The result is valid regardless of the neutral earthing method.
In this case, the resistive part of the measured admittance is due to leakage losses of
the protected feeder. As they are typically very small, the resistive part is close to zero.
Due to inaccuracies in the voltage and current measurement, the small real part of the
apparent neutral admittance may appear positive. This should be considered in the
setting of the admittance characteristic.
Fault in the forward direction, that is, inside the protected feeder.
Unearthed network:
GUID-5F1D2145-3C0F-4F8F-9E17-5B88C1822566 V1 EN (Equation 16)
≈ ⋅
−
j
I I
U
eTot eFd
ph
GUID-0B7C9BA9-B41B-4825-9C1B-F8F36640B693 V1 EN (Equation 17)
Compensated network:
GUID-F3810944-D0E1-4C9A-A99B-8409F4D3CF05 V1 EN (Equation 18)
≈
+ ⋅ ⋅ −
( )
−
( )
I j I K I
U
Rcc eTot eFd
ph
1
GUID-208EA80C-62B6-46E0-8A5B-DC425F0FE122 V1 EN (Equation 19)
High-resistance earthed network:
GUID-F91DA4E4-F439-4BFA-AA0D-5839B1574946 V1 EN (Equation 20)
≈
+ ⋅ −
( )
I j I I
U
Rn eTot eFd
ph
GUID-CAA0C492-20CF-406C-80AC-8301375AB454 V1 EN (Equation 21)
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 16 shows that in case of a fault inside the protected feeder in unearthed
networks, the measured admittance equals the admittance of the background network.
1MRS758755 A Section 4
Protection functions
REC615 and RER615 331
Technical Manual
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