Improving the performance of protection schemes in three level IGCT-based neutral point clamped converters

High power IGCT-based converters are currently used in a wide range of drive applications. The protection of such converters against internal faults has been addressed in the technical literature, but the main focus has been on the solutions to disconnect the converter from the mains. In this paper, the short circuit current paths through the power devices in the three level NPC bridge are identified and studied from the perspective of keeping the overall repair cost, after a fault leading to the shoot-through mode, to a minimum. It is shown in this paper that a preferred short-circuit current path that limits the thermal energy in all IGCTs to safe levels exists. Furthermore, it is demonstrated that it is possible to guarantee, through proper busbar geometry and/or device selection, that the short-circuit current stress can always be moved to the power diodes. Simulation results are presented to support all the discussions and conclusions.