Optimal Control Strategy Of Junction Temperature Fluctuation Based On MMC And Its Reliability Analysis

MMC has low switching frequency,high voltage quality and exce llent characteristics in loss control.It has become the most concerned topology in the field of flexible HVDC transmission.At present,MMC scheme is adopted in many projects under construction or planning.The reliability of modular multi-level converter is determined by the reliability of IGBT of sub-module.At present,the reliability of modular multi-level converter is generally evaluated based on the junction temperature of IGBT.The thermal stress of IGBT module determines the failure rate of the device.The failure of IGBT module is basically caused by fatigue.At present,the loss analysis of MMC sub-module is quite mature,but how to change the modulation strategy to control the temperature of MMC sub-module is still in the exploratory stage.Because the MMC sub-module is in a redundant state when it is put into operation,it will inevitably lead to differences in junction temperature consistency of IGBT module in MMC sub-module.At this time,optimizing the junction temperature fluctuation of IGBT can improve the reliability of the system.Firstly,this paper introduces the topology and control strategy of MMC.The common methods of MMC reliability are analyzed.Coffin-Manson life model and linear cumulative damage theory are used to evaluate the re liability of MMC sub-module and the reliability of the system.Secondly,under the recent level approximation control strategy,the loss distribution of MMC IGBT sub-module is analyzed,and the loss of MMC sub-module in the control cycle is calculated.The temperature distribution of IGBT module in MMC is calculated based on the theory of electrothermal analogy.The loss of the system under different voltage levels is analyzed.The r eliability of the system can be improved by choosing voltage levels reasonably.Finally,how to reduce the difference of junction temperature distribution of MMC sub-module and keep the fluctuation of capacitor voltage of sub-module within a certain range is analyzed,and the possibility of junction temperature fluctuation can be reduced only by changing the switching process of sub-module.Taking the voltage fluctuation,the temperature fluctuation difference of sub-module and the temperature limit as control objectives,the optimal control strategy of temperature fluctuation is given,which improves the reliability of the system.