Environmental Engineering Reference
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100
90
Island_demand-15 min
Island_demand-30 min
Island_demand-60 min
Island_demand-2 h
Island_demand-4 h
Island_demand-8 h
Island_demand-12 h
80
70
60
50
40
30
20
10
0
0
10
20
30
40
50
60
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100
Power fluctuation (±% available capacity)
Figure 5.20
Ireland system demand fluctuations - cumulative distribution
demand tend to be random and uncorrelated - 99 per cent of variation lies within
6 and 11 per cent of system demand, respectively. Particularly over the time scale
of 4-8 hours the probability of significant variation is quite high, with 30 per cent
variation in system demand occurring 17 per cent (4 hours) and 42 per cent
(8 hours) of the time. However, this variation is due mainly to the daily morning
rise and evening fall in demand pattern. System demand can be predicted with high
confidence (1-2 per cent error), and, as discussed in Section 5.2, generating units
are scheduled to cope with this variation.
Comparing Figure 5.18 (wind power fluctuations) and Figure 5.20 (system
demand fluctuations) suggests initially that large-scale variation in wind production
across the island is much more frequent than large-scale variation in system
demand. However, assuming an average annual system demand of 3,600 MW
and an average installed wind farm capacity of 300 MW over the year, enables
the relative (percentage) variations to be approximated to absolute (MW) variations.
Over 15 minutes, 90 per cent of system demand variation is less than 120 MW, and
90 per cent of wind variation is less than 6 MW. Similarly, over 60 minutes, 4 hours
and 12 hours, 90 per cent of the expected variation in system demand/wind
production are 390/20, 1,200/57 and 1,950/120 MW, respectively. Clearly, the
system demand variations dominate. In particular, short-term (15-30 minutes)
variations in wind production can deplete regulatory reserve, which will be increas-
ingly evident with increased wind penetration levels.
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