scheider electric MiCOM P746, Technical Manual

Page: 148 / 530

P746/EN AP/G31 Application Notes
(AP) 6-
10
MiCOM P746
During the internal fault:
•
the bias current is: Icc min (1 bar) + (n-1) x IloadMax (1 bar)
•
the differential current is: Icc min (1 bar)
Thus the biggest slope for the Check Zone to detect the fault is:
Icc min (1 bar)
((Independent bars - 1) x IloadMax (1 bar)) + Icc min (1 bar)
If for example:
There are 3 buses and Icc min = IloadMax, the slope must be below 33%
For a one and half breaker scheme there are:
•
2 bars (Independent bars)
•
A minimum internal short-circuit value (Icc min (1 bar))
•
A maximum load for a bar (IloadMax (1 bar)).
Feeder Feeder Feeder
Feeder
Feeder
Feeder
lload
lload
lload
lload
P0582ENa
AP
The worst case is:
•
when the busbar is split in 2 and goes as well through the opposite bar
•
the maximum load is on the 2 buses (biggest bias current)
•
The internal short-circuit value is minimum.
During the internal fault:
•
the CZ bias current is: 2 x Icc min (1 bar) + 4 x IloadMax (1 bar)
•
the CZ differential current is: Icc min (1 bar)
Thus the biggest slope for the Check Zone to detect the fault is:
Icc min (1 bar)
(4 x IloadMax (1 bar)) + Icc min (1 bar)