ABB REG316 4 Series - Page 73

REG 316*4 1MRB520049-Uen / Rev. F ABB Switzerland Ltd
3-35
According to the equation for the restrain current, I
H
becomes:
for (
a = 0) :
IIxI
H12
=
and for internal faults
a) fed from on side (I
2
= 0) : I
H
= 0
b) fed from both sides (
a = 180°) : I
H
= 0
High through-fault currents can cause c.t. saturation and for this
reason, the gradient of the characteristic is switched to infinity for
I
H
/I
N
> b.
When measuring the operating characteristic, it should be noted
that the gradient of the characteristic is only switched to infinity, if
apart from I
H
, I
1
and I
2
are also higher than b.
g
b
0 0,5 1 1,5
HEST 905 003d C
0,75
0,25
0,5
I
D
I
N
I
H
N
I
Fig. 3.5.1.2 Operating characteristic of the transformer differ-
ential protection for high through-fault currents
This characteristic, however, would scarcely be able to detect
faults in the protected zone at through currents as low as the
load current. Therefore if one of the windings is conducting a
current which is less than the setting of “b”, i.e.
b
I
I
or
I
I
N
2
N
1
<
the characteristic is switched back to the gradient according to
“v”.