Operation of power systems - Wind Power Integration: Connection and System Operational Aspects - page 180

generation-demand balance, conventional plant will reduce their output in sympathy
with the variation in wind production, and so fuel is saved. Wind generation thus
appears as negative load, causing an over-prediction of system demand. Further fuel
could be saved if one or more of the generating units was de-committed, but this
action is not taken for fear of a wind power lull. The result is a very secure power
system, with high levels of reserve, enhanced load-following capability (due to the
number of part-loaded plant) and a solution that is easily implemented. However,
there are a number of notable disadvantages.
There will at times be an over-commitment of conventional plant, with wind
production having to be curtailed to ensure that the minimum output restrictions
of generators are not violated. This issue can be particularly significant for smaller
systems, where at times of low demand, and ignoring the likelihood of wind
generation, a relatively small number of large (high efficiency) generators
would probably be employed. Wind curtailment may be required should there
be significant wind power import. A more expensive solution, but offering greater
load following flexibility would be to operate a larger number of small
(lower efficiency) conventional units. Furthermore, many generators will now
be part-loaded, causing an increase in unit heat rate (which relates to the fuel
consumption rate), and so CO 2 savings (from fuel saved) may not be fully rea-
lised. For example, a 5-10 per cent absolute reduction in thermal efficiency for
fossil-fired plant could be considered typical (Leonhard and M¨ ller, 2002).
Similarly, CCGTs offer the advantage of high efficiency and low emissions when
operating under full-load conditions, but both can drop off significantly if units
are part-loaded. At higher outputs, fuel and air can be pre-mixed before com-
bustion. However, when running at part-load ( < 50-60 per cent) this is no longer
possible, in order to maintain flame stability, and oxides of nitrogen (NO x )
emissions increase dramatically (CIGRE, 2003).
Looking at the longer term, the fuel saver mode effectively assumes a zero
capacity credit for wind. It follows that there will be no reduction in (conventional
plant) capital costs to meet anticipated growth in demand. The fuel saver approach
is thus simple to implement, but potentially expensive. However, despite all the
drawbacks, such conservative unit scheduling has been the natural response of most
system operators - it is probably the best approach when wind energy penetration is
low, say less than 5 per cent of annual energy supplied.
5.3.4.2 Wind forecasting mode
A wind energy penetration figure of 10 per cent is often quoted as a threshold
figure after which the fuel and emissions cost of part-loading fossil-fuel genera-
tion compels integration of wind production into the daily scheduling process.
Allowing for wind forecast errors and wind variability would suggest, however,
that system reserve levels would have to be increased slightly if no-wind system
reliability levels are to be maintained. Unit scheduling following a wind fore-
casting approach therefore tends to favour smaller, more flexible generation plant
which may be required to start up and shut down more quickly compared with the
fuel saver mode.