Research On The Online Monitoring Method Of The State Parameters Of MPPF Capacitor In MMC Sub-module

Modular Multilevel Converter(MMC)is widely used in high-power and highvoltage applications.The high DC side voltage of the MMC is supported by a number of submodule capacitors.The reliability of sub-module capacitor is very important to the stable operation of MMC system.Metallized polypropylene film capacitors(MPPFCAPS)are widely used due to their good pressure and current resistance,long service life and other characteristics.In order to ensure the stable and reliable operation of MMC,it is necessary to add some preventive protection measures on the basis of fault emergency protection measures.In this paper,the MMC sub-module MPPF capacitor state parameter on-line monitoring system is designed and implemented.Firstly,in order to determine the monitoring state parameters,this paper analyzes the influence of the "self-healing" characteristic and the bad end contact of the MPPF capacitor in the long-term operation,and selects the capacitance value and the equivalent series resistance value as the monitoring state parameters.In order to provide a verification platform for the proposed state parameter estimation method,a 21-level2.1k VA MMC simulation platform is built,which includes MMC main circuit topology,modulation strategy,basic control,submodule capacitor voltage equalization control and circulation suppression control.Then,a model reference adaptive control(MRAC)based estimation method of MPPF capacitor state parameters is proposed,which takes the capacitance value and equivalent series resistance value of the capacitor as the estimation target.In this method,the capacitance value estimation model is established based on the current and voltage relationship of capacitor,and the equivalent series resistance value estimation model is established based on the active power balance relationship.The estimation results are obtained under the adjustment of the adaptive law.In this paper,the MMC simulation platform was used to verify the method.The simulation results show that the estimation method proposed in the paper can accurately estimate the capacitance value and the equivalent series resistance value.At last,this paper designs and implements the capacitor on-line monitoring system for the purpose of estimating the capacitor state parameters.Based on the technical requirements of the capacitor working condition simulation test platform,this paper uses Python language to write the monitoring software,and realizes the visualization of the user interface through the Qt framework.Then,the MPPF capacitor condition monitoring experiment is carried out by using the system.The experimental results show that the error of the monitoring result is less than 1%,and it can monitor the slight change of the capacitance value.Can accurately monitor the resistance of 0.5Ω.