ABB RET650 Applications Manual page 93

1MRK 504 169-UEN A
Transformer protection RET650 2.2 IEC
Application manual
CT 300/5
Star
20.9 MVA
69/12.5 kV
YNd1
(YD )
AC
CT 800/5
Star
IEC06000554 V1 EN-US
Figure 32: Two differential protection solutions for star-delta connected power
transformer
For this particular power transformer the 69 kV side phase-to-earth no-load
voltages lead by 30 degrees the 12.5 kV side phase-to- earth 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 32, 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 star connected CTs make sure how they are stared (that is, earthed) to/from
protected transformer.
4. Enter the following settings for all three CT input channels used for the LV side
CTs see table 10.
Section 6
Differential protection
CT 300/5
in Delta
(DAC)
20.9 MVA
69/12.5 kV
YNd1
(YD )
AC
CT 800/5
Star
en06000554.vsd
87