IEEE Transactions on Electrical Insulation
Vol. 28 No. 5, October 1993
Aging Models and
G. C. Stone
Iris Power Engineering, Mississauga, Canada
The insulation community has a great interest in predicting
the life of insulation at normal operating stresses, from rela-
tively rapid tests performed at higher than normal operating
stresses. Life prediction at normal stresses requires a math-
ematical model to extrapolate the accelerated aging data to
lower stresses. Unfortunately, since the lifetimes of apparently
identical specimens tested under the same conditions can vary
by over an order of magnitude from specimen to specimen,
there is considerable difficulty in selecting the best model and
objectively calculating the aging model parameters. In par-
ticular, there is little agreement on the aging models for all
stress-life combinations except perhaps simple thermal aging.
This paper describes the consequences of the inherent high
variability of life data, and reviews some objective methods to
aid in the design and analysis of aging experiments. Before
accelerated aging tests and proposed aging models can acquire
credibility with HV equipment designers, testing of many more
specimens and improved analysis methods are needed. The re-
cent availability of sophisticated computer programs will aid in
the analysis of aging data.
has been documented in the other papers in this Di-
gest, solid and liquid electrical insulation is exposed
to a wide variety of stresses which can deteriorate the
insulation over time, leading to failure. Aging stresses in-
clude temperature, voltage, and mechanical force. These
stresses can be continuous, cyclic and/or intermittent. In
addition, factors (sometimes referred to as environmental
stresses) such as the presence of radiation, water, corro-
sive materials (gaseous, liquid or solid) can lead directly
to insulation breakdown, or accelerate the aging process
from voltage, temperature and/or mechanical stresses.
The combination of all these stresses and factors results
in a large number of possible insulation failure processes.
Specific aging mechanisms are described in other papers
in this Digest.
Since aging of electrical insulation appears to be in-
evitable, and such aging results in insulation breakdown,
a practical question for equipment and components us-
ing electrical insulation is: ‘how long does it take for the
insulation to age before failure occurs’. That is, what is
the life of the insulation under the stresses and factors
experienced during operation of the equipment. Insula-
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