neutral admittance protection: admittance characteristic is set to cover
the value Yo = – Y
Fdtot
with a suitable margin.
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.
~
~
~
A B C
Uo
E
A
E
B
E
C
Io
Reverse
Fault
Y
Fd
L
cc
R
cc
Y
Bg
I
eFd
(I
eTot
- I
eFd
)
R
n
Protected feeder
Background network
Im(Yo)
Re(Yo)
Reverse fault:
Yo ≈ -j*I
eFd
/U
ph
Figure 280: Admittance calculation during a reverse fault
R
CC
Resistance of the parallel resistor
L
CC
Inductance of the compensation coil
R
n
Resistance of the neutral earthing resistor
Y
Fd
Phase-to-earth admittance of the protected feeder
Y
Bg
Phase-to-earth admittance of the background network
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.
Protection functions
1MRS759142 F
494
REX640
Technical Manual