  The incidence of widespread low-wind conditions is important to the
reliability and economics of electric grids with large amounts of wind power.
In order to investigate a future in which wind plants are geographically
widespread but interconnected, we examine how frequently low generation levels
occur for wind power aggregated from distant, weakly-correlated wind
generators. We simulate the wind power using anemometer data from nine
tall-tower sites spanning the contiguous United States. We find that the number
of low-power hours per year declines exponentially with the number of sites
being aggregated. Hours with power levels below 5% of total capacity, for
example, drop by a factor of about 60, from 2140 h/y for the median single site
to 36 h/y for the generation aggregated from all nine sites; the standard
deviations drops by a factor of 3. The systematic dependence of
generation-level probability distribution "tails" on both number and power
threshold is well described by the theory of Large Deviations. Combining this
theory for tail behavior with the normal distribution for behavior near the
mean allows us to estimate, without the use of any adjustable parameters, the
entire generation duration curve as a function of the number of essentially
independent sites in the array.
