Megger DELTA 4000, Reference Manual

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ZM-AH02E DELTA 4000 25
3 TESTING POWER SYSTEM COMPONENTS
and is not appreciably affected by conditions external to the
bushing.
Test connections (UST)
Figure 11: UST-R, test on transformer bushing
Connect a ground wire from the test set to the host appara-
tus for the bushing under test.
1]
Connect the high voltage lead from the test set to
the center conductor of the bushing. If the bushing
under test is in a transformer, jumper all the bush-
ings of the same winding. Also jumper the bush-
ings of the other windings and connect them to
ground. Make sure the bare connector on the high
voltage lead extends away from the bushing under
test to avoid contact with the bushing porcelain.
2]
Connect the low voltage lead from the test set to
the test tap. Test tap accessibility will differ with
the bushings’ style and rating. Some test taps are
terminated in a miniature bushing mounted on
the grounded mounting flange of the bushing.
The tap is grounded in normal service by a screw
cap on the miniature bushing housing. By remov-
ing the screw cap the tap terminal is available to
perform the tests. Most taps are readily accessible,
but a special probe is necessary to make contact
with the tap in certain bushing designs.
3]
The tap housing may contain a small amount of oil
or compound. Care must be taken when remov-
ing the screw cap to catch the oil. Be sure the oil is
replaced after testing is completed.
Test procedure
For all power factor testing, the more information you
record at the time of testing will ensure the best compari-
son of results at the next routine test. Test data should be
compared to the nameplate data. If nameplate or factory
readings are not available, compare the results of prior tests
on the same bushing and results of similar tests on similar
bushings.
Always observe safety rules when conducting tests. Have a
conference before testing begins and make sure all person-
nel understand the danger areas.
1]
Power factor testing is extremely sensitive to
weather conditions. Tests should be conducted in
favorable conditions whenever possible.
2]
The c1 main insulation test is normally performed
at 10kV in the UST test mode. Always refer to the
name plate voltage rating of the bushing under
test. If 10kV exceeds the rating of the bushing, test
at or slightly below the voltage rating.
3]
Proceed with the test and record the results.
4]
Identify each set of readings with the bushing se-
rial number. Record manufacturer, type or model
and other nameplate ratings. Especially be aware
to record nameplate c1 capacitance and power
factor values. Note any special or unusual test con-
nections or conditions.
5]
Record actual test voltage, current, Watts, power
factor and capacitance.
6]
Record ambient temperature and relative humidity
and a general indication of weather conditions at
the time of the test.
7]
Correct the power factor readings to 20°C. If the
bushing is mounted in a transformer, use an aver-
age of the top oil temperature and the ambient.
Test results
Interpretation of capacitance and dissipation factor
measurements on a bushing requires a knowledge of the
bushing construction since each type bushing has its own
peculiar characteristics. For example, an increase in dissipa-
tion factor in an oil-filled bushing may indicate that the oil
is contaminated, whereas an increase in both dissipation
factor and capacitance indicates that the contamination is
likely to be water. For a condenser type bushing which has
shorted layers, the capacitance value will increase, whereas
the dissipation factor value may be the same in comparison
with previous tests.
Except for the specific purpose of investigating surface
leakage, the exposed insulation surface of the bushing
should be clean and dry to prevent surface leakage from
influencing the measurement. The effects of surface leak -
age are eliminated from the measurement when testing by
the UST test method.
Temperature correction curves for each design of bush-
ing should be carefully established by measurement and all
measurements should be temperature corrected to a base
temperature, usually 20 ° C. The temperature measurement
should be based on that at the bushing surface. The air
temperature should also be recorded. When testing a bush-
ing by the grounded specimen method, the surface of the
bushing should be at a temperature above the dew point to
avoid moisture condensation.
General guidelines for evaluating the C1
power and dissipa-
tion factor test data are as follows:
▪
Between nameplate tan delta and up to twice nameplate
tan delta - bushing acceptable