Operation of power systems - Wind Power Integration: Connection and System Operational Aspects - page 169

100

(over 12 h)
(over 4 h)
(over 15 min)

90

80

70

60

50

40

30

20

10

0

0 10 20 30 40 50 60 70 80 90 100

Variation reduction due to interconnection (% power fluctuation)

Figure 5.23 Northern Ireland/Republic of Ireland net wind power transfers -

cumulative distribution

wind output could be forecast perfectly, the task of operating the power system with
a time varying generation source would remain. This may affect the scheduling
of conventional generation units and the requirements for spinning reserve and
other ancillary services. At a system level, it is probably of greater interest to
consider the net demand variability rather than the wind variability. The net demand
may be defined as the system demand requirement to be met by conventional
generation, the rest being provided from wind generators or other variable renew-
able sources. There may be periods when an increase in wind output is offset by an
increase in the system load, while there may be other periods when a fall in wind
output is reinforced by an increase in demand. Analysis of data from western
Denmark suggests that most of the time (52 per cent) wind power production rises
with the system demand and falls with the system demand (Milborrow, 2004).
Often the greatest concern for a utility is that a large fall-off in wind production will
coincide with the daily morning rise. Section 5.3.2 illustrates that, for the Ireland
system, net demand variability is very similar to demand variability for current
wind penetration levels. Similar results have been obtained for other power systems
(Hudson et al. , 2001). This follows, since the short-term variance of the net demand
variations, n net demand , is given as the sum of the variances of the wind time series
and system demand time series, n wind and n system demand :

n net demand ¼ n wind þ n system demand

This assumes that they are statistically independent processes. Since the system
demand is dominated by the morning rise and evening fall-off in load, the net