Failures-tolerance and remedial strategies of a PWM multicell inverter

The aim of this paper is to explain the intrinsic short-circuit tolerance of an IGBT multicell inverter when a commutation failure occurs. Such a failure may either be a wrong gate voltage (malfunctioning of the driver board, auxiliary power supply failure, dv/dt disturbance) or an intrinsic IGBT failure (over-voltage/avalanche stress, temperature overshoot). IGBT stresses are studied and show that no opening of the bonding can appear and consequently no risk of explosion. That is why, owing to the imbricated cells structure, an IGBT short-circuit failure may be withstood for a few switching periods, with nevertheless nonoptimized output waveforms. The design, the lab-test of a sensor able to perform monitoring as well as the failure diagnosis are also presented. This real-time diagnosis allows either a safe stop or a remedial control strategy based on the reconfiguration of the PWM modulator. The reconfiguration strategy enables decrease of internal stresses and optimization of the output shape. A fail-safe operating may be gained for high power applications.