Study On Simulation Model Of MMC Pre-charging And The Capacitor Voltage Balancing Method Among Sub-module

The normal start of MMC is the prerequisite for the reliable operation of the whole project.However,in the process of precharging,the capacitor voltage control algorithm has not been run because the converter is locked,and the capacitance voltage of each module is not controlled.When the consistency of capacitor voltage is poor,it is very easy to bypass the sub module because of over and under voltage protection,which affects the normal operation of the system.In this thesis,±350kV flexible DC converter valve is taken as the research object.Aiming at the phenomenon that the capacitor voltage of the precharging process is inconsistent,the equivalent circuit model,parasitic capacitance parameter extraction,simulation and speed up method,simulation verification and voltage equalization method are studied.First of all,this thesis introduces the electrical design of the ±350kV DC converter valve tower in detail,and explains the relative position of the conductor of the shield shield of the valve tower and the potential relationship.The valve charging test content,analysis of the measured uncontrolled charging voltage distribution curve reaches a steady state,and combined with the experimental curve,analyzes the influence of different factors on the capacitor voltage,and puts forward some ideas of establishing the equivalent circuit model of MMC precharge process.Then,the thesis builds the MMC pre charge equivalent circuit model,the model considering the parasitic capacitance between the conductor structure parameters of the actual valve tower,as well as the influence of sub module to supply the equivalent resistance parameters of sub module voltage equalization,port capacitance extraction method.To solve the problem of low computational efficiency in the number of modules,the equivalent method of MMC single leg and the fitting method of charging curve are proposed.The simulation and validation of the model and speed up method are carried out by PSCAD/EMTDC simulation software.The validity and reliability of the proposed model and speed increasing method are verified,and the equilibrium characteristics and distribution rules of sub module capacitor voltage are analyzed.Finally,based on the proposed equivalent circuit model and simulation speed raising method,the voltage balancing characteristic of pre charged electronic module capacitor is simulated and forecasted for a ±800kV flexible DC project that has not yet been put into operation.On this basis,further analyzing the valve hall layout,full bridge sub modules ratio,valve tower parasitic capacitance,take effect of equivalent resistance,pressure resistance,power supply module sub module capacitance of the capacitor voltage balancing characteristics,provide a reference for engineering design.