Wind power technology - Wind Power Integration: Connection and System Operational Aspects - page 73

reduction in mechanical loads achieved with variable-speed operation. Variable-
speed wind turbines provide the following key advantages (M¨ller et al ., 2002):

'They are cost effective and provide simple pitch control. At lower wind speed,
the pitch angle is usually fixed. Pitch angle control is performed only to limit
maximum output power at high wind speed.

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They reduce mechanical stresses; gusts of wind can be absorbed, i.e. energy is
stored in the mechanical inertia of the turbine, creating an ''elasticity'' that
reduces torque pulsation.

●

They dynamically compensate for torque and power pulsations caused by back
pressure of the tower. This back pressure causes noticeable torque pulsations at
a rate equal to the turbine rotor speed times the number of rotor wings.

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They improve power quality; torque pulsations can be reduced due to the
elasticity of the wind turbine system. This eliminates electrical power varia-
tions, i.e. less flicker.

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They improve system efficiency; turbine speed is adjusted as a function of
wind speed to maximise output power. Operation at the maximum power point
can be realised over a wide power range.

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They reduce acoustic noise, because low-speed operation is possible at low
power conditions.'

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Presently the most common variable-speed wind turbine configurations are:

Doubly fed induction generator (DFIG) wind turbine ●

Wide-range variable-speed wind turbine based on a synchronous generator ●

3.6.1 Doubly fed induction generator wind turbine
A typical configuration of a DFIG wind turbine is shown schematically in
Figure 3.18. It uses a wound rotor induction generator with slip rings to take current
into or out of the rotor winding and variable-speed operation is obtained by
injecting a controllable voltage into the rotor at slip frequency (Holdsworth et al .,
2003). The rotor winding is fed through a variable frequency power converter,
typically based on two AC/DC IGBT-based voltage source converters (VSCs),
linked by a DC bus. The power converter decouples the network electrical fre-
quency from the rotor mechanical frequency, enabling the variable-speed operation
of the wind turbine. The generator and converters are protected by voltage limits
and an over-current 'crowbar'. VSCs and related power electronic devices are
described in the Appendix 1.
A DFIG system can deliver power to the grid through the stator and rotor,
while the rotor can also absorb power. This is dependent on the rotational speed of
the generator. If the generator operates in super synchronous mode, power will be
delivered from the rotor through the converters to the network, and if the generator
operates in sub synchronous mode then the rotor will absorb power from the net-
work through the converters.
These two modes of operation are illustrated in Figure 3.19, where w s is the
synchronous speed and w r is the rotor speed.