Neutral admittance calculation produces certain values during forward and reverse
faults.
Fault in reverse direction, that is, outside the protected feeder.
Yo
Y
Fdtot
= −
(Equation 28)
≈ − ⋅
j
I
U
eFd
ph
(Equation 29)
Y
Fdtot
Sum of the phase-to-earth admittances ( Y
FdA
, Y
FdB
, Y
FdC
) of the protected
feeder
I
eFd
Magnitude of the earth-fault current of the protected feeder when the fault
resistance is zero ohm
U
ph
Magnitude of the nominal phase-to-earth voltage of the system
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
.
is valid regardless of the neutral earthing
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
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.
Protection functions
1MRS757644 H
432
620 series
Technical Manual
