Analysis of high di/dt current pulses in three-level NPC inverters using series connected IGCTs and RC snubbers

High di/dt current pulses are harmful to the safety of IGCTs, especially to the low current conducting or freewheeling conducting IGCTs. This paper comprehensively analyzes the current pulses that flow through the inner switch of a three-level NPC inverter with series connected IGCTs and RC-snubbers. Four cases of high di/dt current pulses are addressed. The moments at which the high di/dt current pulses occur are clearly demonstrated. The value of the corresponding di/dt is estimated as well. The analysis is verified through simulation results using PSCAD/EMTDC. Based on the comprehensive analysis, the methods to handle these high di/dt current pulses are given respectively. An external re-trigger system is designed for a 6 kV/1800 kA three-level NPC inverter-fed medium voltage drive using series connected IGCTs. A more safe and reliable operation is reached for the series connected IGCTs in the three-level NPC inverter.