Research On Operation Control Technology Of Modular Multilevel Converter

With the development of new energy power generation and power grid,new requirements for large capacity long-distance transmission have been put forward.The appearance of Modular Multilevel Converter(MMC)promotes the development of HVDC.It is the priority topology structure of flexible HVDC system,which has great flexibility,expansibility and output characteristics.And to deal with the unbalanced conditions and failures of the power grid can also be well dealt with.Therefore,it is of great practical significance and engineering value to study the key control technology of MMC converter.This paper first introduces the topology structure of MMC main circuit and its submodules,and then analyzes its working principle and mathematical model.Several modulation methods that are often used in MMC are studied.We focus on analyzing Carrier Phase Shifted SPWM(CPS-SPWM)modulation.Through theoretical analysis,we discussed the situation where MMC shifted N+1 level and 2N+1 level of phase voltage,and their effects on converter characteristics.Then a circuit model is built in the software for simulation analysis,and the results verify the correctness of the theoretical analysis.Next,the basic operation control of the MMC converter system is studied,including direct current control,submodule voltage equalization control and circulation suppression.The generation mechanism of the internal circulation of the converter is analyzed.Combining the characteristics of the internal circulation and the trap,a scheme is designed to extract the double frequency component of the circulation for suppression,which can effectively reduce the content of the internal circulation.Finally,in order to verify the effectiveness of the control strategy,the relevant circuit model is built in,and the simulation results prove the feasibility and effectiveness of the control strategy.Then the problem of capacitor voltage ripple of the submodules is studied,the analytical expression of the capacitor voltage of the submodules is derived,the components and characteristics of the capacitor voltage ripple of the submodules are analyzed,and the power fluctuation caused by the fundamental frequency component of the current of the bridge arm and the fundamental frequency component of the voltage of the bridge arm is concluded from the perspective of the power of the bridge arm.Therefore,a ripple suppression strategy based on power decoupling compensation is proposed to eliminate the power fluctuation of the bridge arm to suppress the capacitor voltage ripple of the submodules,and has a certain effect on the circulation,which improves the current characteristics of the bridge arm and reduces the switching stress on the bridge arm and the system loss.The correctness of the scheme is verified by simulation.Finally,the design of experimental platform and experimental scheme is introduced.The main circuit is built in the Hardware in the Loop(HIL)experimental platform,and the control algorithm and modulation strategy are realized through DSP and FPGA chip.The experimental platform was used to verify the correctness of the theoretical analysis and the effectiveness of the proposed scheme.