periodic measurements of insulation resis-
tance, it is necessary either to take each
measurement at the same temperature, or
to convert each measurement to the same
base temperature. A correction curve for
conversion of each measurement can be
made by taking two successive dielectric
absorption curves, such as "C" and "D" in
tance, it is necessary either to take each
measurement at the same temperature, or
to convert each measurement to the same
base temperature. A correction curve for
conversion of each measurement can be
made by taking two successive dielectric
absorption curves, such as "C" and "D" in
temperatures by the standard procedure
outlined in
outlined in
. The first or "hot"
dielectric absorption curve "C" should be
obtained from a test made soon after
shutdown from a stable, full-load
temperature condition. The second or
"cold" curve should be obtained after the
equipment has cooled to a considerably
lower temperature. Using only the two 10-
minute points of the two curves, plot a
straight line on semilog paper as in
obtained from a test made soon after
shutdown from a stable, full-load
temperature condition. The second or
"cold" curve should be obtained after the
equipment has cooled to a considerably
lower temperature. Using only the two 10-
minute points of the two curves, plot a
straight line on semilog paper as in
Extend the straight lines through a
temperature range of 20 to 75
temperature range of 20 to 75
E
C. As
recommended by IEEE, 40
E
C should be
used as the base temperature to which all
measurements are corrected. Once this
conversion curve for the dry winding is
established for a machine, the curve can
be used as long as no rewinding or major
repair work is done. By means of this
curve, insulation resistance measurements
taken at any temperature can be converted
directly to any other temperature within the
range of the curve. If this temperature
conversion curve is not available,
approximate conversion factors can be ob-
tained from
measurements are corrected. Once this
conversion curve for the dry winding is
established for a machine, the curve can
be used as long as no rewinding or major
repair work is done. By means of this
curve, insulation resistance measurements
taken at any temperature can be converted
directly to any other temperature within the
range of the curve. If this temperature
conversion curve is not available,
approximate conversion factors can be ob-
tained from
base temperature of 40
E
C. Careful
measurement of temperature is important in
m a k i n g i n s u l a t i o n r e s i s t a n c e
measurements. For generators with
resistance temperature detectors, the
average of the readings from all detectors
should be used. Otherwise, the average
reading of several strategically placed
thermometers should be used. The elapsed
time during removal of load, disconnection
of associated equipment, and preparation
for test will help to minimize the
temperature gradient between insulation
and temperature measuring device, but the
m a k i n g i n s u l a t i o n r e s i s t a n c e
measurements. For generators with
resistance temperature detectors, the
average of the readings from all detectors
should be used. Otherwise, the average
reading of several strategically placed
thermometers should be used. The elapsed
time during removal of load, disconnection
of associated equipment, and preparation
for test will help to minimize the
temperature gradient between insulation
and temperature measuring device, but the
time lapse should not exceed 1 hour. For
transformers, a delay of about 1 hour after
shutdown is recommended to reduce the
temperature gradient. Any type of forced
cooling should be shut off at the same time
load is removed.
transformers, a delay of about 1 hour after
shutdown is recommended to reduce the
temperature gradient. Any type of forced
cooling should be shut off at the same time
load is removed.
10. Effect of Moisture.
- Moisture, which
can enter the insulation of a generator or
motor winding from damp air or which can
enter the winding of a transformer from wet
oil, will make a surprisingly large difference
in the insulation resistance. This is clearly
shown by the curves of
motor winding from damp air or which can
enter the winding of a transformer from wet
oil, will make a surprisingly large difference
in the insulation resistance. This is clearly
shown by the curves of
Coulee generator. Curve "A" was taken
shortly after the generator was placed in
service, at a temperature of 36
shortly after the generator was placed in
service, at a temperature of 36
E
C. Curve
"C" was taken after a dry-out run of 168
hours on the generator. The generator
winding was, therefore, more thoroughly
dried out in curve "C" than in curve "A,"
although evaporation of the volatile content
of the insulation or other curing or aging
effect may have had an appreciable effect.
Low insulation resistance resulting from
exposure to moisture does not mean that the
insulation is unsuitable for operation,
particularly if the insulation resistance value
is comparable to that obtained from recent
periodic tests. Dry out of thermosetting
insulation is not as big a factor, and is
sometimes not done, except to cure field
applied insulation.
hours on the generator. The generator
winding was, therefore, more thoroughly
dried out in curve "C" than in curve "A,"
although evaporation of the volatile content
of the insulation or other curing or aging
effect may have had an appreciable effect.
Low insulation resistance resulting from
exposure to moisture does not mean that the
insulation is unsuitable for operation,
particularly if the insulation resistance value
is comparable to that obtained from recent
periodic tests. Dry out of thermosetting
insulation is not as big a factor, and is
sometimes not done, except to cure field
applied insulation.
11. Effect of Age and Curing.- Insulation
with semisolid binder, such as asphalt-mica,
undergoes a curing process with time. This
curing process increases the dielectric ab-
sorption current taken by the insulation, and
thus insulation resistance meter or high-
voltage, dc test measurements show a
decrease in insulation resistance with
increasing age. The more noticeable effect
of age on the leakage current is mainly the
development of cracks and contamination.
with semisolid binder, such as asphalt-mica,
undergoes a curing process with time. This
curing process increases the dielectric ab-
sorption current taken by the insulation, and
thus insulation resistance meter or high-
voltage, dc test measurements show a
decrease in insulation resistance with
increasing age. The more noticeable effect
of age on the leakage current is mainly the
development of cracks and contamination.
Solid insulations, such as porcelain, do not
undergo a curing process and thus age, in
itself, does not change their dielectric
absorption or leakage current components
undergo a curing process and thus age, in
itself, does not change their dielectric
absorption or leakage current components
(FIST 3-1 12/91)
4