DC/DC converter,this article focuses on the research work of constant voltage ratio control strategy,soft start control strategy and the related parameters’ design for the DC transformer.Firstly,the resonant condition working mode and operating mechanism of the CLLC resonant converter are both analyzed in this thesis.The relationship equation between the voltage conversion ratio of the DC transformer and the CLLC parameters is derivated by the fundamental wave equivalent analysis method.The effects of inductance ratio,resonant circuit quality factor and degree of deviation from resonant point on voltage ratio are studied.Taking account to the principle of the least impact of parameter varibale on the voltage conversion ratio,the resonant circuit parameters are optimized and selected.It improves the robustness of the voltage conversion ratio of the DC transformer to the circuit component parameters.Secondly,a variable frequency constant voltage ratio control strategy is proposed by considering the manufacturing deviation,aging and environmental influences of the resonance circuit CLLC parameters and high-frequency transformer parameters which cause the resonant converter to deviate from the resonance state.The frequency of transistors is adjusted in real time according to the actual voltage ratio.In combination with the proposed control method,on the one hand,it could achieve the constant voltage conversion ratio.On the other hand,it makes the switching frequency close to the natural resonance frequency.By this way,the DC transformaer realizes the automatical resonant frequency tracking,stabilizes the voltage conversion ratio and improves the operating efficiency.The simulatated results have verified the effectiveness of the resonant tracking strategy.Lastly,to solve the problem of large inrush current at the starting point of the DC transformer,the characteristics of variable frequency voltage regulation method and phase shift voltage regulation method for the DC transformer are analyzed and compared in this thesis.The optimal variation curve of phase-shifting Angle with time is given under the condition of starting current limitation,and the soft starting of DC transformer is realized by the optimal combination of frequency conversion and phase-shifting.The effectiveness of the DC transformer soft start strategy has been validated by the simulated results.A set of DC transformer experimental platform based on CLLC resonant DC/DC converter topology is contructed.The automatic resonant frequency tracking and soft start experiments are carried out.The experimental results show that the resonant frequency automatic tracking control strategy based on the actual voltage ratio feedback adjustment has the effective performance for the DC transformer.What’s more,the soft start strategy based on variable frequency and phase shift hybrid voltage regulation is feasible.The experimental results have a good match to the theory analysis.