Torque and power limitations of variable speed wind turbines using pitch control and generator power control

Variable speed operation of wind turbines has the potential to in crease energy capture and reduce fatigue damage, compared with fixed speed operation. Cage induction generators with their brush-less and rigid structure are at present widely used for fixed speed wind systems. To make the cage induction generators widely used also at variable speed systems, proper control strategies should be developed as well as the development or low cost and reliable power electronic devices. At high wind speeds, as is conventional fixed speed system, It is very important to limit the variable speed system to its ratings without mechanical and electrical stresses induced by wind gusts and/or control process. Ibis paper introduces a configuration of variable speed system which includes a pitch controllable horizontal axis wind turbine, a cage Induction generator, a PWM (pulse width modulation) converter and a PWM inverter, the last one being connected to utility grid. Also, the paper proposes and investigates a class of control strategies for limiting the system to its ratings, from the view point of avoiding axial torque and/or generator power fluctuations induced by wind gusts and control errors. The class adopts parallel operation of pitch angle control and electrical power control. The pitch control alms at limiting turbine rotational speed, while generator power control alms at eliminating torque and/or power fluctuations. Simulation results In time domain are presented to verify the effectiveness of the class of controls.