Progress of a storm

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Winds of hurricane force which swept across Denmark in December last year gave the country's utilities an unexpected opportunity to register how a power system is affected when large numbers of wind turbines stop turning. Actual wind measurements, part of Denmark's offshore wind plant development program, were also compared with power system control room data.

Wind monitoring at one of the proposed offshore sites, Horns Rev on the Jutland west coast, revealed wind speeds of 58 m/s. The wind data established that an offshore wind plant at that location would have run at full load for over 40% of the time -- considerably more than previously estimated and not far from the energy utilisation expected of a coal plant when not running as a combined heat and power facility. For a 12 hour period at the Horns Rev site, average wind speeds of more than 25 m/s were recorded -- the cut out wind speed for most wind turbines.

Indeed, all wind turbines in the entire area managed by Eltra, the power system operator for the main Jutland peninsula and large island of Funen, stopped during the storm -- but not all at once. The wind turbines cut-out and re-started sequentially as the storm rolled over the country.

At 10 a.m. on December 3, the 4370 turbines with a combined capacity of 1320 MW in operation that day were producing 200 MW of power. During the following three to four hours production increased to 1100 MW before half the turbines cut-out during the afternoon. Between 6 p.m. and 9 p.m. production fell to 400 MW. From that point it steadily picked up again as the wind turbines restarted.

Eltra notes that the storm's effect on wind output differed from region to region over a relatively small geographical area. In southwest Jutland the wind increased from 5 m/s to 20 m/s from 10 a.m. to 3 p.m. and then nearly doubled in strength up to 7 p.m. before dropping. In North Jutland wind speeds were 10 m/s throughout the afternoon, dropped around 6 p.m. and then picked up again.

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