OMICRON 35
Setup and connection
3.5.4 Measurement on bushing-type CTs
Applicable for CT Test mode and Advanced CT Test mode.
Measurement on a bushing-type CT
Proceed as follows to perform measurements on a bushing-type CT (refer to Figure 3-10). Observe the
general safety rules in section 3.1 on page 23 and the safety instructions given in chapter 1 "Safety
instructions" on page 9.
1. Disconnect all utility lines from the transformer (i.e., isolate the transformer from the energized power
system).
2. Connect all transformer terminals that are not used for measurement (in this example H2 and H3) to
protective earth (PE) in order to minimize the external disturbances. External disturbances can
influence the measurement results because the bushings are acting as an antenna.
3. Connect terminal H0 to protective earth.
4. Connect one secondary side terminal of the CT to protective earth.
5. Connect the secondary side of the CT to the OUTPUT sockets and the SEC sockets of CT Analyzer:
â–º Connect that side of the CT that is connected to PE to the black sockets of CT Analyzer.
â–º Connect that side of the CT that is open to the red sockets of CT Analyzer.
6. Connect the primary side of the CT to CT Analyzer input PRIM. Make sure that the polarity is correct
(same colors on same polarity).
7. Short-circuit and ground the free winding on the measured transformer leg to reduce the impedance
of the winding that is connected in series to the primary side of the CT. The input impedance of
CT Analyzer may influence the measurement results, if no winding of that leg is short-circuited.
8. If the transformer has a tap changer installed, the position of the tap changer should be changed to
a position where the regulation winding is completely bridged in order to ensure that the regulation
winding cannot act as a voltage divider together with the main winding of the transformer.
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