Investigation of UPQC for sag compensation in wind farms to weak grid connections

In the last years, the wind power generation incorporated into standard grids has been increased significantly. This situation forced the revision of grid connection code requirements, to guarantee the reliability in systems with high wind power penetration. In case of three phase short circuit, voltage sags are observed near the point of failure, and characterized by a sudden voltage reduction and lagging phase jump. For induction generator based wind farms connected to weak grids, such sag may lead to wind farm outage, due to limited low voltage ride through capability of induction generators. In this work a voltage sag compensation strategy is proposed for amplitude and phase jump restoration, and compared with amplitude only compensation strategy. These strategies were implemented using an Unified Power Quality Compensator UPQC. Unlike other Custom Power Devices like DVR and D-Statcom, the UPQC has the feature of active power sharing between shunt and series converters through DC-link; thus, series voltage injection with any phase angle may be maintained without the need of power source installed in DC bus. Results shows a better wind farm performance in proposed strategy than that found in magnitude only compensation schemes. Thus, considering the improvement in performance, the proposed strategy is recommended in retrofitting the existing installed fixed speed induction generators based wind farms.