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2 PRODUCT DESCRIPTION
After the pickup setting of 87 has been established for an application, a check should be made to determine the minimum
internal fault current that just cause the unit to operate. If this value is less than the minimum internal fault current expected,
the pickup setting is suitable for the application. The following expression can be used to determine the minimum internal
fault current required for the particular 87 pickup setting.
Where:
I
= Minimum internal fault current to trip 87
MIN
n =
Number of breakers connected to the bus (i.e. number of CTs per phase)
I =
Secondary excitation current of individual CT at a voltage equal to pickup level of 87
I
=
Pickup level in 87 unit
R
I
=
Current in the MOV unit at 87 pickup level
1
N =
CT ratio.
The values of (I)
, (I)
, etc. are obtained from the secondary excitation characteristics of the respective CTs. The first term
1
2
in equation (2) reduces to N*Ix if it is assumed that all CTs have the same excitation characteristics.
The current drawn by the MOV unit can be obtained from that curve in
GEK-106426B
MIB High Impedance Bus Differntial Relay
2.5 CALCULATION OF SETTINGS
2.5.2 MINIMUM FAULT TO TRIP 87
2-6
that applies to the relay being used.
2
2-9
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