A new protection scheme for three-level NPC converter based DFIG using zero state control

This paper describes a new protection scheme for secure ride-through of doubly-fed induction generator (DFIG) based wind turbines (WTs) connected to a grid through a back-to-back three-level neutral point clamped (NPC) converter. The proposed scheme initiates the machine side converter (MSC) to be at zero state switching, where upper and lower switching devices are turned-off while inner switching devices are turned-on when overshoot occurs in rotor circuit during grid faults. This scheme is only applicable in multilevel converter and not in two-level converter. The scheme is compared to other recent protection scheme with dc-link chopper and the characteristic behavior of the DFIG is demonstrated using simulation results. The new scheme showed a very similar behavior with the conventional scheme and can be more preferable due to its simplicity and less component count. However, the need for the inner switching devices with about two times higher current rating will significantly increase the cost of the MSC and has to be considered.