Snubberless switching-off characteristics of IGCTs equipped in high power three-level neutral point clamped converters

Along with the improved design and broad applications of multi-level topologies, as a new commercial power switch device in recent years, IGCT has led the rapid development of high power converters in many applications, such as medium voltage adjustable speed drive and FACTs. Since the power electronic device is the base of these converters, the switching-off characteristics of IGCTs have become one of the key points in converters´ design and analysis, especially when snubber circuits are absent. This paper focuses on the snubberless switching-off characteristics of IGCTs equipped in the high power three-level neutral point clamped (NPC) converter. According to the physical characteristics of switch devices and commutating mechanism between different switching processes in the converter, the switching-off characteristics of IGCTs in different topology positions under different switching conditions are described, simulated and tested. The results of simulation and experiments are compared, especially the voltage spikes by IGCTs´ switching off. The relation between the switching-off characteristics and the other elements of the converter is also discussed in detail, such as topology position, PWM strategy and mechanical structure. Finally, some helpful conclusions for high power converters´ design and analysis are drawn