Wind Power Generation - Renewable Energy: Technology, Economics and Environment - page 313

In this case the gearbox usually is part of the power train, which joins wind tur-
bine shaft and generator shaft. It divides the drive system into the "slow" and the
"fast" generator shaft. The gearbox is located inside the nacelle of the wind energy
converter and often also serves as the main rotor bearing.
Currently, single or multiple-stage spur and planetary gears are applied. For
blade-controlled converters, spur gears offer the advantage of reaching the rotor
hub through the main shaft by means of e.g. feed lines. Disadvantages are larger
building mass and the wider nacelle that is usually required. Planetary gears are
comparatively more compact and lighter; however, the respective blade adjust-
ment design is much more expensive.
Efficiency amounts to about 98 % per gear level. Energy losses occur due to
the inevitable gearwheel friction causing heat transmission and sound emissions.
The latter may constitute a limiting factor with regard to wind energy exploitation
in view of the acceptance by the population. However, sound emissions may be
reduced by appropriate design measures. Particularly transmission of sound waves
from the gearbox to the wind energy converter body (i.e. nacelle, tower) has to be
avoided in order to prevent system components from acting as resonance bodies.
As for the automotive industry, transmission can, for instance, be prevented by a
corresponding rubber buffer of the gearbox fastening.
Gearless wind energy converters are also increasingly used. A multi-pole ring
generator is applied which is operated at variable revolutions using a direct cur-
rent intermediate circuit. Since revolution transmission is no longer necessary un-
der the described conditions, these kinds of wind energy converters do not require
any gearbox.
Generator. The generator converts the mechanical rotation energy of the power
train into electrical energy (Fig. 7.11). For this purpose slightly adapted commer-
cially available generators are used for conventional converters while especially
designed three-phase alternators are applied for gearless converters. The main
commonly applied generator types are synchronous and asynchronous generators
/7-6/.
Synchronous generator. Synchronous generators are equipped with a fixed stator
at the outside and a rotor at the inside located on top of a pivoting shaft. In most
cases direct current is transmitted to the rotor by slip rings. Direct current creates a
magnetic field inside the rotor winding (excitation). When driving the shaft, a cer-
tain voltage is created by the rotating magnetic field inside the stator whose fre-
quency matches exactly the rotational speed of the rotating field of the rotor. To
prevent expensive maintenance, slip rings are often avoided by the application of
so-called brushless synchronous generators, whose pivoting shafts are provided
with small rotating exciters.
If a synchronous generator is connected to a stable grid, as for the European
grid which is operated at a frequency of 50 Hz, it can only be operated at the num-
ber of revolutions pre-determined by the grid (Fig. 7.12). This is not desirable for