Post-fault reconfiguration for a versatile and hybrid 4 Leg NPC-flying capacitor topology

Enhancing power converter reliability is an important issue for multilevel converters. With 2 level topolgies, one IGBT failure leads to the whole system breakdown. Since multilevel converters are featured by a high number of semiconductor switches, it is possible to reshuffle the topology when an IGBT failure occurs. Remedial measurments are then possible and a safe continuity operation after a power device failure is guaranteed. This paper investigates a hybrid 4 Leg NPC-FC converter: it is composed by 3-level NPC-based 3 phases and a fourth FC-based one. The FC leg regenerates an active neutral point and, consequently, avoids bulky capacitors bank. When one or more IGBTs are damaged, the FC leg is controlled in order to ensure a post-fault operation. An appropriate LC filter is proposed to connect this leg to the NPC inverter. Experimental results show the effectiveness of the proposed LC filter design methodology. A loss-based comparison study between healthy and faulty operating modes is carried out.