time schedules for various insulation
ratings The data recorded should include
the following:
ratings The data recorded should include
the following:
Date of test
Time
Test voltage (kV)
Test current (
Time
Test voltage (kV)
Test current (
F
A)
Resistance (MO)
Temperature of winding(
Temperature of winding(
E
C)
Identification of apparatus under test,
and test instruments
Parts of winding, bus, or equipment
included in test
and test instruments
Parts of winding, bus, or equipment
included in test
Relative humidity
29. Test Technique.
- The machine
winding should be grounded for at least 1
hour immediately preceding the test, thus
ensuring that any absorbed charge is
drained off. The phases should be
separated and tested individually. Lightning
arresters and capacitors must be discon-
nected. Cables and/or buswork should be
disconnected if it is convenient to do so. If
the separation of phases is unusually
difficult, the separation may be made once
for the benchmark tests, and thereafter the
phases may be tested together until
deviation from normal is detected. Similarly,
a winding and its cable or bus lead should
be separated and tested individually. If
separation is difficult and the leakage of the
cable and of the winding are of the same
order (one not more than twice the other),
they may be tested together thereafter until
deviation from normal is detected.
hour immediately preceding the test, thus
ensuring that any absorbed charge is
drained off. The phases should be
separated and tested individually. Lightning
arresters and capacitors must be discon-
nected. Cables and/or buswork should be
disconnected if it is convenient to do so. If
the separation of phases is unusually
difficult, the separation may be made once
for the benchmark tests, and thereafter the
phases may be tested together until
deviation from normal is detected. Similarly,
a winding and its cable or bus lead should
be separated and tested individually. If
separation is difficult and the leakage of the
cable and of the winding are of the same
order (one not more than twice the other),
they may be tested together thereafter until
deviation from normal is detected.
30. Polarization Index.-
The voltage
should be raised abruptly to the first
voltage level with the start of timing for the
test. Readings should be recorded of
insulation current at 1 minute, at 10
minutes, and at other 2- or 3-minute
intervals to the final reading The ratio of the
1-minute to the l0-minute reading of
insulation current will afford useful
indication of polarization index. This gives
the test engineer an idea of insulation
dryness early in the test. (Even though
500 volts has been the standard for
determination of polarization index, the val-
ues yielded at the first voltage steps of the
respective schedules are very close to
those at standard voltage.)
voltage level with the start of timing for the
test. Readings should be recorded of
insulation current at 1 minute, at 10
minutes, and at other 2- or 3-minute
intervals to the final reading The ratio of the
1-minute to the l0-minute reading of
insulation current will afford useful
indication of polarization index. This gives
the test engineer an idea of insulation
dryness early in the test. (Even though
500 volts has been the standard for
determination of polarization index, the val-
ues yielded at the first voltage steps of the
respective schedules are very close to
those at standard voltage.)
31. Reason for Voltage/Time Sched-
ules.
ules.
- The absorption current will seldom
subside to a stable value within any practical
allowable time. Besides leakage current,
some absorption current will be present in the
final readings. This will not confuse results,
however, if sufficient time is allowed at each
voltage for the absorption current to subside
to a proportional value. This requires a
schedule of progressively shorter absorption
times at successive steps of voltage,
because at the beginning of each voltage
step after the first, part of the absorption has
already been satisfied by the preceding
steps. Consequently, the proportional
absorption is reached in less time. Test
schedules with steps chosen to be conve-
nient for insulation of various voltage ratings
are shown in
allowable time. Besides leakage current,
some absorption current will be present in the
final readings. This will not confuse results,
however, if sufficient time is allowed at each
voltage for the absorption current to subside
to a proportional value. This requires a
schedule of progressively shorter absorption
times at successive steps of voltage,
because at the beginning of each voltage
step after the first, part of the absorption has
already been satisfied by the preceding
steps. Consequently, the proportional
absorption is reached in less time. Test
schedules with steps chosen to be conve-
nient for insulation of various voltage ratings
are shown in
32. Applicability of Test Schedules.
--
The test schedules are arranged to include a
minimum of three points up to and including
approximately normal peak operating voltage.
Smaller voltage steps at the higher voltages
are chosen as this affords indication of any
weakness with less danger of unexpected
puncture. The additional points occasioned by
the smaller voltage steps add very little to the
total time of the test. Therefore, the same
schedules are practical for routine tests.
Departures from the schedules for finer steps
when weakness is detected will seldom be
necessary. The voltage steps are sufficiently
conservative that the gradual raising of
voltage in the former 30 minute test schedule
is no longer necessary. The schedules of
minimum of three points up to and including
approximately normal peak operating voltage.
Smaller voltage steps at the higher voltages
are chosen as this affords indication of any
weakness with less danger of unexpected
puncture. The additional points occasioned by
the smaller voltage steps add very little to the
total time of the test. Therefore, the same
schedules are practical for routine tests.
Departures from the schedules for finer steps
when weakness is detected will seldom be
necessary. The voltage steps are sufficiently
conservative that the gradual raising of
voltage in the former 30 minute test schedule
is no longer necessary. The schedules of
rapidly. To protect the microammeter during
the rapid voltage change, it should be
switched to the next higher range.
the rapid voltage change, it should be
switched to the next higher range.
Schedules employing only 10 minutes on the
first voltage step, as shown in
first voltage step, as shown in
tended to supersede the former schedules
which employed 20 to 30 minutes on the first
step. The advantages of the change are:
shorter overall time for testing, adequate
indication of weaknesses, the resistance
results for all steps will be on 10-minute basis
comparable to 10-minute insulation
resistance meter readings for which
there is much background, and the
exponent of -0.8 on which the schedule is
which employed 20 to 30 minutes on the first
step. The advantages of the change are:
shorter overall time for testing, adequate
indication of weaknesses, the resistance
results for all steps will be on 10-minute basis
comparable to 10-minute insulation
resistance meter readings for which
there is much background, and the
exponent of -0.8 on which the schedule is
11 (FIST3-1 12/91)
calculated is more representative of the