2-8
MIB High Impedance Bus Differntial Relay
GEK-106426B
2.5 CALCULATION OF SETTINGS
2 PRODUCT DESCRIPTION
2
2.5 CALCULATION OF SETTINGS
The formulas and procedures described in the following paragraphs for determining
relay settings assume that the relay is connected to the full winding of the differentially connected CTs. It is further assumed
that the secondary winding of each CT has negligible leakage reactance and that all the CTs have the same ratio. If all of
these conditions do not exist, please refer the application to the nearest GE Multilin Sales Office.
2.5.1 SETTING OF THE DIFFERENTIAL ELEMENT
It is assumed that an external fault causes complete saturation of the CT in the faulted circuit. The current forced through
this secondary by the CTs in the in feeding circuits will be impeded only by the resistance of the windings and leads. The
resulting IR drop will be the maximum possible voltage that can appear across the MIB relay for an external fault. The
setting of the high impedance differential unit is expressed as follows:
Where:
I
R
=
pickup setting of the 87 unit
R
S
=
DC resistance of fault CT secondary windings and leads to housing terminal
R
L
=
single conductor DC resistance of CT cable for one way run from CT housing terminal to junction point (at
highest expected operating temperature.
P =
1 for three phase faults; 2 for single phase to ground faults
I
F
=
external fault current – primary RMS value
N =
CT ratio
1.6 =
margin factor
R
E
=
stabilizing resistance (2000)
Calculations need only be made with the maximum value of I
F
for single phase and three phase faults. If the relay is
applicable for these conditions, it will perform satisfactorily for all faults.
The pessimistic value of voltage determined by the equation, for any of the methods outlined, is never realized in practice.
The CT in the faulted circuit will not saturate to the point where it produces no assisting voltage. Furthermore, the condition
that caused the fault CT core to saturate also tends to saturate the cores of the CTs in the in-feeding circuits, which results
in a further decrease in voltage across the MIB. These effects are not readily calculated.
The value of the 87 setting established by the equation is the minimum safe setting. Higher settings will provide more safety
margin, but will result in somewhat reduced sensitivity.
The method of utilizing the above equation is outlined below:
•
Determine the maximum three phase and single phase to ground fault currents for faults just beyond each of the
breakers.
•
The value of R
L
is the one-way cable DC resistance from the junction point to the fault CT being considered.
•
For each breaker in turn calculate I
R
separately utilizing the associated maximum external three phase fault current
with P=1 and the maximum external single phase to ground fault current with P=2.
•
Use the highest of the values of I
R
so obtained in c) above.
It is desirable for the pickup current of the 87 unit multiplied by 2000 ohm. to plot below the knee of the excitation curve (i.e.
point on the excitation curve where the slope is 45º) of all the CTs in use.
