As the penetration of wind generation increases on power systems
throughout the world, the effects of wind variability on power systems
are of increasing concern. This study focuses on sustained occurrences
of low wind speeds over durations ranging from 1 h to 20 days. Such
events have major implications for the variability of energy yields from
wind farms. This, in turn, influences the accuracy of wind resource
assessment. The frequency analysis techniques commonly used to study
wind variability cannot represent the autocorrelation properties of wind
speeds and thus provide no information on the probabilities of
occurrence of such sustained, low wind events. We present two
complementary methods for assessing wind variability, runs analysis and
intensity–duration–frequency analysis, both with emphasis on
characterising the occurrence of continuous, extended periods (up to
several days) of low wind speeds. Multi-annual time series of hourly
wind speeds from meteorological stations in Ireland are analysed with
both techniques. Sustained 20-day periods corresponding to extremely low
levels of wind generation are found to have return periods of around 10
years in coastal areas. Persistent, widespread low wind speed
conditions across the entire country are found to occur only rarely.