ABB RELION REX640 - Page 626

(Equation 133)
where
I
eNet
= Uncompensated earth-fault current of the network taking into account the
decentralized compensation.
3I> = Setting of the low-set overcurrent stage of the protected feeder
The magnitude of the minimum expected cross-country fault current can be
coarsely estimated based on the knowledge of the maximum earthing resistance
values,
Maximum earthing Ris , of all the feeders in the substation, and using the
following equation:
(Equation 134)
where
I
F_XCmin
= Minimum expected cross-country fault current
U
n_PE
= Nominal phase-to-earth voltage
Y
F1
= 1 / Maximum earthing Ris = Admittance corresponding to the set maximum
earthing resistance value of the protected feeder
Y
F2
= 1 / max( Maximum earthing Ris_i ) = Admittance corresponding to the
maximum of the set earthing resistance values of other feeders in the substation
X
k
= Sum of short-circuit reactances of the incoming HV-network and main
transformer referred to MV-side
R
k
= Sum of short-circuit resistances of the incoming HV-network and main
transformer referred to MV-side
Example 1:
• U
n_PE
= 11.547 kV
• Maximum earthing Ris = 15 ohm
• Maximum earthing Ris_i = 10 ohm
• X
k
= 2.2 ohm
• R
k
= 0.2 ohm
• 3I> = 200 A
• I
eNet
= 320 A
Using the above parameters I
F_XCmin
equals 0.8 kA in order of magnitude, which
is also higher than the uncompensated earth-fault current of the network and the
setting of the low-set overcurrent stage of the protected feeder. So, setting XC
stage A Str Val can be selected between I
eNet
= 320 A and I
F_XCmin
= 800 A.
Assuming phase CT-ratio 400/1 A, the setting XC stage A Str Val is selected to be
700 A (1.75*In).
Protection functions
1MRS759142 F
626 REX640
Technical Manual