Powering Europe - European Wind Energy Association

grid, such as large conventional generators suddenly
dropping off line and errors in demand forecast. Obviously,
as illustrated in Figure 3, wind energy’s share
of production – which can be quite high - determines
how much system operation will be affected by wind
variability.
Variableversusintermittentgeneration
Wind power is sometimes incorrectly considered
to be an intermittent generator. This is misleading.
At power system level, wind power does not
start and stop at irregular intervals (which is the
meaning of intermittent, and which is a characteristic
of conventional generation). Even in extreme
events such as storms it takes hours for most of
the wind turbines in a system area to shut down.
For example in the storm of 8 January 2005, it
took six hours for the aggregated wind power in
Western Denmark to shut down from 90% to 10%
production. Moreover, periods with zero wind power
production are predictable and the transition to
zero power is gradual over time. It is also worthwhile
considering that the technical availability of
wind turbines is very high (98%) compared to other
technologies. Another advantage of wind power
in this respect is its modular and distributed installation
in the power system. Breakdown of a
single unit has a negligible effect on the overall
availability. Thus, the term intermittent is inappropriate
for system wide wind power and the qualifier
variable generation should be used.
Short-term variability
For grid integration purposes, the short-term variability
of wind power (from minutes to several hours) is
the most important. It affects the scheduling of generation
units, and balancing power and the determination
of reserves needed. The short-term variability
of wind power, as experienced in the power system,
is determined by short-term wind variations (weather
patterns), and the geographical spread of wind power
plants.
chApTEr 2 Windgenerationandwindplants:theessentials
The total variability experienced in the power system
is determined by simultaneous variations in loads for
all wind power plants and other generation units. The
impact of the short-term variation of wind power on a
power system depends on the amount of wind power
capacity and on many factors specific to the power
system in question (generation mix, degree of interconnection),
as well as how effectively it is operated
to handle the additional variability (use of forecasting,
balancing strategy).
Analysing the available power and wind measurements
at typical wind plant locations allows the variations in
net power output expected for a given time period, i.e.
within a minute, within an hour or over several hours,
to be quantified. The distinction between these specific
time scales is made because this type of information
corresponds to the various types of power plants
for balancing. Experience and power system analyses
show that the power system handles this short-term
variability rather well.
Variationswithintheminute:notanoticeable
impact
The fast variations (seconds to one minute) of aggregated
wind power output as a consequence of turbulence
or transient events are quite small as can be
seen in the operational data of wind farms. As a result
they are hardly felt by the system.
Variationswithinthehourarefeltbythesystemat
largerpenetrationlevels
These variations (10-30 minutes) are not easy to
predict, but they even out to a great extent with geographic
dispersion of wind plants. Generally they remain
within ±10% of installed wind power capacity
for geographically distributed wind farms. The most
significant variations in power output are related to
wind speed variations in the range of 25 – 75% of
rated power, where the slope of the power curve is the
steepest. The variations within an hour are significant
for the power system and will influence balancing capacities
when their magnitude becomes comparable
to variations in demand; in general this will be from
wind energy penetration levels of 5 to 10% upwards.
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