Page 388 - Wind Energy Handbook

362 CONCEPTUAL DESIGN OF HORIZONTAL-AXIS TURBINES

6.9.2 Variable-speed operation

By interposing a frequency converter between the generator and the network, it is
possible to decouple the rotational speed from the network frequency. As well as
allowing the rotor speed to vary, this also allows the generator air-gap torque to be
controlled.
Variable-speed operation has a number of advantages:

• below rated wind speed, the rotor speed can be made to vary with wind speed to
maintain peak aerodynamic efficiency;

• the reduced rotor speed in low winds results in a significant reduction in
aerodynamically-generated acoustic noise – noise is especially important in low
winds, where ambient wind noise is less effective at masking the turbine noise;

• the rotor can act as a flywheel, smoothing out aerodynamic torque fluctuations
before they enter the drive train – this is particularly important at the blade
passing frequency;

• direct control of the air-gap torque allows gearbox torque variations above the
mean rated level to be kept very small;
• both active and reactive power can be controlled, so that unity power factor can
be maintained – it is even possible to use a variable speed wind farm as a source
of reactive power to compensate for the poor power factor of other consumers on
the network; variable speed turbines will also produce a much lower level of
electrical flicker.

In practice, losses in the frequency converter may amount to several per cent,
counteracting the increased aerodynamic efficiency below rated. In terms of energy
capture, there is often little to choose between a two-speed and a variable-speed
machine. The load reduction possibilities, however, mean that most large MW-scale
turbines now use variable speed in some form. Variations in aerodynamic torque at
blade passing frequency are particularly significant in larger turbines because of the
size of the rotor compared to the lateral and vertical length scales of turbulence.
Clearly there is a significant cost associated with the variable-speed drive or
frequency converter, which must be weighed against the advantages. Other draw-
backs include increased complexity, although there is no particular evidence of
reduced availability due to power converter problems, and the generation of
electrical noise and harmonics by the inverter system. Modern PWM inverters
produce much lower levels of undesirable harmonics than earlier devices because
of the high switching frequencies which can be achieved. Electrical noise can be a
problem for control signals within the turbine if insufficient care is taken to shield
cables. Fibre optic transmission is increasingly being used, and this is not affected.
There are two principal methods of achieving variable speed operation: ‘broad
range’ variable speed, in which the generator stator is connected to the network via
the frequency converter, and ‘narrow range’ variable speed where both the gen-