354
10 Wind Turbines
stall, but the blade angle is adjusted such that the power gets exactly limited to the
rated power. This system is applied with wind turbines turning at fixed rotational
speed (see further: fixed and variable rotational speed) of the 1–2 MW order. In this
power range, an exactly limited generator power is important, just as feathering in
stormy weather is important too. Why active stall control instead of pitch control
is preferred with certain turbines is explained in the further discussion of fixed and
variable rotational speed.
For the case that connection to the grid would be interrupted, it shall be possible
to stop the turbine at full speed. Machines with pitch control and active stall control
are provided with a mechanical brake on the fast shaft of the gearbox (more advantageous than on the slow shaft). This brake also serves as a parking brake, i.e. to keep
the rotor in standstill with very low wind speed. Additionally, pitch control is applied to decrease rotor power. Pitch control is much slower than the brake however.
Turbines with passive stall control may be provided with a second brake on the slow
shaft. With passive stall control there is always an aerodynamic brake as well. This
functions by setting the rotor blade tip perpendicularly to the blade speed, reducing
the power to a low value. In all cases, there is the possibility to turn the rotor 90° out
of the wind, as an ultimate measure, by means of the yaw mechanism. At very low
wind velocities (lower than the cut-in speed, see Sect. 10.4), the rotor is immobilised
by the brake. A turbine with stall control is switched on by releasing the brake. The
rotor then starts spontaneously. This is possible if the design value of the tip speed
ratio is not too large. No problem arises with values up to λ = 6 at optimum rotor operation, corresponding to λ = 4.5 at rated wind velocity (see paragraph 10.4). Rotors
with higher design tip speed are normally not self-starting because the blades are
mounted more tangentially. A turbine with pitch control does not start when releasing the brake. The blades must be turned away from the feathered position.
With the simplest systems, the generator is an asynchronous machine. Pole
changing is very often applied, allowing the generator to turn at lower speed with
low wind velocities. With a fixed rotational speed, the optimum power coefficient
is only reached at one single wind speed. In order to optimise the energy yield, this
wind speed shall be about 75 % of the wind speed for rated power (see Sect. 10.4:
wind regime). At a fixed rotor blade pitch, as with stall control (pitch always fixed
with passive stall control and with power lower than rated with active stall control),
the yield is lower than with pitch control where the rotational speed may be kept optimal through a certain wind speed range (see Sect. 10.4: wind regime). Pitch control
cannot easily be applied with a fixed speed generator. The wind speed always has
fluctuations, growing at higher average wind speed. Pitch control cannot be realised
so fast that it neutralises gust effects on torque and power. Many turbine parts are
thus subjected to fatigue and power may fluctuate strongly. The gust problem is obviously most important at high wind speed. Stall controlled turbines (both actively
and passively) are much more tolerant to gusts, that partly are absorbed by increase
and decrease of the rotor stall area. With higher power, a variable rotational speed
is more advantageous to improve the energy yield and to attenuate the fatigue load
on the rotor blades and on the gearbox (if present). Turbines above about 2 MW are
10 Wind Turbines
stall, but the blade angle is adjusted such that the power gets exactly limited to the
rated power. This system is applied with wind turbines turning at fixed rotational
speed (see further: fixed and variable rotational speed) of the 1–2 MW order. In this
power range, an exactly limited generator power is important, just as feathering in
stormy weather is important too. Why active stall control instead of pitch control
is preferred with certain turbines is explained in the further discussion of fixed and
variable rotational speed.
For the case that connection to the grid would be interrupted, it shall be possible
to stop the turbine at full speed. Machines with pitch control and active stall control
are provided with a mechanical brake on the fast shaft of the gearbox (more advantageous than on the slow shaft). This brake also serves as a parking brake, i.e. to keep
the rotor in standstill with very low wind speed. Additionally, pitch control is applied to decrease rotor power. Pitch control is much slower than the brake however.
Turbines with passive stall control may be provided with a second brake on the slow
shaft. With passive stall control there is always an aerodynamic brake as well. This
functions by setting the rotor blade tip perpendicularly to the blade speed, reducing
the power to a low value. In all cases, there is the possibility to turn the rotor 90° out
of the wind, as an ultimate measure, by means of the yaw mechanism. At very low
wind velocities (lower than the cut-in speed, see Sect. 10.4), the rotor is immobilised
by the brake. A turbine with stall control is switched on by releasing the brake. The
rotor then starts spontaneously. This is possible if the design value of the tip speed
ratio is not too large. No problem arises with values up to λ = 6 at optimum rotor operation, corresponding to λ = 4.5 at rated wind velocity (see paragraph 10.4). Rotors
with higher design tip speed are normally not self-starting because the blades are
mounted more tangentially. A turbine with pitch control does not start when releasing the brake. The blades must be turned away from the feathered position.
With the simplest systems, the generator is an asynchronous machine. Pole
changing is very often applied, allowing the generator to turn at lower speed with
low wind velocities. With a fixed rotational speed, the optimum power coefficient
is only reached at one single wind speed. In order to optimise the energy yield, this
wind speed shall be about 75 % of the wind speed for rated power (see Sect. 10.4:
wind regime). At a fixed rotor blade pitch, as with stall control (pitch always fixed
with passive stall control and with power lower than rated with active stall control),
the yield is lower than with pitch control where the rotational speed may be kept optimal through a certain wind speed range (see Sect. 10.4: wind regime). Pitch control
cannot easily be applied with a fixed speed generator. The wind speed always has
fluctuations, growing at higher average wind speed. Pitch control cannot be realised
so fast that it neutralises gust effects on torque and power. Many turbine parts are
thus subjected to fatigue and power may fluctuate strongly. The gust problem is obviously most important at high wind speed. Stall controlled turbines (both actively
and passively) are much more tolerant to gusts, that partly are absorbed by increase
and decrease of the rotor stall area. With higher power, a variable rotational speed
is more advantageous to improve the energy yield and to attenuate the fatigue load
on the rotor blades and on the gearbox (if present). Turbines above about 2 MW are