ABB 615 series Technical Manual page 384

Section 4
Protection functions
378
I j I I
( )
+ ⋅ −
Rn gTot gFd
≈
V
ph
GUID-C6108088-7C33-47E0-9F8C-21BAA16B0898 V1 EN
Y Sum of the phase-to-ground admittances (Y
Bgtot
Y Admittance of the grounding arrangement (compensation coil and parallel resistor)
CC
I Rated current of the parallel resistor
Rcc
I Magnitude of the ground-fault current of the protected feeder when the fault resistance is zero ohm
gFd
I Magnitude of the uncompensated ground-fault current of the network when Rf is zero ohm
gTot
K Compensation degree, K = 1 full resonance, K<1 undercompensated, K>1 overcompensated
I Rated current of the neutral grounding resistor
Rn
Equation 29 shows that in case of a fault inside the protected feeder in ungrounded
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 177.
Equation 31 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
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 33 shows that in case of a fault inside the protected feeder in high-resistance
grounded systems, the measured admittance equals the admittance of the background
network and the neutral grounding resistor. Basically, the imaginary part of the measured
admittance is due to the phase-to-ground capacitances of the background network, and the
resistive part is due to the neutral grounding resistor and the leakage losses of the
, Y , Y ) of the background network
BgA BgB BgC
Figure
1MAC059074-MB A
(Equation 34)
177.
615 series ANSI
Technical Manual