Impact of the Thermal-Interface-Material Thickness on IGBT Module Reliability in the Modular Multilevel Converter

The reliability of the Modular Multilevel Converter (MMC) is of great interest in industrial applications, where the dominant failure mechanism of IGBT modules in an MMC system is temperature-related. In this regard, thermal modeling is critical to map the power losses to thermal profiles and then to predict the lifetime. Even though the Thermal Interface Materials (TIMs) in IGBT modules have a considerable influence on the thermal resistance, the thickness of TIMs is often spuriously considered as a constant according to the thermal conductivity or information in the datasheet. This may lead to misleading results in the lifetime prediction. Hence, this paper investigates the impact of the TIM thickness on the estimated lifetime of IGBT modules in an MMC system for offshore wind power applications, including the starting assembly thickness and the Bond-Line Thickness (BLT) of TIMs. In a 30-MW MMC case study, the lifetime of the IGBT modules is discussed with respect to two values of starting thickness and variable BLT from 20 μm to 60 μm. Experiments are also carried out on a scaled-down system to validate the impact of the TIM thicknesses on the reliability prediction.