Wind Power Generation - Renewable Energy: Technology, Economics and Environment - page 312

Besides power control the blade adjustment mechanism also transfers the rotor
blades into feathered pitch (position without any tangential force), evacuating
them from the wind in case of a forced rotor standstill. Since, for the sake of
safety, wind energy converters are provided with redundant safety systems (thus
equipped with braking features), the blade adjustment mechanism can also be ap-
plied for breaking in addition to the mechanical brakes (see also /7-3/, /7-4/, /7-5/,
/6-6/).
The main components of the blade adjustment mechanism are rotor blade bear-
ing, blade adjustment drive, energy supply and an emergency adjustment system,
if required.
− Rotor blade bearing. Generally, rotor blade bearings hinged at the rotor blade
root of the hub. In principle, trunnion and moment bearings are distinguished.
Unlike for other typical bearings, this bearing does not need to be optimised
with regard to contortions, but regarding static and dynamic strains. Some
blade designs only allow for adjustment of the outer blade range (so-called
blade tip adjustment). In this case the bearing is located in the outer blade sec-
tions.
− Blade adjustment drive. Rotor blades are either adjusted electro-mechanically
or hydraulically. For hydraulic systems, for instance, centering actuators are in-
stalled inside the rotor hub, which enable rotation either directly or by means of
corresponding reversing levers. For systems provided with electric motors
blades are adjusted by mechanical components driven by a centralised electric
motor (e.g. by head spindles, toothed gearing). For more recent designs each
rotor blade is provided with its own drive to adjust every blade optimally to-
wards wind direction.
− Energy supply. The energy supply, also applied for blade adjustment, is located
inside the nacelle. To ensure shut-down of the wind energy converter also in
case of malfunction, the nacelle is provided with corresponding energy accu-
mulators (e.g. pressure accumulators in case of hydraulic systems and batteries
for electric-mechanical drives) (see Fig. 7.11).
− Emergency adjustment system. In case of a power failure or other kind of mal-
function the emergency adjustment system safely locks the rotor. Rotors are,
for instance, moved into feathered pitch, thus avoiding "racing" of the no-load
rotor. Since such safety systems must by all means be redundant, usually addi-
tionally a mechanical brake is installed.
Gearbox. To convert the kinetic energy of the rotor into electrical energy, for
conventional converters equipped with common four or six-pole synchronous or
asynchronous generators, generally revolutions of 1,000 or 1,500 r/min are re-
quired when adhering as much as possible to grid specifications (50 Hz). Current
rotor revolutions of 10 to 50 r/min with wind energy converters of installed ca-
pacities ranging from several 100 kW up to the multi-megawatt range thus require
a transmission gear if no specific generators are applied (Fig. 7.11).