ABB RELION 650 Series Applications Manual page 81

1MRK 504 158-UEN A
CT 400/5
60 MVA
115/24.9 kV
Dyn1
(D Y)
AB
CT 1500/5
IEC06000555 V1 EN-US
Figure 30: Two differential protection solutions for delta-star connected power
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 30, 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 30) must be used for 24.9 kV CTs in
order to put 115 kV & 24.9 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 LV delta connected CTs are DAB connected.
3. For star connected CTs make sure how they are 'star'red (that is, earthed) to/from
protected transformer.
4. Enter the following settings for all three CT input channels used for the HV side CTs, see
table 13.
Table 13:
Setting parameter
CTprim
CTsec
CTStarPoint
5. Enter the following settings for all three CT input channels used for the LV side CTs, see
table "CT input channels used for the LV side
Application manual
Star
Star
transformer
CT input channels used for the HV side CTs
Selected value for both solutions
400
5
ToObject
CT 400/5
Star
60 MVA
115/24.9 kV
Dyn1
(D Y)
AB
CT 1500/5
in Delta
(DAB)
CTs".
Section 6
Differential protection
en06000555.vsd
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