Wind power technology - Wind Power Integration: Connection and System Operational Aspects - page 63

shows a wind turbine blade viewed from its tip. It is useful to consider the blade
vertical with the viewer looking down on to the tip.
U is the relative wind speed seen by the blade and is formed by the incident
free wind speed V and the rotational speed of the blade w R . By definition F d (the
drag force) is parallel and F l (the lift force) is perpendicular to U (Figure 3.8b). The
lift and drag forces can then be resolved into the rotational force that turns
the blades, F r , and the thrust force on the rotor that has to be resisted by the
structure, F q .
For a fixed-speed wind turbine the angular velocity of the rotor w is fixed by
the electrical generator, which is locked on to the network. Hence it may be seen
that there are two ways of reducing the rotational force F r at above rated wind
speeds. The angle of incidence a may be reduced by mechanically turning the
blade. As shown in Figure 3.7 this reduces the lift coefficient C l , with only a small
effect on C d . Reducing the lift coefficient then reduces the lift force F l , which in
turn reduces the rotation force, F r . This is pitch regulation , as it requires a change
in the pitch of the blades by mechanical rotation.
Alternatively the blades can be fixed to the hub at a constant angle. Recalling
that w R is constant, then as the free wind speed V increases it can be seen that the
angle of incidence a also increases. As shown in Figure 3.7, once the blade has
stalled, the lift coefficient and hence the torque decreases. This is stall regulation ,
which does not require any physical change in the pitch angle of the blades.
It is obvious that stall regulation is attractive, as no moving parts are required.
Unfortunately it has proved rather difficult to predict the wind speed at which
blades stall, as it appears that the stall effect is a complex three-dimensional phe-
nomenon involving a degree of hysteresis. Pitch regulation is easier to predict but
requires a control mechanism, as shown in Figure 3.9, to alter the pitch angle of the
blades. The output power of the wind turbine is measured with an electrical power
transducer and used as the input to a control system that alters the pitch angle of the
blades. Because stall occurs at various wind speeds at differing radial locations
along the blade, the power curve of a stall-regulated wind turbine is less steep than
that of a pitch regulated blade, with a consequent loss in energy capture. However,
both stall and pitch regulated fixed-speed wind turbines up to 80 m diameter and
2 MW electrical rating are now commercially available.

Generator

Gearbox

Control
system

Actuator

Figure 3.9 Pitch regulation control system