The cyclical nature of customer demand on large scale interconnected networks is a well known phenomena.  Demand varies by time of day and responds to many factors that influence electric usage, including weather, seasonal activities and business cycles. Composite electric load generally behaves in a cyclical fashion for periods of a day and a year.  With the influx of larger amounts of  wind power, another cyclical characteristic is applied to the power system, that of the available wind generation.  Wind that drives turbines for wind farms varies continuously and generally behaves in a cyclical fashion for periods of a day and a year, just like demand.  The output capacity of a wind farm varies according to the prevailing wind.

Whereas demand tends to peak in either winter or summer, wind capacity tends to peak in spring and fall.  Furthermore, while demand tends to be highest at the hottest time of the day (for summer peaking areas), wind capacity tends to be lower during hot and sunny daylight hours.

Both demand and wind capacity have an impact on the thermal loading of transmission systems, and the non-coincidence of their cyclical behavior leads to interesting transmission usage patterns.