Capacitor Voltage Balancing Algorithm For Voltage Step-down Type MMC Based On Full-bridge Sub-Module

MMC is a new topology of voltage source converter,which has obvious advantages in medium and high voltage applications due to its controllability and flexibility.It has been widely used in faraway and high capacity power transmission,partition controlled large power grid and clean energy integration.In this paper,the three-phase full-bridge MMC structure is used as the basic research topology.The full-bridge MMC control system,modulation method and sub-module capacitor voltage banancing control algorithm are carried out from three aspects:basic theoretical analysis,Matlab/Simulink simulation and experimental verification.Firstly,this paper analyzes the switching mode and capacitor harmonic of full-bridge sub-module.Also,the working principle of three-phase full-bridge MMC is analyzed.Then the mathematical model based on port voltage is analyzed and I_d/I_q three-phase active and reactive decoupling control strategy is adopted.At the same time,a two-module simulation model is built in Matlab\Simulink and the above basic theoretical analysis are fully verified.Secondly,this paper compares the various modulation strategies of MMC.For the negative-level output ability of full-bridge sub-module,this paper sets reasonably sub-module switching rules,and then combing with the NLM to calculate the sub-module input number.Thus,the sub-module is switched according to a certain rule,and the purpose of sinusoidal output port voltage and step-down operation under negative input circumstances are realized.Thirdly,for the problem of capacitor voltage fluctuation of full-bridge MMC,a sub-module capacitor voltage balancing control algorithm suitable for full-bridge MMC is studied based on the idea of half-bridge MMC sorting balance algorithm.The algorithm determines the sub-module input degree of freedom after judging the sub-module charging state,and finally the capacitor voltage between the sub-modules is basically the same.At the same time,this paper introduce the input state of the last moment to solve the problem of high switching frequency.Thus,the switching frequency and the switching loss of the converter are reduced.Finally,a multi-module three-phase full-bridge MMC model is built in Matlab/Simulink,which verifies the correctness of the voltage step-down NLM and sub-module capacitor voltage equalization control algorithm.At the same time,the correctness and effectiveness of the above research are further verified by the three-phase full-bridge MMC low-power experimental platform based on FPGA master-slave control.