ABB RELION REX640 Technical Manual page 494

Protection functions
Uo
Figure 280: Admittance calculation during a reverse fault
R
CC
L
CC
R
n
Y
Fd
Y
Bg
For example, in a 15 kV compensated network with the magnitude of the earth-fault
current in the protected feeder being 10 A (Rf = 0 Ω), the theoretical value for the
measured admittance during an earth fault in the reverse direction, that is, outside
the protected feeder, can be calculated.
494
neutral admittance protection: admittance characteristic is set to cover
the value Yo = – Y
Due to inaccuracies in voltage and current measurement, the small
real part of the calculated neutral admittance may appear as positive,
which brings the measured admittance in the fourth quadrant in
the admittance plane. This should be considered when setting the
admittance characteristic.
E
A
~
E
B
~
E
C
~
R
n
L
cc
R
cc
Reverse fault:
Yo ≈ -j*I /U
eFd
Resistance of the parallel resistor
Inductance of the compensation coil
Resistance of the neutral earthing resistor
Phase-to-earth admittance of the protected feeder
Phase-to-earth admittance of the background network
with a suitable margin.
Fdtot
Io
A B C
Protected feeder
Background network
Im(Yo)
Re(Yo)
ph
1MRS759142 F
Y
Fd
Reverse
Fault
Y
Bg
(I - I )
I
eTot eFd
eFd
REX640
Technical Manual