Executive Summary : | Wind energy conversion systems (WECS) have been receiving widespread attention from governments and industry. In order to extract the mechanical power with maximum efficiency, it is necessary to vary the speed of a wind turbine. However, due to the limitations imposed by the generators coupled to the wind turbines and the associated power electronic converters, a majority of the commercially available WECS are of fixed speed or semi-variable speed type. Fixed-speed WECS are equipped with squirrel-cage induction generators directly connected to the grid and are highly inefficient. Semi-variable speed WECS employ slip ring induction machine (SRIM) or doubly-fed induction generator (DFIG). Back-to-back power electronic converters (AC-DC-AC) are employed in the rotor circuit, while the stator is directly connected to the grid in a DFIG-based WECS. The rating of the back-to-back power converter in the rotor circuit can be a fraction of the nominal power depending upon the range of operational speed. However, direct connection of the stator to grid poses challenges pertaining to the power controllability under grid faults. Recent installations of WECS employ permanent magnet synchronous generators (PMSG) coupled directly to the wind turbines. Back-to-back power electronic converters of full rating are used to interface the PMSG with the grid. The direct drive PMSG-based WECS has advantages such as fault-ride-through capability, elimination of gear box and ability to extract maximum power at all speeds. However, higher number of poles required on PMSG for low-speed operation pose challenges in terms of rotor diameter, weight and cost. Slip ring induction machine (SRIM) with voltage source converters on the stator as well as the rotor is an attractive solution to address the issues mentioned above. This configuration offers the flexibility of controlling the frequency on both the stator and the rotor. Further, a double-converter-fed SRIM can develop rated torque upto twice the rated speed. This facilitates maximum power point tracking over a wide range of operational speed and the gear box can be eliminated to improve the power conversion efficiency. This project aims at the development of a wind energy conversion system with double-converter-fed slip ring induction machine as the generator. The stator-side and rotor-side power converters share a common DC link, which is controlled by the grid-side power converter. Characteristics of the wind turbine are emulated using a sensorless vector controlled induction motor drive. The project also aims at developing novel frequency trajectories on stator and rotor for maximum power point tracking and new sensorless control algorithms for low switching frequency operation of the power converters. A comprehensive literature survey will be followed by numerical simulations of the complete system. Experimental investigations will be carried out on a 5-hp laboratory-scale prototype of the proposed system. |