ABB RELION 670 Series Applications Manual page 420

Section 8
Current protection
IEC05000492 V2 EN-US
Figure 256: Step 1 fault calculation 1
This calculation gives the current fed to the protection: 3I
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
IEC05000493 V2 EN-US
Figure 257: Step 1 fault calculation 1
The fault is located at the borderline between instantaneous and delayed operation of the line
protection, such as Distance protection or line residual overcurrent protection. This calculation
gives the current fed to the protection: 3I
The setting of step 1 can be chosen within the interval shown in equation 443.
414
Three phase CT
summated
IN
Single phase-to-
earth fault
Three phase CT
summated
IN
Single phase-to-
earth fault
YN/D or YN/Y
transformer
3I
0
alt
Single CT
>
IEC05000492-en-2.vsd
0fault1
257 is made.
YN/D or YN/Y
transformer
3I
0
alt
Single CT
>
IEC05000493-en-2.vsd
0fault2
1MRK 504 152-UEN B
.
Application manual