Environmental Engineering Reference
In-Depth Information
by only 2 per cent and 4 per cent for wind penetration levels of 10 per cent and
20 per cent, respectively (Holttinen, 2005b). Clearly, the Nordic power system is
more widespread than that of Germany (reducing variability), while market opera-
tion enables party intervention 1 hour ahead only, mitigating forecasting errors.
For isolated power systems, such as Ireland, which are small in terms of both
geographical area (and hence high variability) and electrical demand requirements
(diminished system flexibility), there may be encouragement for greater exploita-
tion of fast-starting OCGTs and diesel generating sets. Even with the advent of
40 per cent efficient OCGTs, the economic and environmental arguments are not
straightforward. Wind curtailment and ramping limits are alternative options.
Variations in wind production 1-12 hours ahead are significant because they
can affect the scheduling of conventional generation. Wind variability should be
compared with the cold start times of conventional generation, which may
require 6-10 hours before operating at full capacity. In Ireland, over 4 and 12 hour
periods, the maximum variations are 64 and 100 per cent of wind farm capacity
(see Figure 5.19). In Germany, the maximum variation 4 hours ahead is 50 per cent
of installed wind capacity, rising to 85 per cent looking 12 hours ahead. In the
Nordic area, even for the longer time horizon, the maximum variation will only be
50 per cent (Holttinen, 2005a). Here, however, wind-forecasting tools can be of
assistance, although the uncertainty of the prediction tends to increase with the time
horizon. Uncertainty can also be associated with the demand prediction, but again
the magnitude of this is likely to be much less. Hence it is not the variability of the
wind over such time horizons that causes scheduling difficulties, but the errors
associated with the wind forecasts.
A scenario that has concerned system operators is the passage of a storm front,
where wind speeds exceed the cut-out speed, resulting in a shut down from full
output to zero within minutes. In practice, this is only ever likely to occur for small
geographical areas. The distributed nature of wind generation implies that shut-
down will take place over hours and not instantaneously. In the United Kingdom
for example, high wind speeds ( > 25 m/s) are extremely rare, with a probability
less than 0.1 per cent at most sites. There has never been an occasion when the
entire UK experienced high winds at the same time (ECI, 2005). In fact, the
windiest hour since 1970 affected around 43 per cent of the United Kingdom - an
event expected to occur around 1 hour every 10 years. Similarly, on 8 January
2005, a storm all over Denmark resulted in wind speeds over 25 m/s, and a
reduction in wind generation approaching 2,000 MW, but over a period of roughly
6 hours (Bach, 2005). Of course, there are parts of the world where the wind
resource is sufficiently good to introduce its own problems. In New Zealand, for
example, sites with average wind speeds in the range 10-12 m/s are common, and
wind speeds can regularly exceed 25 m/s every 3-4 days on average (Dawber and
Drinkwater, 1996). More generally, the expected growth of offshore wind genera-
tion also suggests that the existing benefits of dispersed (onshore) generation may
be degraded. Without intervening control actions, discussed below, generation
from large-scale offshore sites may be lost within about an hour during severe
storm conditions.
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