Introduction to Modern Wind Turbines - Wind Turbine Technology: Fundamental Concepts of Wind Turbine Engineering - page 58

machine, and the frequency of all of the power generated is restored to line frequency through
AC-DC-AC conversion .
Direct-Drive vs. Speed-Increasing Gearboxes for Utility-Scale HAWTs
Large-scale HAWT rotors turn at relatively low shaft speeds, to maintain an optimum
tip-speed ratio (tip forward speed divided by free-stream wind speed) and partially because
of the necessity of limiting blade tip speeds in order to control noise. For example, the GE
Model 1.5s rotor shown in Figure 2-3 operates at speeds from 10 to 22.2 rpm. Thus, the
design of a direct-drive generator for a utility-scale HAWT is, by necessity, unconventional.
The issue of direct-drive vs. gear-driven generators was addressed by Bywater et al. [2005].
These authors observed that direct drives for large-scale wind turbines have attracted in-
creased commercial support because of their simplicity, quiet operation, a small eficiency
advantage, and (most importantly) avoidance of costly gear failures.
Because of its slow shaft speed, a direct-drive generator of a utility-scale HAWT is a
large, heavy machine. To be competitive with relatively light gearbox-driven generators,
designers of direct-drive power trains must employ innovative measures to reduce the size,
weight, and cost of the generator. Generally, the increased size and strength of the structure
of a direct-drive generator can be used to good advantage in supporting the turbine rotor.
Since 1994, the Enercon Corporation has been the principal innovator of large-scale di-
rect-drive HAWTs, producing machines with rated powers from 500 kW to 6.0 MW. Several
other manufacturers have recently introduced direct-drive generators with ratings from 750
kW to 2.0 MW. However, speed-increasing gearboxes remain the dominant design approach
for large-scale HAWTs. A recent survey [Bywater et al. 2005] found that an estimated 85
percent of the worldwide wind power capacity utilizes gearbox-driven generators.
The Nacelle Structure Subsystem
Referring to Figure 2-9, the HAWT nacelle structure is the primary load path from the
turbine shaft to the tower. Figure 2-9(a) illustrates a bed plate assembly that provides a
stiff loor on which to mount the turbine shaft bearings, the power train components, and
the yaw drive mechanism. Nacelle structures are usually a combination of welded and bolt-
ed steel sections which form trusses or box beams, with an enclosure to protect equipment
and maintenance personnel (Figure 2-9(b)). Stiffness and static strength, rather than fatigue
strength, are the usual design drivers of nacelle structure.
HAWTs require a yaw drive mechanism so that the nacelle can turn to keep the rotor
shaft properly aligned with the wind. As shown in Figure 2-9(c), this mechanism includes
a large bearing that connects the bed plate to the tower and serves as a primary load path.
An active yaw drive (one which turns the nacelle to a speciied azimuth) contains one or two
motors (electric or hydraulic), each of which drives a pinion gear against a bull gear , and an
automatic yaw control system with its wind direction sensor mounted on the nacelle. A pas-
sive yaw drive permits wind forces to orient the nacelle.
A two-bladed rotor produces cyclic loads on the yaw drive that are much larger than
those from a three-bladed rotor. Teetering removes some but not all of these additional yaw
loads. The yaw drive mechanism of a two-bladed HAWT, therefore, is a robust component
designed to resist both fatigue and wear. Even then, yaw brakes may be required to hold the
nacelle in position.
HAWTs also require a yaw slip ring or cable-wrap device to transfer electrical power,
control signals, and data from the moving nacelle to stationary cables in the tower. Other