Operation of power systems - Wind Power Integration: Connection and System Operational Aspects - page 157

capacity credit for wind farms (Section 5.3.5). High wind generation at periods
of low demand is also unlikely, lessening the need for wind curtailment
(Section 5.3.6). This beneficial relationship is true for Ireland (and north-western
Europe), but isn't always the case. For example, peak wind generation in New York
State tends to occur at night (when demand is low), while during daylight hours
wind generation tends to be less (Piwko et al. , 2005). Similar behaviour is seen in
southern California, where wind production tends to fall sharply during the morn-
ing before rising in the afternoon (Kahn, 2004). In contrast, in south Australia
electrical demand and wind generation are almost uncorrelated, so that peak wind
production is equally likely at times of high and low demand (AGO, 2003).

5.3.2.2 Individual wind farm variability
The energy production from wind farms varies on the time scales of seconds and
minutes to months and years. Hence, an understanding of these variations and their
predictability is essential for optimal integration. Figure 5.11 depicts the normalised
output for an individual wind farm, utilising fixed-speed turbines, over the period of
1 minute. Local topography and weather patterns are dominant factors in determin-
ing wind variability, and for an individual turbine will affect both instantaneous wind
speed and direction. In addition to the slow drift downwards in electrical output
shown here, superimposed on the power output trace is a low frequency oscillation,
due to the variation in wind speed seen by the turbine blades. A combination of
factors is involved: tower shadowing, wind shear and turbulence. The frequency
of the oscillation will thus depend on the rotor rotational speed, and the number of

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0 10 20 30 40 50 60

Time (s)

Figure 5.11 Single wind farm variability - 1 minute