Megger DELTA 4000, Reference Manual

Page: 29 / 48

ZM-AH02E DELTA 4000 29
3 TESTING POWER SYSTEM COMPONENTS
1]
Connect the high voltage lead to the main conduc-
tor lug of the bushing under test. Ensure that the
cable extends out away from the bushing and does
not rest on the porcelain.
2]
If the bushing is equipped with a test tap, connect
the low voltage lead to the tap. Test tap connec-
tions can be difficult to make on some bushing
styles. Accessibility will differ with the bushings’
style and rating. Power factor taps are usually
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. In some
cases the power factor tap housing may contain
a small amount of oil or compound. Care must be
taken when removing the screw cap to catch the
oil. Ensure that the oil or compound is replaced
after testing is completed.
3]
When the overall GST-GND test is performed, the
low voltage lead is grounded. The test path is
through the high voltage lead, through the bush-
ing to ground.
4]
When the UST UST-R test is performed, the test
path is through the high voltage lead, the C1 main
insulation and the low voltage lead.
Test Procedure
Always observe safety rules when conducting tests. Power
factor testing is extremely sensitive to weather conditions.
Tests should be conducted in favorable conditions when-
ever possible.
With the breaker in the open position , start with the
#1 bushing and perform the GST test. If the bushing is
equipped with a test tap, perform the UST test. Repeat the
tests for all six bushings.
1]
With the breaker in the closed position, perform
the GST test on all three phases.
2]
All tests are performed at 2.5kv or 10kv or a volt-
age suitable for the insulation.
Test Results
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 breaker and results of similar tests on similar
breakers. The following additional information should be
recorded on the test form.
Enter all the nameplate information of the oil circuit
breaker. 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 if available.
1]
Note any special or unusual test connections or
conditions.
2]
Calculate the tank-loss index per formulas below.
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 on the bushings
to 20°C using the ambient temperature.
The specific term tank-loss index has been developed to
assist in evaluating the results of the open and closed oil
circuit-breaker tests.
It is defined for each phase as the measurement Watts of
the closed breaker minus the measured Watts of the two
measurements the breaker open. Referring to Table 7 above
the Tank-loss indexes are
▪
Tank 1: Watts[test 7] – {Watts[test 1] – Watts[test 2]}
▪
Tank 2: Watts[test 8] – {Watts[test 3] – Watts[test 4]}
▪
Tank 3: Watts[test 9] – {Watts[test 5] – Watts[test 6]}
It is defined for each phase as the difference of the mea -
sured open circuit and the closed circuit power, in watts. To
obtain the open circuit value, the individual values mea-
sured on the two bushings of each phase must be summed.
Tank-loss index may have values ranging from positive to
negative which will give an indication of the possible source
of a problem. Positive indexes occur when the closed
circuit values are larger than the sum of the open circuit
values. Conversely, negative indexes occur when the closed
circuit values are smaller than the sum of the open circuit
values.
Comparison of tank-loss indexes taken when an oil circuit
breaker is new and initially installed will give the general
range of values to expect from a good unit. This practice
also will avoid condemning a good unit as the result of the
inherent design of a particular manufacturer that normally
may show tank-loss indexes without the unit being defective
or deteriorated.
The losses in an oil circuit breaker are different between an
open circuit test and a closed circuit test because the voltage
stress on the insulating members is distributed differently.
Tables 3.5 and 3.6 summarize what may be defective based
upon the polarity of the tank-loss index. Once a particu-
lar section has given indications of deterioration, the test
results should be verified by systematically isolating the
suspected insulating member before disassembling the unit.