Megger DELTA 4000, Reference Manual

Page: 27 / 48

ZM-AH02E DELTA 4000 27
3 TESTING POWER SYSTEM COMPONENTS
▪
Compare test results to similar tests on similar bushings.
(note: power and dissipation factor results are generally
around 1%)
Hot collar test
For bushings not equipped with either a test tap or a volt-
age tap, the only field measurement which can be per -
formed is the hot collar test. The dielectric losses through
the various sections of any bushing or pothead can be
investigated by means of the test which generates localized
high-voltage stresses. This is accomplished by using a con-
ductive hot collar band designed to fit closely to the porce -
lain surface, usually directly under the top petticoat, and ap-
plying a high voltage to the band. The center conductor of
the bushing is grounded. The test provides a measurement
of the losses in the section directly beneath the collar and is
especially effective in detecting conditions such as voids in
compound filled bushings or moisture penetration since the
insulation can be subjected to a higher voltage gradient than
can be obtained with the normal bushing tests.
This method is also useful in detecting faults within con-
denser layers in condenser-type bushings and in checking
the oil level of oil-filled bushings after a pattern of readings
for a normal bushing has been established. If abnormal mA
or Watts reading is obtained, the test should be repeated
with the hot collar band wrapped around the porcelain
surface directly under the second petticoat rather than the
first. If necessary, move the band further down on the
bushing to determine the depth that the fault has pro-
gressed. The hot collar measurements are made by normal
GST GROUND test method and the bushing need not be
disconnected from other components or circuits. Make sure
that the collar band is drawn tightly around the porcelain
bushing to ensure a good contact and eliminate possible
partial discharge problems at the interface. Refer to the sec-
tions on “Significance of Humidity” and “Surface Leakage”
if tests are made under unfavorable weather conditions.
Test connections (GST)
Connect a ground wire between the test set and the host
apparatus for the bushing under test.
1]
Install the collar just under the top petticoat of the
bushing under test. Ensure the collar is drawn tight
around the bushing for good contact.
2]
Connect the high voltage lead from the test set to
the collar. Ensure the high voltage cable extends
away from the bushing at a 90 degree angle and
not resting against the porcelain.
3]
Ground the center conductor of the bushing.
Test procedure
Energize the collar to 10 kV, if 10 kV exceeds the rating
of the bushing, test at or slightly below the rating of the
bushing.
1]
Identify each set of readings with the bushing se-
rial number. Record manufacturer, type or model
and other nameplate ratings. Note any special or
unusual test connections or conditions.
2]
Record actual test voltage, current, and Watts.
Power factor and dissipation factor data is not
recorded.
3]
Record ambient temperature and relative humidity
and a general indication of weather conditions at
the time of the test.
Test results
General guidelines for evaluating the hot collar data are as
follows:
▪
Watts-loss values less than 100 mW - bushing acceptable
▪
Watts-loss values of 100 mW or more - bushing
unacceptable (contamination)
▪
Current values within 10% of similar bushings - bushing
acceptable
▪
Current values less than 10% of similar bushings -
bushing unacceptable (low level of liquid or compound)
If Watt-loss values are in the unacceptable range, clean-
ing may be necessary on the exposed insulation surface of
the bushing. Effects of surface leakage can be substantially
minimized by cleaning and drying the porcelain surface and
applying a very thin coat of Dow Corning #4 insulating
grease (or equal) to the entire porcelain surface.
Spare bushing tests
All the tests discussed thus far are for bushings installed
in apparatus. These same tests can be performed on spare
bushings with minor changes in the test criteria. All tests of
spare bushings should be performed on bushings mounted
vertical or at an angle of inclination to the vertical not to
exceed 20 degrees.