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
Figure 185: 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
186 is made.
IN>
alt
Three phase CT
summated
Single CT
YN/D or YN/Y
transformer
Single phase-to-
earth fault
3I
0
IEC05000493-en-2.vsd
IEC05000493 V2 EN
Figure 186: Step 1 fault calculation 1
1MRK 502 071-UEN - Section 9
Current protection
Generator protection REG670 2.2 IEC and Injection equipment REX060, REX061, REX062 349
Application manual
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