Powering Europe - European Wind Energy Association

When considering the impacts of wind power on the
different types of reserve requirements, it is of central
importance to make a clear distinction between the
need for flexibility in longer time scales of several hours
to a day (power plants that can follow net load variation)
and the need for reserves that can be activated
in seconds or minutes (power plants that can follow unpredicted
net load variations – demand minus wind).
Figure 1 illustrates the time scales of relevance.
Primary reserves
Wind power development will have only a small influence
on the amount of primary reserves needed. On
time scales of seconds/minutes, rapid variations in
total wind power capacity output occur randomly, like
the load variations that already exist. When aggregated
with load and generation variations, the increase in
variability due to wind is very small. Furthermore, the
amount of primary reserve allocated to the power systems
is dominated by the potential outages of large
thermal generation plants, so it is more than large
enough to cope with the very rapid variations in wind.
In practice, power plant generation is scheduled to
match the anticipated trends in demand so it can be
balanced with the supply. For any deviations from the
anticipated trends, primary and secondary reserves
are operated continuously to keep system frequency
close to its nominal value (see page 83). In addition,
wind power can provide its own primary reserve.
Secondary and tertiary reserves
On the time scale of 10-30 minutes the impact of wind
power on the need for secondary reserves will only be
significant and increase due to wind energy penetration
levels of more than 10% 2 .
Wind power has a much more significant impact on
the way conventional units are scheduled to follow the
load (hour to day time-scales). In the absence of a
perfect forecast, the unit-commitment decision will be
chApTEr 3 powersystemoperationswithlargeamountsofwindpower
surrounded by uncertainty additional to the normal uncertainty
associated with load and conventional generation
outage forecasting. The result is that sometimes
a unit might be committed when it is not needed, and
sometimes a unit might not be committed when it is
needed. Here, the generation mix of the power system
determines how the scheduling will change according
to the expected wind power production – the more flexible
power units there are, the later the unit commitment
decisions need to be made.
Estimates for the increase in short-term reserve balancing
capacities [Holttinen, 2009] show a wide
range: 1-15% of installed wind power capacity at 10%
penetration (of gross demand) and 4-18% of installed
wind power capacity at 20% penetration.
Discussion of additional reserve
requirements
Differences in the power system’s operational routines
explain a lot of the differences shown in Figure 2, notably
how often the forecasts of load and wind are updated.
If a re-dispatch based on a forecast update is
done in four to six hours, this would lower the reserve
requirements and costs of integrating wind compared
with scheduling based on only day-ahead forecasts.
Emerging intra-day markets take this particularity into
account by giving the opportunity for hourly updates.
The way the power system is operated regarding the
time lapse between forecast schedules and delivery
has a decisive impact on the degree of uncertainty
wind power will bring and so will indirectly determine
the amount of additional reserves required.
It is important to note that an increase in reserve requirements
does not necessarily mean new investments will
have to be made, for example the construction of new
thermal power plants. From analysis of the system and
from experience it follows that the forecast uncertainty
of incidental combinations of wind power generation
and demand is critical for assessing the need for additional
reserves, especially the “low demand high wind”
2 See IEA Task 25 “power Systems with large Amounts of Wind power” http://www.ieawind.org/AnnexXXV.html
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