Very Low Wind Generation Period of Jan-09, and a 1-Year Lookback at Frequency of Very Low Wind Gen Periods

Author: Bonneville Power Authority | Grid, Idaho, Montana, Oregon, Washington

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The very low wind generation period of Jan-09 has ended, after:

• more than 11-1/2 continuous days with total wind gen <50 MW: 14-Jan-09 02:43:00 thru 25-Jan-09 17:13:00 (11 days, 14.5 hours);
and
• more than 8-1/2 continuous days <10 MW: 14-Jan-09 16:59:00 thru 23-Jan-09 07:37:00 (8 days, 14.6 hours)

How often in the past year have we seen others periods of “very low” wind generation?

The next plot shows the Percent of Time by Week Where Total Wind Gen Was <50 MW over the last 56 weeks. The installed wind capacity during this time was ~1,500 MW, so the 50 MW threshold represents ~3% of capacity. The full 56-week average was ~23%, that is, nearly a quarter of the time the total wind gen was less than 3% of total wind capacity.

Other than the very low wind gen period this month, the weeks of 1/21/08 (55.0%) and 10/27/08 (53%) had the next greatest frequency of low wind gen periods. There was a slight seasonality, with the fall months having somewhat more “very low wind gen” periods.

These very low wind gen periods highlight the high correlation (i.e., low diversity) among wind gen plants within the balancing authority area. [Twelve facilities totaling 1,301 MW were on line at the beginning of 2008; during the data period, five more facilities came on line, adding 470 MW; BPA’s grid covers Washington, Oregon, Idaho, and western Montana.]

[The accompanying data file includes average wind generation for every hour in these 56 weeks. The average hourly average was 361.1 MW (24% of 1,500 MW). The aggregate of the BPA wind plant generated at or above that rate 3,623 out of 8,779 hours, or 41.3% of the time. This accords with Rosenbloom’s law of wind turbine output: Whatever the capacity factor, any turbine or aggregate of turbines generates at or above its average rate only 40% of the time.]

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