Rm = V
t
+ 1
where:
Rm = recommended minimum insulation
resistance in megohms at 40 °C
of the entire machine winding
V
Rm = recommended minimum insulation
resistance in megohms at 40 °C
of the entire machine winding
V
t
= rated machine terminal to terminal
potential, in rms kilovolts
The winding insulation resistance obtained by
applying direct potential to the entire winding
for 1 minute must be corrected to 40 °C to be
used for comparison with the recommended
minimum value R
The winding insulation resistance obtained by
applying direct potential to the entire winding
for 1 minute must be corrected to 40 °C to be
used for comparison with the recommended
minimum value R
m
. The insulation resistance of
one phase of a three-phase armature winding
with the other two phases grounded is
approximately twice that of the entire winding.
Therefore, the resistance of each phase, when
the phases are tested separately, should be
divided by two to obtain a value which, after
correction for temperature, may be compared
with R
with the other two phases grounded is
approximately twice that of the entire winding.
Therefore, the resistance of each phase, when
the phases are tested separately, should be
divided by two to obtain a value which, after
correction for temperature, may be compared
with R
m
. If guard circuits are used on the two
phases not under test when each phase is
tested separately, the observed resistance of
each phase should be divided by three to
obtain a value which, after correction for
temperature, may be compared with R
tested separately, the observed resistance of
each phase should be divided by three to
obtain a value which, after correction for
temperature, may be compared with R
m
. For
insulation in good condition, insulation
resistance readings of 10 to 100 times the
value of R
resistance readings of 10 to 100 times the
value of R
m
are not uncommon. It should be
remembered, however, that decreasing values
of insulation resistance obtained from periodic
tests are more indicative of deterioration of the
insulation than low values. Machines rated at
10,000 kV-A or less should have either the
polarization index or the insulation resistance
(at 40 °C) at least as large as the minimum
recommended values to be considered in
suitable condition for operating or for
overpotential tests. Machines rated above
10,000 kVCA should have both the polarization
index and the insulation resis-tance above the
minimum recommended values.
When the end turns of a machine are treated
with a semiconducting material for corona
elimination purposes, the insulation resistance
may be somewhat lower than without such
treatment.
of insulation resistance obtained from periodic
tests are more indicative of deterioration of the
insulation than low values. Machines rated at
10,000 kV-A or less should have either the
polarization index or the insulation resistance
(at 40 °C) at least as large as the minimum
recommended values to be considered in
suitable condition for operating or for
overpotential tests. Machines rated above
10,000 kVCA should have both the polarization
index and the insulation resis-tance above the
minimum recommended values.
When the end turns of a machine are treated
with a semiconducting material for corona
elimination purposes, the insulation resistance
may be somewhat lower than without such
treatment.
20. Transformer Insulation Resistance.
-
Although the foregoing paragraphs apply more
specifically to generator and motor windings, they
also apply, in general, to transformers, except
that no insulation values have been established
for transformers. Also, the technique of
measuring transformer insulation resistance is
not well known or standardized. If the transformer
windings are not immersed in oil, the insulation
resistance will behave much like generator
insulation resistance. The insulation resistance
will be less after adding the oil, because the
insulation resistance of the oil is in parallel with
part of the solid insulation. Therefore, insulation
resistance readings alone cannot be used to
indicate the progress of dry out of the winding
because the winding and the oil resistances
cannot be separated. Tests should run on oil
samples as specified in Facilities Instructions,
Standards, & Techniques Volume 3-5 at the
same time as the test of the transformer winding,
and the oil then filtered, if necessary, to remove
the moisture. The change of insulation resistance
with temperature when the transformer windings
are oil-immersed is similar to that in generators,
and curves similar to those of
also apply, in general, to transformers, except
that no insulation values have been established
for transformers. Also, the technique of
measuring transformer insulation resistance is
not well known or standardized. If the transformer
windings are not immersed in oil, the insulation
resistance will behave much like generator
insulation resistance. The insulation resistance
will be less after adding the oil, because the
insulation resistance of the oil is in parallel with
part of the solid insulation. Therefore, insulation
resistance readings alone cannot be used to
indicate the progress of dry out of the winding
because the winding and the oil resistances
cannot be separated. Tests should run on oil
samples as specified in Facilities Instructions,
Standards, & Techniques Volume 3-5 at the
same time as the test of the transformer winding,
and the oil then filtered, if necessary, to remove
the moisture. The change of insulation resistance
with temperature when the transformer windings
are oil-immersed is similar to that in generators,
and curves similar to those of
figure 3
are useful
for temperature standard-izing. Whether the
slope of these temperature correction curves is
affected by moisture content in the oil is not fully
known. At the present state of the art, it is
believed that the power factor test gives a better
indication of transformer insulation condition than
the insulation resistance test. Tests should be
made between each winding, between each
winding and ground with the other windings
grounded, and between each winding and ground
with the guard circuit connected to the other
windings but not grounded.
slope of these temperature correction curves is
affected by moisture content in the oil is not fully
known. At the present state of the art, it is
believed that the power factor test gives a better
indication of transformer insulation condition than
the insulation resistance test. Tests should be
made between each winding, between each
winding and ground with the other windings
grounded, and between each winding and ground
with the guard circuit connected to the other
windings but not grounded.
21. Cable Insulation Resistance.
-The most
frequently used test on high-voltage cables is
insulation resistance measured by means of an
insulation resistance meter. The most
informative test for high-voltage cables is the dc,
high-voltage test modified to combine a modest
voltage withstand with insulation current/voltage
measurement as described in
insulation resistance measured by means of an
insulation resistance meter. The most
informative test for high-voltage cables is the dc,
high-voltage test modified to combine a modest
voltage withstand with insulation current/voltage
measurement as described in
Insulation resistance testing of cable differs from
the testing of apparatus windings mainly
because of the high capacitance, if the cable is
long, which takes a longer time to charge, and in
the difficulty of obtaining a satisfactory
(FIST 3-1 12/91) 8