ABB RELION REX640 Technical Manual page 626

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