ABB RELION REX640, Техническое руководство, Страница: 626 / 1960

(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