Megger DELTA 4000, Reference Manual

Page: 32 / 48

32 DELTA 4000 ZM-AH02E
3 TESTING POWER SYSTEM COMPONENTS
smaller than any other type of circuit-breaker of equivalent
power and are far less noisy than air circuit-breakers.
Test connections
Test setup is essentially the same for all live tank circuit
breakers. Additional Hot-Collar tests may be conducted on
breakers equipped with gas-filled bushings to detect internal
contamination or exterior cracks and other problems that
may have occurred along the surface of the bushing. See
also section bushing testing.
Connect a ground wire from the test set to the grounded
frame of the breaker.
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]
Connect the low voltage lead to the test tap if
available. Refer to previous discussion on test taps.
Both the overall GST test and the C
1
UST test can
be performed without a lead change.
Test procedure
▪
Tests are performed at 2.5kv or 10kv or a voltage suitable
for the insulation.
▪
Tests 1 - 9 are conducted with the breaker open. Tests 10
- 12 are conducted with the breaker closed.
▪
Some breaker designs have internal insulators to support
other linkage and apparatus inside the tank.
The closed breaker tests verify the insulation integrity of
these components.
Identify each set of readings with the apparatus and/or
bushing serial number. Record the manufacturer, type, style,
model and other nameplate ratings. Note any special or
unusual test connections or conditions.
1]
Measure actual test voltage, current, watts, power
factor and capacitance. Correct current and watts
to a standard test voltage 2.5kv or 10kv if neces-
sary.
2]
Record ambient temperature and relative humidity
and a general indication of weather conditions at
the time of the test.
3]
If the C1 test is performed on the bushings, correct
power factor readings to 20°C.
Test results
High watts loss and power factor results on tests 1 - 6 and
10 - 12 could be related to an excess of moisture on the
internal apparatus. If moisture is the problem, opening
and closing the breaker several times in succession could
improve the test results.
The results of the UST measurements in tests 7 - 9 are
meaningful to breakers that have grading capacitors across
the contacts. High capacitance’s compared to similar tests
may be the result of short-circuited sections in one or more
of the grading capacitors .
Vacuum circuit breakers
These circuit-breakers operate on a different principle from
other breakers because there is no gas to ionize when the
contacts open. They are hermetically sealed; consequently,
they are silent and never become polluted. Their interrupt-
ing capacity is limited to about 30kv. For higher voltages,
several circuit-breakers are connected in series. Vacuum
circuit-breakers are often used in underground systems.
Test connections
Connections are the same as for dead-tank oil breakers.
Additional Hot-Collar tests may be performed on suspect
bushings that have unusually high losses.
Connect a ground wire from the test set to the grounded
frame of the breaker.
1]
Connect the high voltage lead to the main con-
ductor lug of the bushing under test. Ensure that
the cable extends out away from the bushing and
does not rest on the porcelain. All other bushings
should float.
2]
Connect the low voltage lead to the test tap if
available. Refer to the previous discussion on test
taps.
Both the overall GST test and the C
1
UST test can be per-
formed without a lead change.
Test procedure
Tests are performed at 2.5kv or 10kv or a voltage suitable
for the insulation. All tests are performed with the vacuum
breaker in the open position. Tests 1 - 6 are conducted in
the GST mode and tests 7 - 9 are in the UST mode.
Identify each set of readings with the apparatus and/or
bushing serial number. Record the manufacturer, type, style,
model and other nameplate ratings. Note any special or
unusual test connections or conditions.
1]
Measure actual test voltage, current, watts, power
factor and capacitance. Correct current and watts
to a standard test voltage 2.5kv or 10kv if neces-
sary.
2]
Record ambient temperature and relative humidity
and a general indication of weather conditions at
the time of the test.
If the C1 test is performed on the bushings, correct power
factor readings to 20°C.
Test results
Charging currents are expected to be small. Under dry
ambient conditions, power factor results will be small and
dielectric losses close to zero. Higher than normal UST
measurement could be due to a defective vacuum bottle
allowing moisture to enter or surface leakage across the vac-
uum housing. Clean the surface of the vacuum bottle and
retest. Ensure all cabinet heaters are working to maintain a
sufficient temperature surrounding the vacuum bottles.