A Lifetime Estimation Method of MMC Submodules based on the Combination of FEA and Physical Lifetime Model

Due to a series of advantages compared with other existing power converters, modular multilevel converters (MMCs)have been widely used in the last decade. However, in some applications, such as the offshore wind power transmission, the MMC submodules (SMs)are subjected to harsh operating environment and adverse mission profiles which can easily lead to failures in the solder layers of IGBT modules. Therefore, the lifetime estimation is very important to predict the reliability of MMC. This paper studies the fundamental-frequency thermal cycles and proposes a lifetime estimation method of MMC SMs based on the combination of finite element analysis (FEA)and the physical lifetime model. This method provides a deeper physical description of the failure mechanism and considers the thermal coupling among the chips, which make the lifetime calculation more accurate. The simulation time of FE in this research is acceptable. The distribution of lifetime is presented in the form of a colorful cloud map, rather than a single number, from which we can know the most vulnerable part of the solder layers. This can help us find the appropriate measures to improve the reliability of MMC SMs. In this paper, the principle and reliability issues of MMC are first analyzed. Second, the loss of the SM is calculated through simulation and curve fitting. Then, a 3D finite element (FE)model of MMC SM is developed and the FE simulation is performed. Finally, the lifetime of the SM is obtained based on the FEA results and Morrow lifetime model.