Fault-tolerant hybrid four-leg multilevel converter

In this paper a modification of the Neutral-Point-Clamped (NPC) converter, in which a fourth leg has been added, is presented. The main aim of this additional leg is to increase the reliability of the converter and thereby allow it to continue operating when a breakdown in one of the IGBTs occurs. In addition, during normal fault-free operation, the fourth leg also guarantees a correct control of the neutral point voltage, removing completely the low frequency voltage oscillations. A simple and computationally efficient control scheme is proposed for this purpose.