IEEE Power & Energy Magazine - July/August 2015 - 96

Apparatus and Systems), vol. 79,
1960 (February 1961 section), pp.
1165-77.
Reference 72 [1960] is an important reference developed by the Working Group of
the AIEE Subcommittee on Application of
Probability Methods. The paper presented
the results of three separate reliability measures applied to the same problem. The
three methods were designated as
✔ LOLP
✔ loss of energy probability
✔ Interval between outages.
This is a benchmark paper that created some excellent discussion. No specific LOLP criterion was given, and the
paper stated: "The selection of a satisfactory level of reliability or a corresponding index appropriate to any methods of
measurement requires at some point the
exercise of informed judgment."
The paper also stated in regard to the
loss of load probability approach that: "Regardless of the form in which the result is
presented, its significance as a probability

of existence of a specified condition, and
nothing more, must be recognized."
In his discussion of the AIEE Working Group paper, C.W. Watchorn suggested that 0.1 day of failure per year or one
day of failure in ten years is a generally
accepted standard of service reliability.
Virtually all the applicable references
from this point on used an LOLP index of
one day in ten years in their applications.
The creation and movement toward
the use of an LOLP of one day in ten
years or an LOLE of 0.1 days/year as
a generally accepted generating capacity adequacy criterion can be briefly
described in three phases. In Phase 1
from 1947 to 1950, no specific LOLP
was suggested, and authors generally
investigated a range of LOLP values.
Most authors also agreed that the objectives for service reliability would be
different for different systems and that
they had to be determined with judgment and based on local conditions. In
Phase 2 from 1950 to 1960, the LOLP

values presented in various papers began to converge to a narrower range,
i.e. from one day in five years to one
day in 15 years. By 1960, the LOLP
index was in the range of one day in
five years to one day in ten years. In
Phase 3 from 1960 on, the LOLP index
of one day in ten years seems to have
been widely recognized by the electric
power industry, and most papers used
or quoted this value as a standard.
There are obviously many other people in addition to those cited in the bibliography who actively participated and
contributed in the early evolution of probability techniques in generating capacity
reliability evaluation and in the LOLP
criterion discussions. The authors cited,
however, are pioneers who documented
their views and presented them for discussion in established technical forums. It
is interesting to note that C.W. Watchorn
authored or coauthored 11 papers in the
bibliography over a 19-year period from
1945 to 1964.

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