Setting parameter Select value for solution 1 (star
connected CT)
Selected value for solution 2
(delta connected CT)
LocationOLTC1 Not used Not used
Other Parameters Not relevant for this application.
Use default value.
Not relevant for this
application. Use default
value.
1)
To compensate for delta connected CTs
2)
Zero-sequence current is already removed by connecting main CTs in delta
Delta-star connected power transformer without tap charger
SEMOD167854-209 v4
Single line diagrams for two possible solutions for such type of power transformer
with all relevant application data are given in figure 33.
CT 1500/5
in Delta
(DAB)
CT 400/5
Star
60 MVA
115/24.9 kV
Dyn1
(D
AB
Y)
CT 1500/5
Star
CT 400/5
Star
60 MVA
115/24.9 kV
Dyn1
(D
AB
Y)
en06000555.vsd
IEC06000555 V1 EN-US
Figure 33: Two differential protection solutions for delta-star connected power
transformer
For this particular power transformer the 115 kV side phase-to-earth no-load
voltages lead by 30° the 24.9 kV side phase-to-earth 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 33, 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 33) must be used for 24.9 kV CTs in order to put
115 kV & 24.9 kV currents in phase.
1MRK 504 169-UEN A Section 6
Differential protection
Transformer protection RET650 2.2 IEC 89
Application manual
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