ABB REL 356

ABB REL 356 Current Differential Protection
3-10
Settings and Application
Case 2
When the minimum ground fault current is substantially smaller than the minimum three phase fault current, C0 can be
given a value to increase the sensitivity for ground fault.
The recommended settings
C1 = 0.1
C2 = 0.7
C0 = according to Figure 3-1
gives the following sensitivity for different faults:
Three phase faults
min3min3min
1.01
φφ
IICIT ⋅−=⋅−=
Phase-phase faults
min3min3minmin
60
min
33.0
2
3
38.038.0)21(
3
1
φφφφφφ
IIIIeCCIT
j
⋅=⋅⋅=⋅=⋅⋅+−=
±
ο
Single phase to ground faults
The value of C0 is determined from Figure 3-1. Calculate the ratio I
3
F
min
/I
F
Gmin
and find this number on the vertical axis.
Find the C0 for the calculated current ratio based on the curve corresponding to the least sensitive fault type.
Selecting C0 is this way results in the same sensitivity for minimum ground fault current as for minimum three phase
fault current.
Figure 3-1. Ratio of I
3
F
min
/I
F
Gmin
vs. C0 (C1 = 0.1, C2 = 0.7)