Megger DELTA 4000, Reference Manual

Page: 26 / 48

26 DELTA 4000 ZM-AH02E
3 TESTING POWER SYSTEM COMPONENTS
▪
Between twice nameplate tan delta and up to 3 times
nameplate tan delta - monitor bushing closely
▪
Above 3 times nameplate tan delta - replace bushing
General guidelines for evaluating the C1 capacitance data
are as follows:
▪
Nameplate capac/-5% - bushing acceptable
▪
Nameplate capac/-5% to +/-10% - monitor
bushing closely
▪
Nameplate capac/-10% or greater - replace
bushing
Changes in C1 test data are usually contamination issues
caused by moisture ingress, oil contamination or breakdown
and short-circuited condenser layers.
Inverted tap to center conductor test C1
(UST)
The inverted tap test can be performed on bushings with
test taps. The high voltage lead and the low voltage lead are
reversed for this test. The high voltage lead is connected
to the test tap and the low voltage lead is connected to the
center conductor of the bushing. The test tap may have to
be accessed with a special probe as previously described.
This test is normally not performed except on bushings
that have abnormal test results from the standard UST
method. Care must be taken to ensure test voltages do not
exceed the tap rating. All windings must be shorted and test
results recorded as in the standard C1 UST method.
Power and dissipation factor & capaci-
tance test C2
The C
2
test measures only the insulation between the tap
and ground and is not appreciably affected by connections
to the bushing center conductor. The tap is energized to a
pre-determined test voltage and measured to ground in the
grounded specimen test (GST) mode.
Figure 12: C2, GST GND, test on transformer bushing
Always refer to nameplate data or manufacturer’s literature
on the bushing for tap test voltages. Please note that the
power factor tap is normally designed to withstand only about
500 V while a capacitance tap may have a normal rating of 2.5
to 5 kV. Before applying a test voltage to the tap, the maxi-
mum safe test voltage must be known and observed. Typical
test voltages for potential taps are between .5kV and 2kV.
Power factor taps test voltages should not exceed .5kV. If
no information is given, do not exceed .5kV to prevent
inadvertent damage to the insulation. An excessive voltage
may puncture the insulation and render the tap useless.
Some bushings do not have a power factor or capacitance
tap or an isolated mounting flange. These bushings must
be electrically isolated from the apparatus for test. This can
be accomplished by removing the metal bolts and tempo-
rarily replacing them with insulated bolts. The insulating
gasket between the bushing flange and apparatus cover
will normally provide sufficient insulation so that a UST
type measurement can be made on the bushing in the same
manner as for a bushing which has provisions for flange
isolation. Verify isolation with an ohmmeter.
Test connections (GST)
Connect a ground wire between the test set and the host
apparatus for the bushing under test.
1]
Connect the high voltage lead from the test set to
the test tap. Test tap accessibility will differ with
the bushings’ style and rating. Refer to previous
discussion on test taps. Care must be taken to sup-
port the high voltage lead, as the test tap elec-
trode may be fragile.
2]
Connect the low voltage lead from the test set
to the center conductor of the bushing for the
guarded test method.
Test procedure
Before energizing the test specimen, double check that the
test set will initially energize at low or zero potential. Care-
fully increase test set output to desired test voltage.
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, Watts, power
factor and capacitance.
3]
Record ambient temperature and relative humidity
and a general indication of weather conditions at
the time of the test.
4]
Correct the power factor readings to 20°C
Test results
▪
Changes in C2 power/dissipation factor, which is not
usually included on the nameplate, are most commonly
indicative of oil contamination.
▪
Changes in C2 capacitance are typically indicative of
physical change, such as tap electrode problems or tap
connection problems. Nameplate values for C2 are not
typically found on nameplates of bushings rated below
115 kV.
General guidelines for evaluating the C2 power and dissipa-
tion factor data are as follows:
▪
Compare test results to prior tests on the same bushing.