Commercial Wind Turbine Systems and Applications - Wind Turbine Technology: Fundamental Concepts of Wind Turbine Engineering - page 221

torques for large turbines, such as the Clipper Liberty gearbox that drives multiple generators
directly from the planetary gears (see Fig. 3-46).
Even more variations in gearboxes are found in the design of smaller wind turbines.
Enertech uses a cycloidal-drive with no gears on its 5-kW model. Many other small-scale
turbines that operate at higher rotor speeds do not use a gearbox at all, but instead drive the
generator directly with the turbine shaft. A very few medium- and large-scale turbines also
use direct-drive generators, including the EWT Directwind and the Enercon E-126 7-MW
wind turbine. Direct-drive generators will be discussed later in this chapter.
An alternative to the conventional mechanical gearbox is the use of hydraulic trans-
mission systems. Voith Turbo's WinDrive system couples a hydraulic torque converter to a
planetary gearbox to provide variable speed ratios and reduced torque dynamics. This drive
system is used in the DeWind 2-MW wind turbine.
Because the generator is electrically locked to the utility grid ( i.e. , constrained to stay at
the same frequency as the grid) under normal operation, the drive train must also serve to par-
tially dampen torque fluctuations caused by wind gusts. This is generally accomplished by the
use of shock/spring mounts that allow the gearbox to rotate by a few degrees (see Fig. 0-3).
Other approaches to dampen torque fluctuations are () the use of one or more flexible cou-
plings located on either side of the gearbox or (2) the use of a quill shaft , a torsionally flexible
tube between the hub and the gearbox, which is located inside a hollow turbine shaft.
Most manufacturers of wind turbines have experienced failures of the gears, gear sur-
faces, or bearings in their gearboxes. Gearbox problems have generally been the result of
() lack of understanding of the cyclic and random loads imposed by the rotor, resulting in
premature fatigue failures; (2) inadequate alignment of the high-speed shafts, resulting in
seal degradation; (3) inadequate servicing resulting in premature failures from friction and
wear; (4) poor assembly quality control resulting, for example, in excessive bearing endplay
and misalignment; and (5) inadequate control of impact loads imposed by brakes on the
high-speed shaft, resulting in fracture of critical components, and (6) lack of lubrication and
impact loads during rotor parked (stopped) conditions.
As large-scale commercial turbines grow in physical size, power increases while rotor speed
decreases. This trend simultaneously increases both rotor torques into the gearbox and gearbox
speed-up ratios. With turbine sizes continuing to increase in the foreseeable future, the low-
speed end of the drive train will continue to be a significant design and maintenance challenge.
Brakes
Mechanical brakes are typically employed for normal stopping of the rotor or holding
the rotor in place after it has stopped. Mechanical brakes are activated electromechanically,
hydraulically, or pneumatically. The most common brake systems are conventional disc and
caliper types. If the drive train has a gearbox, brakes are usually placed on the generator drive
shaft (gearbox high-speed shaft) or aft on the generator shaft itself, because of the torque-
multiplying effect of the gearbox. In addition to mechanical brakes, several manufacturers
have incorporated circuitry that allows the use of the generator as an electrical brake or, as it
is commonly termed, a dynamic brake .
There is general agreement within the wind turbine industry that if mechanical brakes are
the primary means of stopping the rotor, the turbine must either provide aerodynamic braking
to augment them or provide fail-safe protection (such as redundant brakes) to stop the rotor
should the primary brakes fail.
Mechanical brake wear has been a problem at some sites. Wear is accelerated by exces-
sive start-stop cycles which can occur in areas with high, gusty winds (causing frequent high-
wind shut downs) and in areas where the utility grid is frequently disabled (causing repeated
losses of electrical load followed by emergency stops).