failure in rotating machines is turn-to-turn
failures occurring and progressing into ground-
wall breakdowns.
failures occurring and progressing into ground-
wall breakdowns.
4. Insulation Test Methods.- The principal
methods used today for testing insulation are:
methods used today for testing insulation are:
a. Insulation resistance and dielectric ab-
sorption by high-voltage dc insulation test.
sorption by high-voltage dc insulation test.
b. Insulation resistance and dielectric ab-
sorption by high-voltage, dc (direct-current)
test.
sorption by high-voltage, dc (direct-current)
test.
c. Insulation current by dc, high-voltage,
ramped test.
ramped test.
d. Power factor
e. AC (alternating-current) voltage with-
stand.
stand.
f. DC (voltage withstand)
Methods e. and f. are used primarily in factory
tests and after initial erection of the equipment
in the field. Practically all routine field tests are
made using nondestructive methods a., b., c.,
and d. Neither the insulation resistance nor
power-factor test methods can accurately predict
the breakdown voltage of insulation. The
results must be compared with values when the
insulation was new and of known voltage with-
stand. Therefore, power-factor or insulation re-
sistance measurements must be made
periodically, beginning when the equipment is
new and in good condition, and the trend noted
of the test results. The great amount of data
collected from insulation tests reveals that no
single test method can be relied upon for indicating
all conditions of weakened insulation.
There are cases on record where transformer
insulation resistance tests gave no indication of
moisture, but power-factor tests on the windings
definitely showed moisture. There are other
cases where the reverse was found. Therefore,
to minimize chance of deterioration escaping
notice, it is recommended that the periodic tests
on equipment be made by more than one of the
following tests:
tests and after initial erection of the equipment
in the field. Practically all routine field tests are
made using nondestructive methods a., b., c.,
and d. Neither the insulation resistance nor
power-factor test methods can accurately predict
the breakdown voltage of insulation. The
results must be compared with values when the
insulation was new and of known voltage with-
stand. Therefore, power-factor or insulation re-
sistance measurements must be made
periodically, beginning when the equipment is
new and in good condition, and the trend noted
of the test results. The great amount of data
collected from insulation tests reveals that no
single test method can be relied upon for indicating
all conditions of weakened insulation.
There are cases on record where transformer
insulation resistance tests gave no indication of
moisture, but power-factor tests on the windings
definitely showed moisture. There are other
cases where the reverse was found. Therefore,
to minimize chance of deterioration escaping
notice, it is recommended that the periodic tests
on equipment be made by more than one of the
following tests:
Equipment
Motor and generator stator and
field windings
Power transformer
High-voltage cables
High-voltage bushings
Circuit breakers
Preferred test method
(Numbers show order of preference)
(1) Insulation current by dc high-voltage ramped test
or
(1) Insulation resistance and dielectric absorption
(1) Power factor and (2) Insulation resistance and
dielectric absorption
or
(1) AC voltage withstand of oil and (2) Power factor
(1) Insulation current by dc high-voltage ramped test
and (2) Power factor
or
(1) Insulation resistance and dielectric absorption
and (2) Power factor
(1) Power factor and capacitance and 2) Insulation
resistance and dielectric absorption
(1) Power factor and (2) Insulation resistance and
dielectric absorption
(FIST 3-1 12/91)
2