ABB REL 356

ABB REL 356 Current Differential Protection
3-9
Settings and Application
Sequence Coefficients (C0, C1, C2)
REL 356 uses sequence filters to obtain positive, negative and zero sequence currents. These currents are then com-
bined into one quantity:
currents. sequence negative theand sequence positive thesequence, zero theare and ,
210
210
210
III
where
ICICICIT ⋅+⋅−⋅=
In order to minimize the influence of load, C1 should be set to a relatively low value. We recommend to use C1 = 0.1.
C0 and C2 will both multiply fault currents. Setting recommendations C1 and C2 for three cases are made:
Case 1) Minimum fault current for a ground fault I
F
Gmin
> 50% of minimum fault current for a three phase fault, I
3
F
min
.
Case 2) Minimum fault current for a ground fault I
F
Gmin
< 50% of minimum fault current for a three phase fault, I
3
F
min
.
Case 3) Minimum fault current for a ground fault I
F
Gmin
< 12.5% of minimum fault current for a three phase fault, I
3
F
min
.
Case 1
Either C2 or C0 can be used for fault detection of any unsymmetrical fault. The use of negative sequence, C2, is
preferable as the negative sequence currents are more consistent at the two line terminals. We therefore recommend to
set C0 = 0.
A high value of C2 would increase the sensitivity but in order not to make the relay responding to system unbalances a
“moderate” setting is recommended. Good sensitivity with high security is achieved with setting C2 = 0.7.
The recommended settings
C1 = 0.1
C2 = 0.7
C0 = 0.0
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
minminmin
2.0)021(
3
1
GG
IICCCIT
φφ
⋅=⋅++−=
3.080.023.08.02)(8.02
11minmin
⋅⋅⋅=⋅⋅⋅=−⋅≤ ICITLRESOPOTH
and for single end infeed: