≈
+ ⋅ −
( )
I j I I
U
Rn eTot eFd
ph
GUID-CAA0C492-20CF-406C-80AC-8301375AB454 V1 EN (Equation 26)
Y
Bgtot
Sum of the phase-to-earth admittances (Y
BgA
, Y
BgB
, Y
BgC
) of the background network
Y
CC
Admittance of the earthing arrangement (compensation coil and parallel resistor)
I
Rcc
Rated current of the parallel resistor
I
eFd
Magnitude of the earth-fault current of the protected feeder when the fault resistance 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 overcompensated
I
Rn
Rated current of the neutral earthing resistor
Equation 21 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 158.
Equation 23 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 158.
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 25 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 158.
1YHT530004D05 D Section 4
Protection functions
615 series 317
Technical Manual
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