1.
TYPES OF INSULATION USED ON
ROTATING MACHINES, THEIR
ROTATING MACHINES, THEIR
1.1 Thermoplastic (asphalt-mica).-The useful life
of a thermoplastic (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 operation at high
temperatures; 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 may quickly follow the development of
ensuing cracks, especially when moisture and
contaminants invade the system through the cracks.
of a thermoplastic (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 operation at high
temperatures; 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 may quickly follow the development of
ensuing cracks, especially when moisture and
contaminants invade the system through the cracks.
Another cause of failure in asphalt-mica windings 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
formerly 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 compound had migrated. This is not
a healthy condition, but the only remedies are
reduced load or rewinding the generator.
the migration of the asphalt compound when the coil
operating temperature reaches the flow point of the
asphalt. As the compound migrates, the space
formerly 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 compound had migrated. This is not
a healthy condition, but the only remedies are
reduced load or rewinding the generator.
1.2 Thermosetting (polyester-mica or epoxy-
mica).-The useful life of thermosetting (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
mica).-The useful life of thermosetting (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 slots. Thermosetting
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.
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.
New installation methods, such as the use of spring
type wedge fillers, are being used to control the
tendency of thermosetting systems to become loose
in the slots. It appears that proper installation
techniques may solve this problem.
type wedge fillers, are being used to control the
tendency of thermosetting systems to become loose
in the slots. It appears that proper installation
techniques may solve this problem.
2. MECHANICAL LIMITATIONS
Large and high-voltage hydrogenerators built in
accordance with ANSI C50.12- 1965 may be operated
up to 115 percent load at rated power factor,
frequency, and voltage, with the stator and rotor
temperatures in excess of normal for these machines.
Although this load level does not define the actual
temperature rises, it does define the mechanical limit
as the value on the machine at rated power factor and
115 percent kVA. For example, a generator with a
rated power factor of 0.9 would actually be operating
at 128 percent of the mechanical rating if it was
loaded to 115 percent kVA and unity power factor. If
it is intended to operate units with rated power factors
less than unity at 115 percent kVA and unity power
factor, the Denver Office should obtain the
manufacturer's concurrence that the machine has this
mechanical capability.
accordance with ANSI C50.12- 1965 may be operated
up to 115 percent load at rated power factor,
frequency, and voltage, with the stator and rotor
temperatures in excess of normal for these machines.
Although this load level does not define the actual
temperature rises, it does define the mechanical limit
as the value on the machine at rated power factor and
115 percent kVA. For example, a generator with a
rated power factor of 0.9 would actually be operating
at 128 percent of the mechanical rating if it was
loaded to 115 percent kVA and unity power factor. If
it is intended to operate units with rated power factors
less than unity at 115 percent kVA and unity power
factor, the Denver Office should obtain the
manufacturer's concurrence that the machine has this
mechanical capability.
For other unusual conditions such as short-time
overloads, unbalanced load operation, harmonic
current loadings, etc., industry standards do not make
any specific provisions. Additionally if persistent
vibrations at some fixed frequency during operation
should occur due to hydraulic forces, the resonant
frequencies of rotating parts and associated
components should be investigated because usually
no allowance has been made for this condition in the
original design. Each such case should again be
discussed with the manufacturer to establish safe
limits.
overloads, unbalanced load operation, harmonic
current loadings, etc., industry standards do not make
any specific provisions. Additionally if persistent
vibrations at some fixed frequency during operation
should occur due to hydraulic forces, the resonant
frequencies of rotating parts and associated
components should be investigated because usually
no allowance has been made for this condition in the
original design. Each such case should again be
discussed with the manufacturer to establish safe
limits.
1 (FIST 1-4 3/91)