CT 1500/5
in Delta
(DAB)
CT 400/5
Wye
60 MVA
115/24.9 kV
Dyn1
(D
AB
Y)
CT 1500/5
Wye
CT 400/5
Wye
60 MVA
115/24.9 kV
Dyn1
(D
AB
Y)
en06000555_ansi.vsd
ANSI06000555 V1 EN-US
Figure 51: Two differential protection solutions for delta-wye connected power
transformer
For this particular power transformer the 115 kV side phase-to-ground no-load voltages
lead by 30° the 24.9 kV side phase-to-ground no-load voltages. Thus when external
phase angle shift compensation is done by connecting main 24.9 kV CTs in delta, as
shown in the right-hand side in figure
51, it must be ensured that the 24.9 kV currents
are rotated by 30° in anti-clockwise direction. Thus, the DAB CT delta connection (see
figure 51) must be used for 24.9 kV CTs in order to put 1
15 kV & 24.9 kV currents in
phase.
To ensure proper application of the IED for this power transformer it is necessary to do
the following:
1. Check that HV & LV CTs are connected to 5 A CT inputs in the IED.
2. For second solution make sure that LV delta connected CTs are DAB connected.
3. For wye connected CTs make sure how they are 'star'red (that is, grounded) to/from
protected transformer.
4. Enter the following settings for all three CT input channels used for the HV side
CTs, see table
18.
1MRK 504 163-UUS A Section 7
Differential protection
Transformer protection RET670 2.2 ANSI 151
Application manual
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