Introduction - Wind Power Integration: Connection and System Operational Aspects - page 15

1.7.1 Wind fluctuations
Just as consumer demands are smoothed by aggregation, so is the output from wind
plant, and geographic dispersion dramatically reduces, in proportion, the wind
fluctuations. This is clearly illustrated by data from the German wind programme
(ISET, 1999/2000), and by an analysis of wind fluctuations in Western Denmark
(Milborrow, 2001). The data cover 350 MW of plant in Germany and 1,900 MW in
Western Denmark.
In each case, there were no recorded upward power excursions in an hour
greater than 20% of rated power. However, from individual machines, or a single
wind farm, power excursions up to about 96% of rated power were recorded, albeit
rarely. With a single machine, excursions of 10% of rated power were measured for
about 7% of the time, whereas these fluctuations only occurred about 1% of the
time from the aggregated output. Figure 1.5 compares the output from all the wind
plant in Western Denmark with data from a single wind farm. The latter is based on
power measurements from Bessy Bell wind farm in Northern Ireland, by kind
permission of Northern Ireland Electricity. These data were analysed in the same
way as the Danish data to show the power swings.
The measurements from the German wind programme also indicate that the
power fluctuations are lower than suggested by previous modelling and analyses.
This, in turn, means that the operational penalties associated with running networks
with wind generation may be lower than some early estimates. The extra cost of
running more thermal plant as spinning reserve is modest - an early study sug-
gested it was below 0.1 p/kWh with 2% of wind energy on the system, rising to
around 0.2 p/kWh with 10% wind generation (Dale, 2002). A more recent estimate
suggested that the additional cost to the electricity consumer of 20% wind energy
would be about £286 million per annum, or 0.08 p/kWh to the consumer (National
Grid, 2011). Other studies have yielded similar results, as the key issue is the

10

1 farm

1

Western
Denmark

0.1

0.01

-100 -50 0 50 100

Change (% rated capacity)

Figure 1.5 Changes in average generated output from wind plant, based on

hourly averages