Operation of power systems - Wind Power Integration: Connection and System Operational Aspects - page 165

100

Wind farm
NI Total
All_Island

90

80

70

60

50

40

30

20

10

0

0 10 20 30 40 50 60 70 80 90 100

Generation (% available capacity)

Figure 5.19 Load duration characteristic - single wind farm, Northern Ireland

region, all island

duration curve for a single wind farm, the N. Ireland region and the entire island.
All three curves are broadly similar in shape, and only differ in the low and high
extremes. Considering first a single wind farm, there is a quantifiable period of
time, approximately 20 per cent, when the total production is almost zero, due either
to maintenance outages or becalmed conditions. Similarly, for 2 per cent of the time
the wind farm is operating at maximum output, since above rated wind speed the
electrical output is effectively curtailed. The addition of further wind farms has
the effect of flattening the load duration curve and removing the extremes. Total
wind production equalling 100 per cent of available capacity implies that all wind
farms must be operating at 100 per cent output. Thus, considering the N. Ireland
region, the wind farms occupy 32 per cent of their time at 50 per cent or more output,
and no time above 95 per cent output. For the entire island the trend continues, so
wind production exceeds 50 per cent of available capacity for 29 per cent of the time,
and rarely exceeds 90 per cent. At the same time, total wind production across the
island exceeds 10 per cent of capacity for 78 per cent of the time, while the equivalent
figures for N. Ireland and a single wind farm decrease to 76 and 70 per cent
respectively.
The system demand, like wind power production, is also a highly variable
quantity, and it is equally informative to plot the cumulative distribution of varia-
tions over different time scales, ranging from 15 minutes to 12 hours, akin to
Figure 5.18. In this case, the variation in load, over a particular time horizon, has
been determined as a fraction of the average system demand over that time.
As expected, increasing the time horizon of interest results in a greater likelihood
of increased variation. For example, over 15 and 30 minute periods fluctuations in