Wind turbine modelling and control

The wind turbines discussed in the paper are upwind horizontal axis, grid-connected medium to large scale wind turbines. The configuration of wind turbine is the simplest possible comprising a three-blade rotor with rigid hub, gearbox and induction generator. The generator is connected directly to the grid thereby locking the speed of rotation of the rotor to the frequency of the grid which is of course fixed. For this reason this type of machine is called a constant speed wind turbine. There are two common methods of aerodynamic power limiting. The first is passive regulation. The rotor blades are designed to stall near rated windspeed. Hence, the power generated by the turbine does not rise further with windspeed. The second is active regulation. The torque induced on the rotor by the wind depends on the pitch angle of the blades. Hence, the torque may be reduced by feathering the blades and vice versa. During active regulation, above rated windspeed, the pitch of the blades are continuously set to the angle of pitch at which rated power is generated. The adjustment of pitch angle is usually made in response to power measurement. The capability to vary the pitch angle of the blades may be exploited to achieve much more than simply limiting the level of generation of power. The purpose of the paper is to report on an extensive investigation into the active control of constant speed wind turbines