Review of control strategies for DFIG-based wind turbines under unsymmetrical grid faults

As the penetration of wind power and other distributed generation sources increases, more demanding grid code requirements are introduced by the network operators in order to ensure the stability of the evolving electrical networks. The most challenging requirement for wind energy converter systems is the low voltage ride through capability that expects generators to remain connected during symmetrical and asymmetrical grid faults and to contribute to the system recovery. Asymmetrical voltage conditions and dips in the grid can have significant negative effects on the performance of doubly-fed induction generators. These effects can decrease the lifetime of sensitive components in the wind energy converter in the long term and in extreme cases they can cause damages and tripping of the system, leading to violation of the grid code requirements. Protective measures must be taken so that the wind energy converters remain connected and support the grid without putting the reliability of the system at risk. Various control solutions have been developed to deal with these challenges. Although the most common voltage dips caused by grid faults are asymmetrical, the majority of the control solutions developed so far consider only symmetrical faults and they cannot mitigate the problems faced under asymmetrical conditions. This paper provides a comprehensive overview of different linear vector control methods for wind energy converter systems with doubly-fed induction generators which have been proposed in the literature to deal with the challenge of operating during voltage asymmetry and `riding-through´ all types of voltage dips, symmetrical and asymmetrical.