Wind power technology - Wind Power Integration: Connection and System Operational Aspects - page 56

operate at a higher rotational speed in order to extract the wind energy from the
rotor disk. Although a high rotor speed is attractive in that it reduces the gearbox
ratio required, a high tip speed leads to increased aerodynamic noise and increased
drag losses. Most importantly, three-bladed rotors are visually more pleasing than
other designs and so these are now always used on large electricity-generating
turbines.
The simplest arrangement for electricity-generating wind turbines is to operate
at essentially constant rotational speed using an induction (sometimes known as
asynchronous) generator. It is not practical to use a directly connected synchronous
generator as the tower shadow effect causes large pulsations in the mechanical
torque developed by the aerodynamic rotor. Practical synchronous generators
cannot damp these oscillations and so an induction machine, which has much
greater damping, is used. An alternative approach is to couple the generator to the
network through a frequency converter using power electronics and so allow its
speed to vary. The advantages of this arrangement are that it reduces mechanical
loads, the control of transient torque is easier and the aerodynamic rotor can operate
at its maximum efficiency over a wide range of wind speeds. The penalty is the cost
and electrical losses of the power electronics. Most of the largest wind turbines now
being installed operate at variable speed, as the power electronic converters also
allow much greater control of the output power and it is easier to comply with the
requirements of the power system network operator.

3.4 Energy extraction and power regulation

3.4.1 Energy extraction across the rotor disk
A wind turbine operates by extracting energy from the swept area of the rotor disk
(Manwell et al ., 2002; Burton et al ., 2001), as shown in Figure 3.1.
Power in the airflow is given by

1
2 r AV 3

P air ¼ ð 3 : 1 Þ

where r¼ air density (approx. 1.225 kg/m 3 ); A ¼ swept area of rotor; V ¼ free wind
speed.
This may be seen to be true by considering the kinetic energy of the air passing
through the rotor disk in unit time.

1
2 r AV

V 2

P air ¼

ð 3 : 2 Þ

where r AV ¼ mass flow rate of air.
However, not all the power can be extracted by the turbine and so a power
coefficient ( C P ) is defined. The power coefficient is simply the ratio of power
extracted by the wind turbine rotor to the power available in the wind.