Megger DELTA 4000, Reference Manual

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20 DELTA 4000 ZM-AH02E
3 TESTING POWER SYSTEM COMPONENTS
Table 3.3
Transformer excitation current test connections
Single phase
Measure Test lead connections
Terminal symbol High voltage Red Ground
H1-H2 H1 H2 –
H2-H1 H2 H1 –
Three phase high side “Y”
Measures Test lead connections
Terminal symbol High voltage Red Ground
H1-H0 H1 H0 –
H2-H0 H2 H0 –
H3-H0 H3 H0 –
Three phase high side “Δ“
Measures Test lead connections
Terminal symbol High voltage Red Ground
H1-H2 H1 H2 H3
H2-H3 H2 H3 H1
H3-H1 H3 H1 H2
▪
Transformer excitation current tests are performed on the
high voltage winding to minimize the excitation current.
Problems in the low voltage windings will still be detected
by this method.
▪
The secondary windings are left floating with the
exception of a wye or zig-zag secondary. In this case
the neutral bushing remains grounded as it is in normal
service. Refer to the user manual for test connections for
Single Phase, Three Phase High Side Wye and Three Phase
High Side Delta transformers.
▪
Single Phase: The transformer is energized from the phase
to neutral bushings (ANSI: H1-H2). Test connections can
be reversed for additional data, but test results should be
the same. H2 may also be designated as H0.
▪
Wye – Wye: Observe that the ground wire is removed
from the high voltage side neutral bushing for testing,
but remains connected on the low voltage side neutral
bushing.
Test procedure
▪
Test voltages should be as high as possible, but limited
to 10 kV, without exceeding the rating of the line-to-line
voltages on delta connected transformers and line-to-
ground on wye connected transformers. Also note that in
many cases the maximum applied voltage is limited by the
maximum current output
▪
Test voltage must always be the same as prior tests if any
comparisons are made.
▪
All transformer excitation current tests are conducted in
the UST test mode (normally UST-R, using Red low voltage
lead).
▪
For routine testing, transformers with load tap changers
should have tests performed in at least one raise and one
lower position off of neutral. The no-load tap changer
should be in the normal in service position.
▪
For new transformers, excitation tests should be
performed in every tap position for both the load and no-
load tap changers.
▪
The more information that is recorded at the time of
testing will ensure the best comparison of results at the
next routine test.
▪
Temperature corrections are not applied to transformer
excitation current tests.
Test results
Compare test results to previous tests on the same trans-
former, or to manufacturers’ data if available. Tests can also
be compared to similar type units. It is essential that identi-
cal test voltages be used for repeat tests on a transformer.
Fluctuation in the test voltage will produce inconsistent
current readings. Three phase transformers should have the
individual windings energized at both ends if the original
test appears abnormal.
Transformer excitation current tests on the high voltage
winding should detect problems in the secondary winding
if they exist. Winding resistance testing in addition to the
excitation tests could be helpful in isolating either a core or
winding defect.
Test results on three phase transformers, especially wye-
connected windings, could produce high but similar read-
ings on two phases compared to the third phase. This is