REG 316*4 1MRB520049-Uen / Rev. F ABB Switzerland Ltd
3-89
Where the rated c.t. current I
N1
differs from the rated current I
GN
of the protected unit, compensating the measurement to achieve
a match is recommended. This is done by correcting either the
reference value of the A/D input or the setting.
For example, assuming I
GN
= 800 A and I
N1
= 1000 A, the setting
to pick up at 1.5 I
GN
= 1200 A would be
2,1
A1000
A800
5,1
I
I
5,1
1N
GN
==
Characteristic angle
Determining the phase-angle of the current provides an addi-
tional criterion for preserving discrimination compared with non-
directional overcurrent protection. The directional sensitivity is
±180° in relation to the reference voltage. This is illustrated in the
following diagrams. The angles given apply for connection ac-
cording to the connections in Section 12.
U
ST
I
R
U
R
U
S
U
T
U
RS
U
ST
U
TR
I
R
j
a
= 45°
Operation:
L
L
w
M
a
x
.
s
e
n
s
i
t
i
v
i
t
y
Restraint:
ja
’
cos ( - ) = neg.
j
’
ja
’
cos ( - ) = pos.
HEST 005 001 C
b)a)
j’ = phase-angle between current and voltage
(positive angle)
a = Characteristic angle
L = Border line between operating and restraint areas
a) Definition of current
and voltage
b) Operating characteristic
Fig. 3.5.7.2 Vector diagram for a fault in the forwards direction on
R phase
To Next Page
Loading...
