Megger DELTA 4000, Справочное руководство

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18 DELTA 4000 ZM-AH02E
3 TESTING POWER SYSTEM COMPONENTS
8]
Correct the power factor readings of the trans-
former to 20°C using individual temperature cor-
rection or standard tables
9]
Identify each set of readings of the transformer
bushings with a serial number. Record manufac-
ture, type or model and other nameplate ratings.
Especially be aware to record nameplate C
1
capaci-
tance and power factor values if available. Correct
the power factor readings on the bushings to 20°C
Test results
Power factor results should always be compared to manu-
facturers’ tests, or to prior test results if available. It is im-
possible to set maximum power factor limits within which
all transformers are acceptable, but units with readings
above 0.5% at 20°C should be investigated.
Oil-filled service-aged transformers may have slightly
higher results and should be trended to identify significant
changes.
Bushings, if in poor condition, may have their losses
masked by normal losses in the winding insulation There-
fore, separate tests should be applied to them.
Increased power factor values, in comparison with a previ-
ous test or tests on identical apparatus, may indicate some
general condition such as contaminated oil. An increase in
both power factor and capacitance indicates that contami-
nation is likely to be water. When the insulating liquid is be-
ing filtered or otherwise treated, repeated measurements on
windings and liquid will usually show whether good general
conditions are being restored.
Oil oxidation and consequent sludging conditions have a
marked effect on the power factors of transformer wind-
ings. After such a condition has been remedied, (flushing
down or other treatment) power factor measurements are
valuable in determining if the sludge removal has been ef-
fective.
Measurements on individual windings may vary due to dif-
ferences in insulation materials and arrangements.
However, large differences may indicate localized deteriora-
tion or damage. Careful consideration of the measurements
on different combinations of windings should show in
which particular path the trouble lies; for example, if a mea-
surement between two windings has a high power factor,
and the measurements between each winding and ground,
with the remaining winding guarded, gives a normal read-
ing, then the trouble lies between the windings, perhaps in
an insulating cylinder.
Three-winding transformers
Testing of three-winding transformers is performed in
the same manner as two-winding transformers with the
additional tests of the tertiary winding. In some cases
transformers are constructed so that the inter-windings are
shielded by a grounded electrostatic shield or a concentric-
winding arrangement. This could provide test results that
capacitance is almost non-existent or even a negative power
factor. The transformer manufacturer should be contacted
12]
Transformer windings must remain shorted for all
bushing tests. Windings not energized must be
grounded.
13]
For transformers that have wye-wye configuration,
and the neutrals internally cannot be separated,
1-3 and 5-7 cannot be measured. In this case short
the high voltage bushings and the low voltage
bushings together and perform a GST test. Test
voltage should be suitable for the rating of the
low voltage winding.
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 transformer or results of similar tests on
similar transformers. If at all possible, power factor and
capacitance readings should be taken on all new transform-
ers for future benchmarking.
Field measurements of power-factor and capacitance can
differ from measurements made under the controlled
conditions in the factory. Therefore, the power-factor and
capacitance should be measured at the time of installation
and used as a base to compare future measurements. Power
factor testing is extremely sensitive to weather conditions.
Tests should be conducted in favorable conditions whenev-
er possible. All tests are performed at 10kV. If these values
exceed the rating of the winding, test at or slightly below
the rating.
1]
Follow the test sequence of the Two-Winding
Transformers Test Connections. Tests 1, 2 and 3 can
be completed without a lead change.
2]
Test 4 is a calculation subtracting the capacitance
and watts results in test 2 from test 1. The results
should compare with the UST measurement for the
C
HL
insulation
3]
Reverse the test leads for tests 5, 6 and 7. Test volt-
age should be at a level suitable for the secondary
winding of the transformer.
4]
Test 8 is a calculation by subtracting test 6 from
test 5. Results should compare with the UST meas-
urement in test 7 for the C
HL
insulation.
5]
Enter all the nameplate information of the trans-
former. Note any special or unusual test connec-
tions or conditions.
6]
Enter ambient temperature and relative humidity
and a general indication of weather conditions at
the time of the test.
7]
Enter the insulation temperature (top oil or wind-
ing temperature)