TRANSIENT ANALYSIS OF A GRID BASED DOUBLY FED INDUCTION GENERATOR (DFIG) VARIABLE SPEED HYDRO TURBINE

Abstract

In view of increasing demand of energy and world's depleting fossil fuel reserves, which provide the major source of energy, the development of renewable energy sources has got great importance. Small. hydropower (SHP) is an important renewable energy source. In order to increase the efficiency of the generation the power output spectrum should be wide. This can only be possible if the generation works according to the availability of the water, and there comes the concept of Variable Speed Constant Frequency (VSCFG). Under this scheme a Doubly fed induction generator (DFIG) is introduced which delivers output at +30% to -30% of synchronous speed, without the use of governor under grid connected variable speed turbine. And although the system is reliable the study under fault is very necessary, hence the ground fault at a distance of 10 km on the grid is simulated in Simulink Tool of Matlab R2008a. And the results were studied. Hydro turbines use a doubly-fed induction generator (DFIG) consisting of a wound rotor induction generator and an AC/DC/AC IGBT-based PWM converter. The stator winding is connected directly to the 50 Hz grid while the rotor is fed at variable frequency through the AC/DC/AC converter. The DFIG technology allows extracting maximum energy from the flow of water for low speeds by optimizing the turbine speed, while minimizing mechanical stresses on the turbine. The optimum turbine speed producing maximum mechanical energy for a given speed is proportional to the torque. For speeds lower than 1200 rpm the rotor is running at sub synchronous speed . At high speed it is running at hyper synchronous speed. The turbine mechanical power as function of turbine speed is displayed for speeds ranging from1100 to 1300 rpm . The DFIG is controlled in order to follow the reliable power output under steady sate and in voltage sag condition. Another advantage of the DFIG technology is the ability for power electronic converters to generate or absorb reactive power, thus eliminating the need for installing capacitor banks as in the case of squirrel-cage induction generators.