ABB RET670 - Page 439

51N
Three phase CT
summated
Single CT
YN/D or YN/Y
transformer
Single phase-
to-ground fault
ANSI05000492_3_en.vsd
0
3I
alt
ANSI05000492 V3 EN
Figure 196: 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
197 is made.
1MRK504116-UUS C Section 3
IED application
433
Application manual