ABB REG670 Applications Manual page 146

Section 7
Differential protection
140
CT 300/5
Star
20.9 MVA
69/12.5 kV
YNd1
(YD )
AC
CT 800/5
Star
IEC06000554 V1 EN
Figure 55: 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 55, 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 15.
Generator protection REG670 2.2 IEC and Injection equipment REX060, REX061, REX062
1MRK 502 071-UEN -
CT 300/5
in Delta
(DAC)
20.9 MVA
69/12.5 kV
YNd1
(YD )
AC
CT 800/5
Star
Application manual
en06000554.vsd