Key Technologies Research On Modular High Frequency Chain DC Solid State Transformer

Along with the gradual implementation of the "double carbon" goal,the requirement of distributed energy efficiency increases continuously.Hence,DC transmission and distribution and DC microgrid will become more important.DC bus with different voltage levels in DC system need power converters for power exchange,energy storage system and DC load also need to be connected to DC bus of different voltage levels through power converter.DC solid-state transformer(DCSST)will be the key equipment,which can realize DC bus interconnection of different voltage levels,and the power transmission management of distributed energy,energy storage system and DC load.Due to the limitation of voltage and power level of power electronic power devices,it is difficult to meet the requirements of high-voltage and high-power DC system by a single module power converter.Therefore,high voltage and large capacity DCSST based on modularization is the inevitable trend of future development.At present,the DC/DC link of modular DCSST high-frequency chain mainly includes dual active bridge(DAB)converter and bidirectional series resonant converter(CLLLC),which have been widely applied.This thesis mainly studies the topology and control strategy of modular high-frequency link DCSST based on DAB converter and CLLLC converter.The main contents and innovations are shown below:(1)Since there is no normalized analysis of DAB converter model in the existing literature,which considering the conduction,inductance power loss and input-output filter circuit at the same time.A new mathematical modeling method of DAB converter is proposed.The model is modeled in TPS modulation mode,considering conduction,inductance,transformer loss and input-output filter circuit.It is suitable for all modulation modes,which can solve the problem of single application range of the model,and effectively improves the accuracy of the model.In this study,a detailed model for predicting large signal transients is established,the small signal model and output control transfer function are deduced,the small signal analysis in frequency domain is carried out,and the complete mathematical description and simulation experimental parameters of the mathematical model are given.The open-loop dynamic response,frequency domain analysis and simulation are consistent with the experimental results,which verifies that the mathematical model of DAB converter in this thesis has high accuracy.The mathematical model of high-precision DAB converter is helpful to improve the robustness and generalization ability of modular high-frequency link DCSST control system.(2)In order to solve the problem of unbalanced voltage and low fault tolerance of multi module series in the front stage of modular DCSST,the voltage equalization control strategy generally needs to be coupled with the control strategy of high-frequency DC/DC link.A new DCSST topology based on bidirectional voltage balance converter(VBC)is proposed for simplification in this thesis.According to the innovative idea of voltage self-balancing characteristics of the converter,the multi module series voltage balancing strategy in the front stage of modular DCSST can be omitted,and the whole system control strategy is simplified.In addition,when some modules fail,the failed modules can be removed without adding auxiliary equipment such as bypass switch,and the input voltage of each module is kept balanced,which improves the fault handling ability and fault tolerance of DCSST.The switching characteristics of VBC are analyzed and compared with other converters,and the superiority of the VBC is well proved from the aspect of switching efficiency and switching frequency.The experimental results verify the feasibility of the proposed DCSST topology.(3)For the problem of low efficiency of bidirectional soft switching characteristics of DCSST high-frequency link DC/DC link CLLLC converter when the input voltage is wide,a new DCSST topology based on the cascaded sub modules of Buck/Boost converter and CLLLC converter is proposed by ultilizing the respective advantages of Buck/Boost converter and CLLLC converter.The DCSST topology can effectively adapt to the wide input and output application system.The CLLLC converter always works at the best working point,which improves the overall efficiency of the full power range of the system.Furthermore,this converter solves the low conversion efficiency problem caused by the deviation of the switching frequency from the quasi-resonant working state.Aiming at the novel proposed DCSST topology,the upwarping control strategy is adopted to realize the natural voltage sharing control of DCSST and simplify the complexity of system control.Simulation and experimental results verify the feasibility of the proposed DCSST topology and control strategy.