IEEE Power & Energy Magazine - July/August 2015 - 90

Development of the
LOLP Index
The bibliography indicates that interest
in the use of probability methods in the
determination of generating capacity requirements became evident in 1933 [reference 1]. This was followed by a series
of publications [references 2-10] between
1933 and 1946. The first coordinated attempt to discuss the application of probability techniques in generating capacity
reserve assessment is considered to have
occurred at an American Institute of
Electrical Engineers (AIEE) Conference
in Chicago, Illinois, in 1947, where the
following four papers [references 11-14]
were presented on this subject. All four
papers proposed the application of probability theory to consider generating unit
outages, and they presented some examples of a loss of load (LOL) index but did
not suggest a criterion for an acceptable
level of reliability.
11.
"Generating Reserve Capacity Determined by the Probability Method,"

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G. Calabrese, AIEE Trans., 1947, vol.
66, pp. 1439-50.
12. "Calculating Probability of Generating Capacity Outages," W.J. Lyman, AIEE Trans., vol. 66, 1947,
pp. 1471-77.
"Outage Expectancy as a Basis for
13.
Generator Reserve," H.P. Seelye, AIEE
Trans., vol. 66, 1947, pp. 1483-88.
14.
"Probability Methods Applied to
Generating Capacity Problems of a
Combined Hydro and Steam System,"
E.S. Loane and C.W. Watchorn, AIEE
Trans., vol. 66, 1947, pp. 1645-57.
Reference 11, [1947] by G. Calabrese (Consolidated Edison Company of New York), presented the basic
methodology and illustrated the LOLP
using daily values of 0.00001, 0.0001,
and 0.001 but did not specifically indicate a criterion value.
Reference 12, [1947] by W.J. Lyman
(Duquesne Light Company), proposed
the use of megawatt outages and did
not consider the LOLP.
Reference 13, [1947] by H.P. Seelye
(Detroit Edison Company), suggested
that a reasonable interval between the occurrences of generating capacity deficiencies might be once in 20, 30, or 50 years
but did not mention an LOLP value.
Reference 14, [1947] by E.S. Loane
(Duquesne Light Company) and C. W.
Watchorn (Pennsylvania Water and
Power Company), noted an acceptable lower LOLP limit of one day in
27 years and an upper limit of one day
in 58 years and implied that one day in
8.66 years would perhaps be undesirable. (The average of LOLP values of
0.001 and 0.0001 on a natural logarithmic scale is 0.000316, which corresponds to one day in 8.66 years).
It is important to note that all the
authors suggested that the choice of an
appropriate level of service reliability
for a system should be based on personal judgment and local conditions.
The four papers [references 11-14]
presented at the 1947 AIEE meeting
in Chicago can be considered as the
starting point in the evolution and application of probabilistic techniques
in generating capacity reliability assessment. These papers also initiated

23/03/13 10:38 AM

the determination of the generally accepted loss of load expectation (LOLE)
index criterion of 0.1 days/year presently in use. To track the origin of this
criterion, the papers listed in the 1966
bibliography that were deemed to be
related to generating capacity planning
or reserve requirements were reviewed.
This review focused on the following
requirements:
✔ Did the paper mention the application of LOLP, and if yes, was a
criterion provided?
✔✔ If a criterion was provided or a value quoted, was there a justification
given for the criterion or value?
The following papers indicated the
use of LOLP and any suggested criterion or value. References 11-14 are
covered above.
17. "Probability Theory Helps Determine System Reserve Generating
Capacity," G. Calabrese, Power,
New York, New York, vol. 92, July
1948, pp. 423-26.
Reference 17, [1948] by G. Calabrese, discussed LOLP but did not
indicate any specific value.
22.
"The Determination and Allocation of the Capacity Benefits Resulting from Interconnecting Two
or More Generating Systems,"
C.W. Watchorn, AIEE Trans., vol.
69, Pt. II, 1950, pp. 1180-86.
Reference 22, [1950] by C.W. Watchorn, utilized the LOLP approach
and suggested a criterion of one day in
eight to ten years and used one day in
8.66 years as an example in his calculation. He also stated: "It is believed that
a reasonable level of service reliability,
when the effect of probable load forecasting error is included in the evaluation, is the probability of failure to
carry the load in the order of an average rate of one day in 8 to 10 years."
23. "Determination of Reserve Capacity by the Probability Method," G.
Calabrese, AIEE Trans., vol. 69, Pt.
II, 1950, pp. 1681-89.
Reference 23, [1950] by G. Calabrese, suggested that an LOLP of one
day in 50 years after voltage reduction
should be satisfactory. He also indicated that the reliability level may be
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Table of Contents for the Digital Edition of IEEE Power & Energy Magazine - July/August 2015