Research On Loss Characteristics And Junction Temperature Prediction In Modular Multilevel Converter

With the continuous rising of voltage level and transmission capacity of VSC-HVDC project,the converter loss draws more attention.In terms of loss calculation of modular multilevel converters(MMC),current researches mostly focus on the total loss rate of the system,however,there are few studies concern about device loss and chip junction temperature in the converter sub-module,which is another significant aspect contributing to the long-term safety of sub-modules and devices inside.Therefore,this paper mainly focuses on the loss and junction temperature characteristics of each component of the MMC sub-modules,also considers the effect of the heat dissipation system.The loss and junction temperature characteristics under normal and fault conditions are analyzed respectively based on the accurate calculation of chip loss and junction temperature.Firstly,this paper introduces the operating principle of MMC half-bridge sub-module and the equivalent thermal model of the cooling system under different operating conditions.Then,an electro-thermal combined calculating method based on junction temperature feedback is proposed.This method combines the electrical output of the electrical simulation with the equivalent thermal circuit model extracted by the thermal field simulation through the iterative calculation of loss and junction temperature,which is convenient and efficient.Secondly,considering the thermal coupling between chips in the IGBT power module,the electro-thermal combined calculation method is applied to the MMC normal operating condition and fault transient process.Under normal operating conditions,the influence of thermal coupling between the chips on the loss and the junction temperature of the device is studied,which indicates that the effect of junction temperature feedback on the loss is relatively large,and the thermal coupling only has a certain influence on the device junction temperature.While in the fault transient process,time recursive convolution algorithm is introduced to improve the efficiency of the electro-thermal combined calculation.The electrical transient processes under three typical faults are simulated and the transient junction temperatures of IGBT and diode are predicted and analyzed.