WARNING !
WARNING !
After performing test don't forget to short terminals which have opened for
test. Energizing CT while secondary terminals are open circuit is dangerous
and there is a risk of explosion and damage.
6.2 Measurement setup for built-in CT
The most accurate turns ratio results are achieved if the CT under test can be connected as
depicted in Figure 8 and measured discretely. In case this is not possible - because the CT under
test is built into a power transformer and the high current taps are not accessible - it can be
measured as a part of the power transformer. Special attention has to be paid to not measuring
any additional voltage.
Figure 9 shows a built-in CT under test and the power transformer’s associated primary and
secondary windings. A jumper is used to short circuit the primary winding of the power transformer
and thus eliminating the additional voltage U induced (due to the turns ratio measurement) into the
secondary winding.
90225-1.30 Instruction Manual CT-T1
Page 26 of 62
Figure 9: Example measurement with a built-in three taps CT under test connected to the power transformer
on the secondary winding. The primary winding of the power transformer is short circuited to prevent any
voltage U on the secondary winding of the power transformer
6.3 Measurement setup for built-in CT in Δ winding power transformer
If the CT under test is built into a Δ winding power transformer and the according winding can not
be short circuited, a compensation factor needs to be applied. Figure 10 and Figure 11 show the
measurement setup and the equivalent circuit of the high current side.
Turns ratio factor is determined depending on the connection of the H1-H2 cables of the CT-T1.
A turns ratio correction factor of 2/3 needs to be applied if the H1-H2 cables of the CT-T1 are
connected to the CT under test and the associated coil (Figure 10).
And a turns ratio correction factor of 1/3 needs to be applied if the H1-H2 cables of the CT-T1 are
connected to an independent coil (Figure 11).
90225-1.30 Instruction Manual CT-T1
Page 27 of 62
Figure 10: CT under test built into Δ power transformer (TR factor 2/3)
Figure 11: CT under test built into Δ power transformer (TR factor 1/3)
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