IEEE Power & Energy Magazine - July/August 2015 - 98

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Based on the review of the papers listed in the bibliography, it
appears that C.W. Watchorn was the person who first proposed an
LOLP of one day in ten years as a reasonable criterion for reserve capacity planning. It is interesting to see how his thinking evolved over
time as expressed in his publications. In 1947, he commented that one
day in 27 years and one day in 88 years, respectively, were acceptable
lower and upper limits with one day in 8.66 years being in the unacceptable range [reference 14]. By 1950, he suggested one day in eight
to ten years and used one day in 8.66 years in an example calculation.
He did not provide any justification and wrote in reference 22:
"It is believed that a reasonable level of service reliability, when
the effect of probable load forecasting errors is included in the evaluation, is the probability of failure to carry the load of in the order
of an average rate of one day in from eight to ten years," he said.
By 1957 he seemed to consider an LOLE of 0.1 days/year as a
criterion that corresponded to an economic standard of service reliability. His discussion on the 1960 AIEE Working Group report
[Reference 72] suggests that the use of 0.1 days/year is now a tradition but "the tremendous advantage of money saving might warrant
a break from this tradition." By 1964, Watchorn appears to quote the
use of an LOLE of 0.1 days/year without any qualification.
It should be noted that the numerical value of the LOLP produced in a particular generation adequacy study is highly dependent
on the factors incorporated in the analysis. As an example, consider
two studies of a given system. In the first study, no load forecast uncertainty is considered. In the second study, significant load forecast
uncertainty is included. The calculated LOLP in the second case
will be higher than that in the first case, and the system load carrying capability at the criterion risk will be lower than in the first case.
There are a number of important factors that create similar effects.
The use of a particular LOLP criterion such as one day in ten years
as the system criterion should include recognition of the standard
factors that are incorporated in the analysis.

Epilogue
There have been many utility applications, studies, reports, and published papers involving the LOLP index over the almost 50 years
since the 1966 Bibliography was presented at the IEEE 1966 Winter
Power Meeting. Many reports published since the mid-1960s indicate the use of an LOLP index of one day in ten years in generating capacity adequacy assessment. The National Electric Reliability
Study, Final Report, authored by the U.S. Department of Energy,
Office of Energy Emergency Operations, in 1981 noted that electric
utility system reliability criteria have been established on the basis
of historical reliability levels that provided trouble-free service in
the past. The reliability criterion used in planning is that the LOLP
should not exceed one day in ten years. These reports and many
others that utilize this LOLP index do not attempt to justify this criterion value but seem to accept it as a universal standard.

For Further Reading
R. Billinton, "Bibliography on application of probability methods in the evaluation of generating capacity requirements," in
Proc. IEEE Winter Power Meeting, New York, Jan. 30-Feb. 4
p&e
1966, paper no. 31 CP 66-62.
july/august 2015

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