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Power Measurements

A new way to measure what matters most: how close a unit comes to meeting its total potential profit.
Fortnightly Magazine - December 2004

the extent to which the unit was operated according to the perfect solution. Because the metric ranges between 0 and 100 (measured in percentages), a plant operator can obtain no credit, perfect credit, and everything in between.

The score for each hour is effectively weighted by price. Hours when the market price is very high or very low are weighted higher than average prices. This is an attractive characteristic of this statistic because these are the hours when you make a killing or lose your shirt and therefore probably should be taken more seriously. Another attractive characteristic is that an operator can get as much credit for avoiding losses by turning the unit off during low price periods as it gets for running the unit when the prices are strong.

Examples of the EDF in Action

Figure 2 shows an example of the dispatch of PSEG's Kearny Generating Station 9, an 18.5-MW GT unit in Hudson County, N.J. During the period shown in the chart, the unit posted a 17 percent capacity factor. If the unit were to have generated only when it was the least bit profitable, it would have posted a 35 percent capacity factor. While the unit did not generate every hour that it was profitable to do so, and it did operate during some hours when it lost money, on the whole the dispatch pattern results in an EDF of about 32 percent. Because the EDF is nearly twice the capacity factor, it is clear that the unit did a better job of choosing when to generate than PSEG did if one were to measure total generating hours alone.

Consider all the results for the peaking unit, Kearny Generating Station 9 in 2003 (). During that year, it only ran for 49 hours and had a capacity factor of less than 1 percent, but it picked the right hours to generate, capturing many of the peak price periods. There were 335 hours when the price was higher than the marginal cost but the unit did not run. This did not affect the EDF much, however, because for this particular unit, the best way to obtain a high score is to avoid the losses that would accrue because of relatively low prices. This unit did just that. As a result, this unit scored a 98.61 percent EDF in 2003.

In Table 2 we see the 2003 operating statistics for the Bergen Generating Station CC. Note that for most of the year this combined-cycle unit was generating when the price was favorable. From the capacity factor, it's clear that this unit did not generate at full capacity most of the year. The unit did a good job of ramping its generation to near full capacity when the prices were highest. Note that the unit did generate for 6,146 hours when price was less than marginal cost. That seems like a bad idea, but what these numbers miss is the fact that the unit ramps down to its minimum threshold during most of these low price periods. The EDF score reflects