GE P743 - Page 173

A
pplication Notes P74x/EN AP/N
a7
MiCOM P74
1, P742, P743 (AP) 6-
7
AP
• when all these buses are independent (bus sectionalizers open)
• the maximum load is on all the buses (biggest bias current)
• The internal short-circuit value is minimum.
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P0871ENa
Max Load
Max Load
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)).
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lload
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P0872ENa
The worst case is:
• when the is split in 2 and goes as well through the opposite bar