Omicron Lab CT Analyzer, User Manual

Page: 34 / 254

CT Analyzer User Manual
32
2.5.4 Measurement on Bushing-Type CTs
Measurement on a Bushing-Type CT
Proceed as follows to perform measurements on a bushing-type CT (refer to
Figure 2-10). Observe the general safety rules in section 2.1 on page 21.
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 the CT Analyzer :
• Connect that side of the CT that is connected to PE to the black sockets
of the CT Analyzer .
• Connect that side of the CT that is open to the red sockets of the
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 input "Prim" is
only approx. 330k
Ω
and can thus 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.