Omicron Lab CT Analyzer, User Manual

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CT Analyzer User Manual
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6.4 Residual Magnetism Card
Note: Residual magnetism is an optional feature. The functionality described
below is only available if you have purchased a corresponding license. For
further information, please contact your OMICRON sales contact or the
OMICRON office nearest you.
The Residual Magnetism card is only available if it is enabled on the
Select Cards page (Default Test Card Selection or Select Cards soft key in the
CT-Object card).
High-transient currents or DC currents applied to the primary side of a CT can
cause saturation effects inside the CT with a possible subsequent displacement
of the operating point on the CT’s excitation curve. The CT then has a residual
magnetic flux in its core even if no current is applied on the primary side. The CT
has residual magnetism.
Residual magnetism in CTs may cause erroneous tripping or prevent tripping of
the connected protection relay since the behavior of the CT is no longer as
specified and expected.
Using the Residual Magnetism card it is possible to measure the residual
magnetism of CTs.
The CT Analyzer performs demagnetization of the CT after the test is
finished. Please note that the residual magnetism the CT had prior to
testing gets lost through the test.
Figure 6-6 Residual Magnetism card
Note: The wiring for residual magnetism measurement is identical to the wiring
for a normal CT test (see section 2.4.1 on page 24).
Note: Gapped cores normally have very low residual magnetism. The
CT Analyzer is possibly not able to determine the knee point of gapped cores.