1. Purpose.- The purpose of this volume is to
briefly cover the methods available for deter-
mining the insulating value of the insulation on
generators, motors, transformer windings, ca-
bles, circuit breakers, and high-voltage
bushings. This volume is based on operating
experience of Reclamation and on a number of
articles listed in the bibliography, which should
be studied for more detailed information.
briefly cover the methods available for deter-
mining the insulating value of the insulation on
generators, motors, transformer windings, ca-
bles, circuit breakers, and high-voltage
bushings. This volume is based on operating
experience of Reclamation and on a number of
articles listed in the bibliography, which should
be studied for more detailed information.
2. Insulation Life and Deterioration.-The
useful life of a thermoplastic e.g., (asphalt-
mica) insulation system is practically ended
when the insulation has become brittle enough
to develop cracks under the mechanical
stresses to which it is subjected. A direct
cause of embrittlement is from operation at high
temperatures over a period of time; cracking
results from mechanical stresses imposed upon
conductors having brittle insulation. The
mechanical stresses are caused by (a) short-
circuit currents, (b) thermal expansion and
contraction of the conductors, and (c) vibration.
The dielectric strength of insulation is not
significantly reduced by brittleness alone;
however, electrical breakdown will quickly
follow the development of ensuing cracks. For
this reason, close inspection for cracked
insulation should be made at frequent intervals,
and repairs made as necessary.
useful life of a thermoplastic e.g., (asphalt-
mica) insulation system is practically ended
when the insulation has become brittle enough
to develop cracks under the mechanical
stresses to which it is subjected. A direct
cause of embrittlement is from operation at high
temperatures over a period of time; cracking
results from mechanical stresses imposed upon
conductors having brittle insulation. The
mechanical stresses are caused by (a) short-
circuit currents, (b) thermal expansion and
contraction of the conductors, and (c) vibration.
The dielectric strength of insulation is not
significantly reduced by brittleness alone;
however, electrical breakdown will quickly
follow the development of ensuing cracks. For
this reason, close inspection for cracked
insulation should be made at frequent intervals,
and repairs made as necessary.
The age and service history of the insulation
will provide some indication of its probable
condition. For example, a generator with class
B insulation that has been fully loaded 24 hours
every day for 15 years at a temperature of 110
will provide some indication of its probable
condition. For example, a generator with class
B insulation that has been fully loaded 24 hours
every day for 15 years at a temperature of 110
E
C (as measured by resistance-temperature
detectors), probably has brittle insulation.
insulation life versus continuous operating
temperature.
temperature.
permissible overloading of generators and
transformers, respectively, without undue
acceleration of insulation embrittlement.
transformers, respectively, without undue
acceleration of insulation embrittlement.
Another cause of failure in asphalt-mica wind-
ings is the migration of the asphalt compound
when the coil operating temperature reaches
the flow point of the asphalt. As the compound
migrates, the space occupied by the compound
ings is the migration of the asphalt compound
when the coil operating temperature reaches
the flow point of the asphalt. As the compound
migrates, the space occupied by the compound
becomes a void, subject to localized interior
corona deterioration and resultant failure.
This type of deterioration is more
predominant in the phase terminal end of the
winding where the voltages to ground are
high enough to produce corona discharges.
Evidence of asphalt compound migration
would be bulges in coil tape, usually at the
lowest point in the coil. It has been known for
coils to remain in service, without failure, for
years even if all the asphalt com-pound had
migrated. This is not a healthy condition, but
the only remedy is reduced load or rewinding
the generator.
corona deterioration and resultant failure.
This type of deterioration is more
predominant in the phase terminal end of the
winding where the voltages to ground are
high enough to produce corona discharges.
Evidence of asphalt compound migration
would be bulges in coil tape, usually at the
lowest point in the coil. It has been known for
coils to remain in service, without failure, for
years even if all the asphalt com-pound had
migrated. This is not a healthy condition, but
the only remedy is reduced load or rewinding
the generator.
The useful life of thermosetting e.g.,
(polyester-mica or epoxy-mica) insulation
systems has not yet been determined as
they have not been in service long enough to
determine the eventual effects of aging. To
date, the main cause of failure of
thermosetting insulation systems has been
vibration due to looseness in the slots.
Newer epoxy-mica insulation systems have
proven to be especially prone to developing
loose wedges and slot discharge because
the "hard" nature of the insulation system
does not mold itself to fit the slot as the old
asphalt-mica system did. Therefore, slot
discharge tests and visual inspection of the
installation for loose wedges and indication
of vibration are very important for
thermosetting insulation systems and should
be performed more frequently than
previously done for thermoplastic systems.
(polyester-mica or epoxy-mica) insulation
systems has not yet been determined as
they have not been in service long enough to
determine the eventual effects of aging. To
date, the main cause of failure of
thermosetting insulation systems has been
vibration due to looseness in the slots.
Newer epoxy-mica insulation systems have
proven to be especially prone to developing
loose wedges and slot discharge because
the "hard" nature of the insulation system
does not mold itself to fit the slot as the old
asphalt-mica system did. Therefore, slot
discharge tests and visual inspection of the
installation for loose wedges and indication
of vibration are very important for
thermosetting insulation systems and should
be performed more frequently than
previously done for thermoplastic systems.
3. Causes of Insulation Failures. - Many
failures of insulation are caused by the
entrance of moisture. Organic insulating
materials used on generator and transformer
windings have a high affinity for moisture
from the surrounding atmosphere or oil.
Therefore, it is of primary importance that
close periodic inspections be made of the
equipment to make sure no water comes in
contact with the insulation through loose
joints, poor gaskets, leaky cooling coils, or
moisture-contaminated oil. Contamination of
exposed insulation by dirt and oil also
weakens the insulation and may cause
failure. Another cause of ground insulation
failures of insulation are caused by the
entrance of moisture. Organic insulating
materials used on generator and transformer
windings have a high affinity for moisture
from the surrounding atmosphere or oil.
Therefore, it is of primary importance that
close periodic inspections be made of the
equipment to make sure no water comes in
contact with the insulation through loose
joints, poor gaskets, leaky cooling coils, or
moisture-contaminated oil. Contamination of
exposed insulation by dirt and oil also
weakens the insulation and may cause
failure. Another cause of ground insulation
1 (FIST 3-1 12/91)