Page 386 - Wind Energy Handbook

360 CONCEPTUAL DESIGN OF HORIZONTAL-AXIS TURBINES

6.8.3 Mechanical brake options

As noted in Section 6.8.1, the duty of the mechanical brake need only be that of a
parking brake on machines where the aerodynamic brakes can be actuated inde-
pendently. However, on pitch-regulated machines where blade position is con-
trolled by a single actuator, full independent braking capability has to be provided
by the mechanical brake. It is worth noting that several manufacturers of stall-
regulated machines fitted with independent tip brakes ensure that the mechanical
brake can stop the rotor unassisted. This may be to satisfy requirements in certain
countries that two independent braking systems of a different type are provided.
A wind turbine brake typically consists of a steel brake disc acted on by one or
more brake callipers. The disc can be mounted on either the rotor shaft (known as the
low-speed shaft) or on the shaft between the gearbox and the generator (known as
the high-speed shaft). The latter option is much the more common because the
braking torque is reduced in inverse proportion to the shaft speeds, but it carries with
it the significant disadvantage that the braking torques are experienced by the gear
train. This can increase the gearbox torque rating required by as much as ca 50
percent, depending on the frequency of brake application (see Section 7.4.5). Another
consideration is that the material quality of brake discs mounted on the high-speed
shaft is more critical, because of the magnitude of the centrifugal stresses developed.
The brake callipers are almost always arranged so that the brakes are spring
applied and hydraulically retracted, i.e., fail-safe.
Aerodynamic braking is much more benign than mechanical braking as far as
loading of the blade structure and drive train is concerned, so it is always used in
preference for normal shut-downs.

6.8.4 Parking versus idling

Although a mechanical parking brake is essential for bringing the rotor to rest for
maintenance purposes, many manufacturers allow their machines to idle in low
winds and some do so during high wind shut-downs. The idling strategy has two
clear advantages: it reduces the frequency of imposition of braking loads on the
gear train, and gives the impression to members of the public that the turbine is
operating even when it is not generating. On the other hand, gearbox and bearing
lubrication must be maintained throughout.

6.9 Fixed Speed, Two-speed or Variable-speed
Operation

Wind turbine rotors develop their peak efficiency at one particular tip speed ratio (see
Figure 3.15), so fixed speed machines operate sub-optimally, except at the wind speed
corresponding to this tip speed ratio. Energy capture can clearly be increased by
varying the rotational speed in proportion to the wind speed so that the turbine
is always running at optimum tip speed ratio, or alternatively a slightly reduced