Isolated Multi-port DC-DC Converter Based On Bifurcate MMC Structure

With the world's energy shortages and environmental pollution problems increasing,distributed energy technologies have developed rapidly.Compared to traditional AC grids,DC grids are easier to accept distributed energy sources.Besides,the development and maturity of high-power switching devices and the proposal of new topology have led to the rapid development of high-voltage direct current(HVDC)system.In addition to improving the stability of the power grid and eliminating the need for reactive power compensation,under the same conditions,high-voltage direct current system can transmit more power.When constructing the HVDC systems,a high-voltage DC-DC converter is an important part of it.Modular multilevel converters(MMCs)are widely used in high-voltage DC-DC converters due to their high efficiency,modularity,and redundancy.At present,the research of domestic and foreign research institutions is mainly aimed at the application of modular multilevel converters in double-terminal DC power grids,that is,the converter is suitable for connecting two DC terminals.However,in the HVDC systems,not only simple two-terminal interconnection is required,but also multi-terminal interconnection is required.Facing the occasion of multi-terminal interconnection,the existing solutions often adopt two back-to-back modular multilevel converters or use multi-winding transformers.These solutions have higher costs and more complicated designs.In order to solve the multi-terminal interconnection problem,based on the traditional back-to-back modular multilevel converter,this paper proposes a new isolated multi-port DC-DC converter based on bifurcate MMC Structure.The typical application of this topology is the interconnection of three DC terminals.Through proper expansion,more DC terminal interconnections can be achieved.The advantage of this converter is that it can reduce the number of sub-modules and eliminate the design of multi-winding transformers.The main content of this paper is as follows:First,the basic topology and working principle of isolated multi-port DC-DC converter based on bifurcate MMC Structure are described.Then,the mathematical model of voltage,current and power of the topology under sine wave modulation is analyzed.According to the mathematical model,the corresponding power closed-loop control strategy is proposed,and the corresponding voltage balance method is proposed for the arm.Corresponding simulation models were established in Matlab/Simulink,and the feasibility verification of the topology and control strategy was performed.Then,the mathematical model of voltage,current and power of the topology under two-level modulation is analyzed.According to the mathematical model,the corresponding power closed-loop control strategy is proposed,and the corresponding closed-loop power control scheme is designed.The corresponding voltage balance method is proposed for the arm.The advantages and disadvantages of sine wave modulation and two-level modulation are compared.Finally,an experimental prototype was built to verify the feasibility and the effectiveness of the control method of the isolated multi-port DC-DC converter based on bifurcate MMC Structure under two-level modulation.Experimental results prove the feasibility of the proposed topology and the effectiveness of the two-level modulation method.