ABB RET670 Applications Manual page 117

1MRK504116-UUS C
Application manual
CT 300/5
Wye
20.9 MVA
69/12.5 kV
YNd1
(YD )
AC
CT 800/5
Wye
ANSI06000554 V1 EN
Figure 27: Two differential protection solutions for wye-delta connected power
transformer
For this particular power transformer the 69 kV side phase-to-ground no-load voltages
lead by 30 degrees the 12.5 kV side phase-to- ground no-load voltages. Thus when
external phase angle shift compensation is done by connecting main HV CTs in delta,
as shown in the right-hand side in figure 27, it must be ensured that the HV currents
are rotated by 30° in clockwise direction. Thus the DAC delta CT connection must be
used for 69 kV CTs in order to put 69 kV & 12.5 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 HV delta connected CTs are DAC connected.
3. For wye connected CTs make sure how they are stared (that is, grounded) to/from
protected transformer.
IED application
CT 300/5
in Delta
(DAC)
20.9 MVA
69/12.5 kV
YNd1
(YD )
AC
CT 800/5
Wye
en06000554_ansi.vsd
Section 3
111