1. The ratio for delta connected CTs shall be set √(3)=1.732 times smaller than the
actual individual phase CT ratio.
2. The power transformer
phase-shift shall typically be set as Yy0 because the
compensation for power transformer the actual phase shift is provided by the
external delta CT connection.
3. The zero sequence current is eliminated by the main CT delta connections. Thus
on sides where CTs are connected in delta the zero sequence current elimination
shall be set to Off in the IED.
The following table summarizes the most commonly used wye-delta phase-shift
around the world and provides information about the required type of main CT delta
connection on the wye side of the protected transformer.
IEC vector group ANSI designation Positive sequence no-load
voltage phasor diagram
Required delta CT connection
type on wye side of the
protected power transformer
and internal vector group
setting in the IED
YNd1 YD
AC
IEC06000559 V1 EN-US
DAC/Yy0
Dyn1 D
AB
Y
IEC06000560 V1 EN-US
DAB/Yy0
YNd11 YD
AB
IEC06000561 V1 EN-US
DAB/Yy0
Table continues on next page
Section 7 1MRK 504 163-UUS A
Differential protection
156 Transformer protection RET670 2.2 ANSI
Application manual
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