58. Test Methods for Slot Discharge.-
A method used for detecting slot discharge in
ac generator windings consists of energizing
the winding from a high-potential ac test set
at voltages of approximately 50 to 125
percent of normal rated machine voltage or
running the unit self excited and isolated from
the system and observing on an oscilloscope
the resulting induced potentials from slot dis-
charges. The detecting apparatus is
essentially a band pass filter passing a band
of frequencies between 1,000 and 2,000 Hz
to an oscilloscope. The detecting apparatus
is coupled through a condenser to the
terminal of the winding being tested or to a
probe which is placed ill contact with the
stator coil shielding. With the oscilloscope
sweep set for one-half the applied frequency,
the characteristic hash indication appears at
two or four places along the sweep since the
ac generator windings consists of energizing
the winding from a high-potential ac test set
at voltages of approximately 50 to 125
percent of normal rated machine voltage or
running the unit self excited and isolated from
the system and observing on an oscilloscope
the resulting induced potentials from slot dis-
charges. The detecting apparatus is
essentially a band pass filter passing a band
of frequencies between 1,000 and 2,000 Hz
to an oscilloscope. The detecting apparatus
is coupled through a condenser to the
terminal of the winding being tested or to a
probe which is placed ill contact with the
stator coil shielding. With the oscilloscope
sweep set for one-half the applied frequency,
the characteristic hash indication appears at
two or four places along the sweep since the
discharge occurs at particular parts of the
voltage cycle. Because the detector cannot
distinguish between hash originating in the
winding and that originating in the power
source, it is essential that the stator winding
be energized from a good high-potential test
set or a transformer operated well below its
rated voltage. Voltage of about 50 percent,
normal or below which no corona occurs, is
first applied as a calibration point or base.
The voltage is gradually raised until there is
distinct change in the hash on the
oscilloscope screen. This is the point where
slot discharge is considered to begin. A
winding properly protected from slot
discharge should not produce hash at
voltages below approximately 125 percent of
normal. Experience and special calibration
apparatus are necessary in order to properly
interpret the results.
voltage cycle. Because the detector cannot
distinguish between hash originating in the
winding and that originating in the power
source, it is essential that the stator winding
be energized from a good high-potential test
set or a transformer operated well below its
rated voltage. Voltage of about 50 percent,
normal or below which no corona occurs, is
first applied as a calibration point or base.
The voltage is gradually raised until there is
distinct change in the hash on the
oscilloscope screen. This is the point where
slot discharge is considered to begin. A
winding properly protected from slot
discharge should not produce hash at
voltages below approximately 125 percent of
normal. Experience and special calibration
apparatus are necessary in order to properly
interpret the results.
HIGH-POTENTIAL, AC PROOF TESTS
59. Application.-
High-potential, ac tests
to determine the condition of insulation are
used primarily on new equipment at the
factory, after erection in the field, or after
rewinding in the field. The only positive
means of determining what voltage an
insulation will withstand is to apply voltage
across it until failure occurs. However, this
test may be destructive and the full test
voltage is not recommended for application
to equipment that has been in service. In
deteriorated areas, the deterioration may be
advanced by such tests even though failure
may not occur during the test. Thus, an
unknown part of the useful life may be spent.
Reduced levels are sometimes permissible
as "suitability for service" tests.
used primarily on new equipment at the
factory, after erection in the field, or after
rewinding in the field. The only positive
means of determining what voltage an
insulation will withstand is to apply voltage
across it until failure occurs. However, this
test may be destructive and the full test
voltage is not recommended for application
to equipment that has been in service. In
deteriorated areas, the deterioration may be
advanced by such tests even though failure
may not occur during the test. Thus, an
unknown part of the useful life may be spent.
Reduced levels are sometimes permissible
as "suitability for service" tests.
In any situation where a contract is involved,
have the contractor's representative present
for the tests or obtain written permission
from the contractor for the Government to
perform the tests.
have the contractor's representative present
for the tests or obtain written permission
from the contractor for the Government to
perform the tests.
If the ac proof test is to follow a high-voltage,
dc test, the precaution of allowing adequate
time, about 2 hours, for the absorption
charge to dissipate before application of the
ac over-potential test must be observed.
dc test, the precaution of allowing adequate
time, about 2 hours, for the absorption
charge to dissipate before application of the
ac over-potential test must be observed.
The procedure to be used on all ac proof
tests of stator windings is as follows:
tests of stator windings is as follows:
a. Break the 'Y" connection at the
generator neutral, separating the three
phases from each other and the
powerhouse ground.
generator neutral, separating the three
phases from each other and the
powerhouse ground.
b. Disconnect all potential transformers
and surge-protection equipment, such as
lightning arrestors, capacitors, etc., from
the generator terminal leads.
and surge-protection equipment, such as
lightning arrestors, capacitors, etc., from
the generator terminal leads.
c. Ground both ends of the two phases
which are not to be tested.
which are not to be tested.
d. Connect both ends of the phase to be
tested together with insulated wire and
tested together with insulated wire and
19 (FIST3-1 12/91)