At WECC, we use the distributions shown in the previous sections to find the threshold margin. The threshold margin is calculated as the distance needed between the resource availability and the demand for the tails of the distributions not to overlap beyond a certain amount. The certain amount refers to the industry practice of a reliability threshold of no more than 1 day of loss of load every 10 years, commonly referred to as “1 day in 10.” The tails are the ends of the distributions where the probabilities flatten out. As discussed in the Margin Variability section, if you compare the high end of the demand distribution with the low end of the resource availability distribution, the overlap is where there can be risk of not having enough resources to meet demand for that hour.
The chart shows the same hourly comparison that was shown in the Margin Variability section. However, this time the threshold margin is added to the chart. The difference between resource availability and the threshold are compared to see whether resource availability falls below the threshold margin. If so, there is risk in the system on those hours.
Clicking a region on the map will change the charts to show that area's information. Clicking the same area again will switch the charts back to the overall Western Interconnection results.
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