| Metallized Film Capacitor(MFC)is the mainstream choice of capacitors in modular multilevel converter(MMC)valves for the advantages of high reliability and energy storage density.However,metallized film capacitors will suffer from corrosion failure of electrodes when applied to long-term voltage fluctuation.Conventional reliability assessment methods based on data statistics are difficult to describe the coupling relationship between capacitor aging rate and temperature.Accurate assessment of metallized film capacitor reliability is of academic significance and practical value for its safe operation and active maintenance.Taking the application condition of MMC valve as an example,this paper studies the multi-physical field modeling and reliability modeling of metallized film capacitors,and forms a software platform for reliability evaluation of MMC converter valve sub-models.The main contents of this paper are as follows:(1)In order to accurately analyze the electrothermal behavior of metallized film capacitors,the electrothermal physical field model of multi-component parallel MFC is established.Firstly,based on the actual structure of capacitor elements,a two-dimensional electric field model of capacitor elements is built to calculate the equivalent series resistance of capacitor elements and other parameters.Secondly,the temperature field models of capacitor elements and the multi-component parallel MFC are established by analyzing power loss with the parameters extracted from electric field.Finally,the electric-thermal multi-physical field model of metallized film capacitor is analyzed and verified.(2)Considering the coupling relationship between aging rate and temperature of capacitors,a failure physical model of metallized film capacitors considering aging effect is established.Firstly,based on the corrosion failure mechanism of capacitors,the failure rate model of MFC is established.Secondly,considering the coupling relationship between the equivalent series resistance as well as its aging rate and temperature,the aging failure model of MFC is established.Finally,taking equivalent series resistance as the characterization,the failure rate variation law of MFC under different aging degrees is analyzed.(3)Aiming at the problem that the fixed failure rate is often used to evaluate the reliability of MMC valves currently and it is difficult to reflect the failure mechanism of components,a reliability evaluation model of MMC valves based on MFC aging failure is proposed.Firstly,the electrothermal stress characteristics of key components such as IGBT and MFC under the application conditions of MMC converter valves are analyzed.Secondly,the reliability model of MMC commutation valve assembly is established based on the failure rate model of IGBT,MFC and other key components.Finally,the reliability of MMC converter valve sub-models under the rectifier and inverter operational conditions is evaluated and compared with the results without considering aging effect.The research results provide theoretical support for the design and reliability evaluation of metallized film capacitors,and also lay a foundation for the reliability analysis of MMC converter valve components. |
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