Design and development of megawatt wind turbines - Wind Power Generation and Wind Turbine Design - page 223

Future large turbine blade technology may incorporate elements of dry nano-
technology, hybrid construction (i.e. inboard structure of one material joined
through one or more joints to outboard sections made of alternate materials) or
some form of repeating and panellized spaceframe structures that may include
self-erecting and self-healing features.

4.4.2 Pitch bearing and drive system
The main functions of the blade pitch system are to keep the WT operating
within a designed speed range and to unload the rotor bringing it to a shut-
down condition. This is accomplished by rotating (i.e. “pitching”) the blades
about their longitudinal axis relative to the hub. The pitch bearing is the mov-
able or “slewing” interface that permits this rotation; while at the same time
safely transmits the rotor loads into the hub, main shaft and support structure.
This angular positioning of all the blades for a rotor is more or less coor-
dinated simultaneously throughout the operational range; however there are
some design concepts that deviate from this and deliberately operate each
blade slightly different (largely per revolution) to optimize energy capture and
minimize loads.
Should the blade pitch actuation and control system be hydraulic or electric?
Figure 21 shows the concept of an electrical pitch system where the pitch drive
bull gear is driven by an electric motor through a gearbox ratio suffi cient to ensure
enough drive torque for the proper range of operation meeting the requirements for
blade aero torque and rotor loads transfer across the pitch bearings. Typical electrical
pitch drive systems have pitching rates as fast as 7.5

8°/s.
A little more than half of all MW WTs running today use hydraulic pitch sys-
tems instead of electrical. OEMs appear to make this choice and stick with it for
reasons that are not clearly established. Although value analysis shows that there
may be a slight cost advantage for hydraulic pitch systems, the potential control-
lability advantages for electric systems and lack of environmental concern for
hydraulic fl uid leakage offsets this view.

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Figure 21: Hub and blade pitch gear − blade shown in “running position”.