ABB ret650, Applications Manual

Page: 137 / 392

Setting of step 1
SEMOD55591-35 v5
One requirement is that earth faults at the busbar, where the transformer winding is
connected, shall be detected. Therefore a fault calculation as shown in Figure
56 is
made.
IN
>
alt
Three phase CT
summated
Single CT
YN/D or YN/Y
transformer
Single phase-to-
earth fault
3I
0
IEC05000492-en-2.vsd
IEC05000492 V2 EN-US
Figure 56: Step 1 fault calculation 1
This calculation gives the current fed to the protection: 3I
0fault1
.
To assure that step 1, selectivity to other earth-fault protections in the network a
short delay is selected. Normally, a delay in the range 0.3 – 0.4 s is appropriate. To
assure selectivity to line faults, tripped after a delay (typically distance protection
zone 2) of about 0.5 s the current setting must be set so high so that such faults
does not cause unwanted step 1 trip. Therefore, a fault calculation as shown in
Figure
57 is made.
1MRK 504 169-UEN A
Section 7
Current protection
Transformer protection RET650 2.2 IEC 131
Application manual