| In DC transformer system structure,the multi-module combination structure has a broad application prospect because of its modular design,reduced component stress,and easy maintenance and expansion.In this paper,the CLLLC converter with electrical isolation,bi-directional high efficiency operation and high power density characteristics is used to cascade Buck/Boost converters,treating two converters as a sub-module,and the input and output sides of each sub-module are combined in series and parallel for the bi-directional DC resonant multi-converter system studied in this paper.When the system is boosted,it can be considered as an input-parallel output-series(IPOS)system,and when the system is bucked,it can be considered as an input-series output-parallel(ISOP)system.The main research of this paper is as follows.1.Analyze the working principle of CLLLC converter,establish the equivalent model of converter,analyze the influence of primary and secondary resonance parameters on the gain curve and forward and reverse operating consistency of converter,and design the converter parameters on this basis.2.Aiming at the problems caused by the frequency control of CLLLC converter,the DC Transformer(DCX)characteristics of the converter are made by parameter design,and the voltage regulation function is realized by cascading Buck/Boost converters,and the bi-directional DC resonant multi-converter system is formed by this cascaded converter as a sub-module.3.It is pointed out that for IPOS and ISOP systems,achieving voltage and current equalization is the key to system stability,and the relationship between voltage and current equalization control is further clarified based on the analysis of voltage and current equalization,and the stability of using voltage and current equalization control strategies is analyzed for both systems.4.The small-signal models of Buck/Boost and CLLLC converters are established,and the PI controller parameters are designed based on them.The simulation model of bi-directional DC resonant multi-converter system is built,and the three-loop control strategy is used to realize the voltage and current equalization of IPOS and ISOP systems.5.For the IPOS and ISOP systems,the natural voltage and current equalization of each module is realized by using the interconnectionless control strategy.A small-signal model of the system is established,and the values of the system control parameters are determined according to the generalized root trajectory.A simulation model is built and the simulation results show the correctness and stability of the interconnectionless control strategy.The effectiveness of the IPOS and ISOP system control strategies is verified experimentally. |
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