A Rotor-Tied DFIG Scheme With One Phase Open Circuit Fault-Tolerant Capability

This article proposes a fault-tolerant control scheme for a transformer isolated three-phase rotor-tied doubly fed induction generator (RT-DFIG) suitable for grid connected wind energy conversion system (WECS). The scheme uses back-to-back connected three-phase four-wire two-level converters having a common dc bus with the grid side converter (GSC) connected to the point of common coupling via a delta/star/star isolation transformer whose neutral is joined to the dc bus midpoint. The stator side converter (SSC) is coupled to the star connected stator terminals of the RT-DFIG with the stator neutral joined to the dc bus midpoint. The rotor is connected directly to the grid via the other star winding of the isolation transformer. This fault-tolerant topology along with the control scheme restrict negative sequence current and allows circulation of zero sequence current through the neutral wire via the dc bus midpoint. This enables seamless operation with time-invariant power transfer and balanced grid side line currents during one-phase open-circuit fault in either the GSC or SSC or one phase of transformer secondary or line reactors or one phase of RT-DFIG stator. This proposed scheme uses a simple control without any extra hardware and hence is cost-effective. Simulation and experimental results are provided to validate the proposed concept.