Research On Dynamic Characteristics And Control Strategy Of The Hybrid Modular DC Solid State Transformer

In recent years,with the rapid development of distributed power generation and DC loads,the research work on DC distribution networks has become a current research hotspot.As an interface device that connects DC grids with different voltage levels,DC solid-state transformers play an important role in voltage level conversion,fault isolation,and power voltage control.Traditional DC solid-state transformers mainly adopt MMC type or modular cascade structure,and their research content covers modeling,control strategy optimization and efficiency improvement.Different from previous research,the hybrid modular DC solid-state transformer(HMDCSST)uses sub-modules of two different topologies with phase-shift dual active bridge(PS-DAB)and series resonant dual active bridge(SR-DAB)to constitute the ISOP cascaded DC solid-state converter with a new hybrid topology.Therefore,the HMDCSST has high control flexibility and topology compatibility while improving efficiency.This paper mainly studies its dynamic characteristics and control strategies.Firstly,the HMDCSST was modeled with the large-signal average method,based on which a small-signal linear model was established.The detailed switching model was built using Matlab/Simulink platform,and the accuracy of the small signal model derived in this paper is verified by comparing the unit step response and the frequency response characteristic curve with the detailed switching model.Then,for different application scenarios,this paper proposes a controller structure that can smoothly switch between constant power control and constant voltage control,and designs the controller for both controllers.The effects of filter reactance and filter capacitor on the dynamic characteristics and stability of the system were studied.In order to suppress the influence of input voltage and load power disturbance on the system output voltage and improve the dynamic response speed of the system,in this paper,taking HMDCSST output side as a purely resistive load to work in constant voltage control mode as an example,the predictive fast response control strategy was proposed.Finally,the efficiency and loss breakdown of HMDCSST were measured by building a low-power experimental prototype.The experimental results show that SR-DAB has ZVS and ZCS characteristics in the full power range,and the switching loss and reflow power are basically zero,which is the main reason why HMDCSST can improve the overall conversion efficiency of the system.Compared with pure PS-DAB DC solid-state transformer experiments,the effectiveness of HMDCSST in improving efficiency is verified.The effectiveness was also verified by the constant power and constant voltage control experiment.Compared with traditional PI control,the experimental results show that the fast response control strategy proposed in this paper has faster response speed.