ABB Modures LZ91 - Page 11

- The considerable increase of load current which can occur on the healthy
line when one line of a double circuit is tripped must be taken into
account.
- Balancing currents I in the heal thy phases during an earth fault must
not cause their sta~ters to pick up. The corresponding limit can be
determined as follows:
1
B max
·90°
2 X9 (IA+ 19
max) e J
Us
j900
-
- XAe
-
2Is
-
'90°
U.t:.
A -
x eJ
A
-
-
'90°
U.z.
.~
+ XBej90o
A +
X eJ
B
-
2I
8
Fig.
3.2
- Example
for XA f
XB
IA
I
Bmax
balancing current
max. load current
replica voltage
'90
I ) eJ (S phase)
Bmax
criterion
- I_..
for pick-up: u.1 -L Ur
--
·-+
'90°
--
u6 =
us
-
2
X eJ
IS
A
·-
--
'90°
-
U,c:
us
+
2
X e
1
IS
B
___,...
- Us
with A= ---
2 r;
The limits can be expressed mathematically as follows:
- in grounded systems
u
z
(Ohm/Phase)
9