Equation 13 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 Figure 167.
The result of
Equation 13 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.
1MRS758755 A Section 4
Protection functions
REC615 and RER615 329
Technical Manual
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