ABB REL 356

ABB REL 356 Current Differential Protection
4-10
Measuring Elements and Operational Logic
Figure 4-8. Mho Characteristic for Phase to Ground Faults
Three Phase
Three phase fault detection is accomplished by the logic operation of one of the three ground units plus the 3FF output
from the phase selector. However, for a three phase fault condition, the computation of the distance units will be:
OP = V
XG
- I
X
Z
P
REF = V
Q
Where
V
Q
= V
CB
for phase A
V
AC
for phase B
V
BA
for phase C
V
XG
= V
AG
, V
BG
or V
CG
I
X
= I
A
, I
B
or I
C
Z
P
= zone reach setting in secondary ohms