Large Wind Turbine

Working principle of a wind turbine

Modern wind turbines work on aerodynamic lift principle, just like the wings of an aeroplane. The wind does not "push" the turbine blades, but instead when the wind flows across and past a turbine blade, the difference in the pressure on either sides of the blade produces a lifting force, causing the rotor to rotate and cut across the wind.

Not all the power in the wind can be extracted by the turbine rotor. Theoretically, the maximum amount of power that can be extracted by a wind turbine, according to the Betz Law, is 59.6% of the power in the wind. Most wind turbines can extract about 40% or less of the power in the wind.

A wind turbine mainly comprises of three major parts - a rotor, a nacelle, and a tower. The horizontal axis, three-blade turbine on a free-standing tubular tower is the predominant configuration for large grid-connected wind turbines.

(Source for above: Danish Wind Industry Association)

The rotor blades are made of composite materials. Unlike small wind turbines, the rotors of large wind turbines rotate rather slowly. Simpler wind turbines are fixed speed machines, often with two speeds - a lower speed for weaker wind conditions and a higher speed for stronger wind conditions. For fixed speed machines, the induction generator directly produces alternating current at grid frequency.

Newer designs are variable speed ones (for example the speed range is 14 to 31.4 rpm for Vestas V52-850kW machine). Under variable speed operation, rotor aerodynamic efficiency is improved, leading to better energy capture and bringing about additional benefits such as lower noise in light winds. Variable speed has been gaining ground over fixed speed design.

Sensors mounted on the nacelle detect the wind direction, and a yawing mechanism automatically orientates the nacelle and rotor to face the wind.

The rotational motion of the rotor is transmitted via a gearbox to the electric generator inside the nacelle (or in the case of gearless machine, is transmitted directly to the electric generator). Wind turbines with gearboxes are the common type in the industry. However, the use of purpose-built multi-pole direct-drive generators is also gaining notable development.

A transformer at the base of the tower (or for some designs inside the nacelle) steps up the generator voltage to the grid voltage (11kV in the case of Hong Kong).

All turbines produce a varying power output dependent on the wind speed. The two most common means of limiting the power output (and hence the stress on the rotor) in high winds are "stall regulation" and "pitch regulation". With stall regulation, an increase in wind speed beyond the rated wind speed causes progressive stalling of the air flow over the rotor. Tip brakes are used to brake the wind turbine when the wind gets excessively strong. In the case of pitch-regulated machines, each turbine blade can rotate about its own length-wise axis. The "pitch angle" of the turbine blade varies with the wind speed, altering the aerodynamic performance of the rotor. When the wind gets too strong, the leading edge of the blade actually faces the wind so that the wind turbine brakes.

Lightning protection stripes are embedded inside the turbine blade, to bring the lightning charges to ground when a lightning stroke hits the blade.

Above: Components inside the nacelle of a Vestas V52-850kW wind turbine
(Source: Vestas)