Research On Capacitance Voltage Fluctuation And Circulation Suppression Method Of Modular Multi-level Convert

With the development of HVDC technology,large capacity transformer ushered in the high speed development period,modular multilevel converter due to its unique modular cascade approach of maintenance is convenient,the advantages of easy expansion,make its developing rapidly in recent years,and in the flexible direct current transmission,power grid harmonic governance,and other fields has been widely applied.However,the dispersed capacitors in the submodules will produce harmonic circulation in practice,and the circulation composition will affect the voltage fluctuation of the module capacitance,and reduce the safety margin and economy of the converter.Firstly,the working principle of MMC submodule is analyzed,and its equivalent switching model is established.Then,the steady state mathematical model of MMC harmonic circulation is established by means of the average switching function,the composition of harmonic circulation is analyzed,the influence of harmonic circulation on each electrical quantity is studied,and the circulation suppression strategy based on PI regulator is studied,the controller parameters are designed,and the circulation model is verified by simulation.In view of the problem that the double frequency suppression of the circulation can not effectively reduce the capacitor voltage fluctuation of the submodule under the symmetric grid voltage,the capacitor voltage fluctuation model was established from the perspective of the energy fluctuation of the bridge arm,and the influence of the harmonic circulation on the fluctuation of the submodule was analyzed.A circulation suppressor was proposed to reduce the capacitor voltage fluctuation.The controller adopts a double-loop structure,the outer loop generates the predicted circulation value,the inner loop tracks the reference value of the actual circulation,and designs the relevant parameters,and then verifies the effectiveness of the proposed controller through simulation.Circulating harmonic complexity in asymmetric network under pressure,rise,lead to negative sequence circulation inhibition is no longer applicable,and can produce double frequency dc power fluctuations,to eliminate harmonic of circulation and the dc power fluctuations,design based on the principle of endometrium circulation and dc power fluctuations suppressor,and on the simulation platform to verify the effectiveness of the controller.This paper has 45 pictures,8 tables and 75 references.