Modular multilevel converter-based high voltage current (MMC-HVDC) system is widely utilized in power transmission systems. Insulated gate bipolar transistor (IGBT) is one of the core devices in MMC. The degradation mechanism is of significance to the safe and reliable operation of the MMC-HVDC system. Compared with power module IGBTs (PMIs), high voltage press-pack IGBTs (PPIs) have short-circuit failure mode and double-side cooling advantages. This paper uses power cycling tests and microscopic observation methods to study the condition deterioration characteristics and mechanism of PPIs. Power cycling test is performed on PPIs to obtain the evolution rule of parameters like saturation voltage and gate threshold voltage. The deterioration modes of aged chips are analyzed based on the result of micro-optical observation on the chips’ emitter surface, and the possible reason for the evolution rule of the measured parameters of the PPIs is explained. The results show that the condition deterioration characteristics of PPIs are dispersed. In the early stage of the power cycling tests, since the fretting wear has little effect on the contact thermal resistance and contact resistance, the device saturation voltage and gate threshold voltage remain unchanged. It is inferred that if fatigue cracks appear on the emitter’s surface, the device’s state deteriorates faster and causes the increment of the saturation voltage. The gate threshold voltage of the device is less affected by cracks and remains relatively unchanged.

PPIs;power cycling test;condition deterioration mechanism;saturation voltage;fatigue crack