Commutation strategy for minimized common mode voltage using direct power control

Nowadays the doubly fed induction generator is often used in high power wind energy conversion systems. A common problem of converter fed induction machines are the bearing damages due to the bearing currents. The source of this problem is the common mode voltage caused by the fast switching converters. Instead of a passive solution like EMI filters or insulated bearings, this paper presents a control method based on the direct power control combined with an advanced commutation strategy to avoid the common mode voltage. This control strategy needs no space vector modulation and further no knowledge of system parameters like stator resistance. Only the direction of the phase currents must be determined by an additional circuit. Experimental results for a 4kW DFIG system illustrates the effectiveness of the proposed method.