OMICRON 109
Test cards for Advanced CT Test mode
8.4 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 of the Advanced CT Test mode 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.
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 8-6: Residual Magnetism card
Note: The wiring for residual magnetism measurement is identical to the wiring for a normal CT test (see
section 3.4.1 on page 27).
Note: Gapped cores normally have very low residual magnetism. CT Analyzer is possibly not able to
determine the knee point of gapped cores.
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