Capacitive Reliability Evaluation And Packaging Optimization Of IGBT Module In Modular Multilevel Converter

In the context of "carbon neutrality",State Grid proposes to vigorously develop flexible DC power transmission and distributed energy projects.Modular Multilevel Converter(MMC)is the most commonly used topology mode in flexible DC power transmission because of its advantages of smooth output,easy replacement and low transmission loss.At present,it is also developing rapidly in the field of DC power distribution network.Capacitors and IGBT modules,which are most prone to failure in the converter,affect the reliability of the whole system.In this paper,MMC submodule capacitors and IGBT modules are taken as research objects,and the reliability evaluation of submodule capacitors and the packaging optimization of submodule IGBT module are carried out.The main contents of this paper include:(1)The working principle of MMC is introduced,and the mathematical model of MMC is established.The strategy of nearest level control and capacitor voltage balance is introduced.Then the mechanism of capacitor aging is summarized and the capacitance value is selected as the aging state monitoring parameter.Finally,the failure mechanism of IGBT module caused by heat is described,and the failure causes such as spalling of bonding line and cracking of solder layer are summarized.(2)For the capacitor of MMC submodule,a method of condition monitoring based on mapping of switching frequency and capacitance is proposed.Firstly,the distribution of switching frequency under MMC’s nearest level control and capacitor voltage balance strategy is analyzed.Then,the reason why the switching frequency of submodule will change with the decrease of capacitance is analyzed,and the fitting relationship between the two is constructed by training neural network.Finally,circuit simulation and RTBOX hardware-in-the-loop simulation are used to verify the method.(3)In view of the situation that the upper and lower arms of IGBT module do not have equal power flow,an asymmetric packaging optimization method for IGBT module in MMC submodule is proposed.Firstly,a traditional symmetrical IGBT module is designed based on the internal packaging of commercial products already in production.The number of chips in the upper and lower arms of the module is equal,which wastes part of the current capacity of the chips,and the temperature difference between the chips is too large.The optimized asymmetric IGBT module package is designed to make full use of the asymmetric power of upper and lower bridge arms in rectification and inverter working conditions,and balance the temperature difference between chips as far as possible.Then,finite element simulation is used to verify the temperature of IGBT module before and after optimization through solid heat transfer module.The simulation results show that the optimization method can reduce the total loss of the module,and also reduce the maximum temperature of the module.Finally,two IGBT modules are customized and compared on the MMC comprehensive experimental platform to verify the effectiveness of the proposed method in reducing chip temperature.The research results of this paper can provide theoretical support for the safe operation of the MMC submodule capacitor and reduce the maintenance cost,and provide certain design ideas for the design of IGBT module in the MMC system,so as to make contributions to improve the reliability of THE MMC submodule.