Powering Europe - European Wind Energy Association

network element is important for system operation.
Therefore, on systems with relatively high wind penetration,
there is often a requirement for frequency response
or frequency control. Pitch controlled wind turbines
are capable of such system support only when
they are set in advance at a level below the rated output
and, of course, if wind is available. This allows
them to provide primary and secondary frequency control.
This can take many forms, but the basic principle
is that, when instructed, the wind power plant reduces
its output power by a few percent, and then adjusts its
output power in response to the system frequency. By
increasing power when frequency is low or decreasing
when frequency is high, the wind power plant provides
a contribution to controlling the system frequency.
The problem associated with this type of network assistance
from wind turbines is a reduced output and
hence loss of income, which might not be offset by
the primary control service. This is less of an issue
for conventional power stations, where the lost revenue
will be compensated to some extent by a reduction
in fuel consumption. For wind power this implies
a loss of electricity produced at zero fuel costs, therefore
it is not the cheapest option for the system, and
should only be applied when other more cost effective
options, such as fuel based technology curtailments,
have been exhausted.
protectivedevices
Protective devices such as relays, fuses and circuit
breakers are required in order to protect the wind power
plant and the network from electrical faults. Careful
co-ordination may be required, in order to ensure that
all conceivable faults are dealt with safely and with the
minimum disconnection of non-faulty equipment. Fault
current is a related issue. In the event of an electrical
fault in the network close to the wind power plant,
some fault current will flow from the wind turbines into
the fault. There may be requirements on the maximum
or minimum permitted levels.
powerquality
This term covers several separate issues [IEC, 2008]
that determine the impact of wind turbines on the voltage
quality of an electric power network. It applies in
chApTEr 2 Windgenerationandwindplants:theessentials
principle both to transmission and distribution networks,
but is far more essential for the latter which
are more susceptible to voltage fluctuations on the
generation side.
The relevant parameters are active and reactive power,
including maximum value, voltage fluctuations (flicker),
number of switching operations (and resulting voltage
variations), harmonic currents and related quantities.
The standard for characterising the power quality of
wind turbines and for the measurement of the related
quantities is IEC 61400-21 [IEC, 2008]. The application
of this standard enables a careful evaluation of
the impact of wind power plants on the voltage quality
in electrical networks. Instead of applying simplified
rules which would be prohibitive for wind power,
analysis of IEC 61400-21 methods is recommended
(Tande in [Ackermann 2005] p.79) in order to carry
out the following:
• Load flow analysis to assess whether slow voltage
variations remain within acceptable limits
• Measurements and comparison with applicable
limits of maximum flicker emission which can be
caused by wind turbines starting or stopping, or in
continuous operation
• Assessment of possible voltage dips due to
wind turbine start-up, stops or by energisation of
transformers
• Estimation of maximum harmonic current and comparison
with applicable limits
Visibility
In a power system with large contributions from decentralised
plants, it is essential for the system operator to
obtain on-line information about the actual operational
conditions at the decentralised plants. Access to such
information can, for example, be critical during network
faults when fast decisions have to be made to reschedule
generators and switch network elements. For this
purpose, agreements are made between the system
operator and the wind plant operators on communicating
signals such as active and reactive power, technical
availability and other relevant status signals. On-line information
about wind plants can also be necessary for
system operation for the purpose of short-term forecasting
of the output of wind plants in a region.
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