ABB RELION REX640, Техническое руководство, Страница: 496 / 1960

I
eFd
Magnitude of the earth-fault current of the protected feeder when the fault resist-
ance is zero ohm
I
eTot
Magnitude of the uncompensated earth-fault current of the network when Rf is
zero ohm
K Compensation degree, K = 1 full resonance, K<1 undercompensated, K>1 overcom-
pensated
I
Rn
Rated current of the neutral earthing resistor
Equation 62
shows that in case of a fault inside the protected feeder in unearthed
networks, the measured admittance equals the admittance of the background
network. The admittance is dominantly reactive; the small resistive part of the
measured admittance is due to the leakage losses of the background network.
Theoretically, the measured admittance is located in the first quadrant in the
admittance plane, close to the im(Yo) axis, see
Figure 281
.
Equation 64
shows that in case of a fault inside the protected feeder in
compensated networks, the measured admittance equals the admittance of the
background network and the coil including the parallel resistor. Basically, the
compensation degree determines the imaginary part of the measured admittance
and the resistive part is due to the parallel resistor of the coil and the leakage
losses of the background network and the losses of the coil. Theoretically, the
measured admittance is located in the first or fourth quadrant in the admittance
plane, depending on the compensation degree, see
Figure 281
.
Before the parallel resistor is connected, the resistive part of the
measured admittance is due to the leakage losses of the background
network and the losses of the coil. As they are typically small, the
resistive part may not be sufficiently large to secure the discrimination
of the fault and its direction based on the measured conductance. This
and the rating and the operation logic of the parallel resistor should be
considered when setting the admittance characteristic in compensated
networks.
Equation 66
shows that in case of a fault inside the protected feeder in high-
resistance earthed systems, the measured admittance equals the admittance of
the background network and the neutral earthing resistor. Basically, the imaginary
part of the measured admittance is due to the phase-to-earth capacitances of the
background network, and the resistive part is due to the neutral earthing resistor
and the leakage losses of the background network. Theoretically, the measured
admittance is located in the first quadrant in the admittance plane, see
Figure 281
.
Protection functions 1MRS759142 F
496 REX640
Technical Manual