Introduction - Wind Power Integration: Connection and System Operational Aspects - page 9

In the early years of wind energy development, the majority of wind turbines
operated at a fixed speed and used induction generators, but increasing numbers
now run at variable speed, using power conditioning equipment (Chapter 3).
The advantage of using slower speeds in low winds is that noise levels are reduced
and, in addition, aerodynamic efficiency - and hence energy yield - is slightly
increased. Increasing numbers of machines dispense with a gearbox and use a direct
drive to a multi-pole generator.
A further advantage of using power conditioning equipment is that it avoids the
need to draw reactive power from the electricity network. This benefits the utility
and may mean savings for the wind turbine operator as reactive power is often
subject to a charge. A further benefit accrues if the wind turbine is able to sell
energy with a lagging power factor to the utility at a premium.
The most recent developments in power train technology involve the use
of direct-drive electrical generators. The German manufacturer, Enercon, for
example, has sold a large number of machines of this type, worldwide, and
currently offers machines in a range of sizes from 200 to 7,580 kW. The diameter
of the latter is 127 m.

1.4.3 Summary of principal design options
A summary of the principal options is given in Table 1.1.

Table 1.1 Features of typical electricity-generating wind turbines

Rotor size and rating Up to 164 m and 8 MW

No. of blades Most have three, some have two, a few have one

Blade material Most use glass-reinforced plastic, increasing use of carbon

fibre-reinforced plastic

Rotor orientation Usually upwind of tower; some downwind machines

Rotational speed Constant speed machines rotate at about 10 revolutions/minute at

100 m diameter, faster at smaller sizes, slower at larger sizes
Increasing numbers of variable-speed machines

Power control The most common methods are:

'Pitch control': all or part of the blade rotated to limit power
'Stall control': blades are fixed, but stall in high winds; now
becoming less common

Power train Step-up gearboxes most common, but direct drives (no gearbox)

with multi-pole generators now increasing in popularity

Generator Induction usual, four or six pole; double-fed induction generators were

popular around the turn of the century, but variable speed machines,
with AC/DC/AC power electronics becoming increasingly popular
Direct-drive machines also becoming increasingly popular

Yaw control Sensors monitor wind direction; rotor moved under power to

line up with wind
A few machines respond passively

Towers Cylindrical steel construction most common

Lattice towers used in early machines
A few (large) machines have concrete towers