Postfault Degradation Analysis of IGBT Power Devices in Medium Voltage Converters

Modular design is largely applied to Medium Voltage Converters and provides a simple way to replace a set of damaged devices after failure events, allowing a fast return to operation. Arranging the power semiconductors into converter phase modules for instance, allow the system to be taken out of operation only for the defective phase module replacement, which can be sent to the supplier for repair. However, the indication of the failed power semiconductors inside the phase module is usually only given by the gate driver and simple in-circuit tests performed with the aid of a multimeter during the downtime. In this paper, it is demonstrated that a failure occurring in one flat-pack IGBT power semiconductor of any phase can cause escalated consequences and basic tests may not be enough to spot degraded devices in other phase modules. This is performed by the modeling, simulation, and analysis of two consecutive faults in a real case application of a three-level medium voltage converter. Furthermore, post-fault offline tests are performed in flat-pack IGBT samples that apparently survived the real case fault. The results reveal a relevant parameter shift that support questioning the benefits of the modular design. The deteriorated power devices might not fail immediately, but they will have a shorter lifetime expectancy impacting the overall reliability leading to a subsequent fault of likely larger significance [G. Salvador Ferreira et al. 2023], [G. Salvador Ferreira et al. 2024].