Operation of power systems - Wind Power Integration: Connection and System Operational Aspects - page 186

inevitably lead to greater forecast uncertainty, which must be matched by higher
reserve levels. This can be translated into additional unit start-ups, part loading of
thermal plant and increased capability for ramping up and down. As outlined
in Section 5.2, forecasting of system demand can generally be achieved with a
1-2 per cent error of peak demand over a 24-hour period. This is in contrast to
average wind forecasting errors of 8-10 per cent of wind capacity, with the
forecast generally becoming less valid for longer time horizons. As wind pene-
tration levels increase it is inevitable that increased uncertainties will arise in the
forecast of net demand, i.e. that part of the system demand met by conventional
generation. If the system operator is uncertain exactly how much wind generation
will be available at a particular time, then increased operating reserve must be
carried. It should be noted, however, that the primary reserve requirement will be
largely unaffected even at relatively high wind penetration levels (see Section 5.3.3).
Primary reserve is intended to cover the sudden loss of a large infeed to the system -
it is probable that this will always be an existing conventional generator, with
smaller wind farms scattered around the distribution and transmission networks.
Large-scale reduction in wind output is most likely to occur during storm condi-
tions, and even here may take place over several hours and should be largely
predictable.
In operational practice, utilities will perform the unit commitment task
according to the available wind forecast, acting effectively as a reduction in the
system demand. The unit commitment may be modified/tweaked to ensure that the
generation profile remains viable and economic within high and low confidence
boundaries of the wind prediction. Reserve levels must be increased to
combat wind variations on the time scales of tens of minutes to hours. As seen in
Section 5.3.2 for Ireland and in Section 5.3.3 more generally, although the wind
output cannot be predicted accurately, bounds can be placed on the likely variation
over extending periods of time. For example, taking Figure 5.18 representing wind
variability for Ireland from 1 hour ahead to 12 hours ahead, it can be seen that,
looking 1 hour ahead, a maximum variation of 11 per cent could be expected.
Variability increases to 32 and 50 per cent when looking 4 and 8 hours ahead,
respectively. Advance action plans will also be defined for extreme wind scenarios,
in addition to those normally created to ensure system integrity.
5.3.5 Capacity credit
On 28 February 2005 the Spanish electricity system was near collapse, due to a
combination of cold weather (increasing electrical demand) and a shortage of
conventional generation. Four nuclear power stations were out of service,
and hydroelectric reserves were largely depleted. Five gas-fired units were also
unavailable due to their fuel supply being interrupted for domestic purposes
(increased heating load), while tankers carrying a back-up liquefied natural gas
(LNG) supply could not dock at port due to the poor weather conditions
(Ford, 2005). Over the peak evening period, 4,000 MW of wind production
prevented emergency actions being implemented. The following day only 900 MW
was provided from wind generation, resulting in exports to France and Portugal