Reliability is one of the most important criteria which must be taken
into consideration during the planning, designing, and operating phase
s of any industrial, commercial, or utility power system, Two distinct
reliability assessment methodologies are frequently used to evaluate
the reliability performance of power systems, They are: 1) historical
reliability assessment, i,e,, the collection and analysis of system ou
tage data, and 2) predictive reliability assessment, i,e,, combining h
istorical outage data and theoretical models to estimate the performan
ce of various power system operating configurations, This paper will f
ocus on the historical assessment of the sustained frequency and durat
ion of single circuit and multicircuit delivery point interruptions, D
elivery point reliability levels have a significant impact on the desi
gn of industrial and commercial power systems (e.g., IEEE Gold Book-IE
EE Standard 493), Bulk electricity system average reliability performa
nce indices are defined and statistics based on delivery point sustain
ed interruption data for a five-year period from 1988 to 1992 are pres
ented and discussed in detail in this paper.