51N
Three phase CT
summated
Single CT
YN/D or YN/Y
transformer
Single phase-
to-ground fault
ANSI05000492_3_en.vsd
ANSI05000492 V3 EN-US
Figure 273: Step 1 fault calculation 1
This calculation gives the current fed to the protection: 3I
0fault1
.
To assure that step 1, selectivity to other ground-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
274 is made.
1MRK 504 163-UUS A Section 9
Current protection
Transformer protection RET670 2.2 ANSI 547
Application manual
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