ABB RELION REX640 Technical Manual page 463

1MRS759142 C
REX640
Technical Manual
I
eFd
≈ − ⋅ j
U
ph
GUID-82C36150-43AA-42FC-863B-80C48C964CED V1 EN
Y Sum of the phase-to-earth admittances (Y
Fdtot
I Magnitude of the earth-fault current of the protected feeder when the fault resistance is
eFd
zero ohm
U Magnitude of the nominal phase-to-earth voltage of the system
ph
Equation 47 shows that in case of outside faults, the measured admittance equals the
admittance of the protected feeder with a negative sign. The measured admittance is
dominantly reactive; the small resistive part of the measured admittance is due to the
leakage losses of the feeder. Theoretically, the measured admittance is located in the
third quadrant in the admittance plane close to the im(Yo) axis, see
The result of
method. In compensated networks the compensation degree does not
affect the result. This enables a straightforward setting principle for
the 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.
, Y
FdA FdB
Equation 47 is valid regardless of the neutral earthing
with a suitable margin.
Fdtot
Section 4
Protection functions
(Equation 48)
, Y ) of the protected feeder
FdC
Figure 247.
457