Traditionally, admittance calculation is done with the calculation mode "Normal",
that is, with the current and voltage values directly measured during the fault. As
an alternative, by selecting the calculation mode "Delta", the prefault zero-
sequence asymmetry of the network can be removed from the admittance
calculation. Theoretically, this makes the admittance calculation totally immune to
fault resistance, that is, the estimated admittance value is not affected by fault
resistance. Utilization of the change in Uo and Io due to a fault in the admittance
calculation also mitigates the effects of the VT and CT measurement errors, thus
improving the measuring accuracy, the sensitivity and the selectivity of the protection.
Calculation mode "Delta" is recommended in case a high sensitivity
of the protection is required, if the network has a high degree of
asymmetry during the healthy state or if the residual current
measurement is based on sum connection, that is, the Holmgren
connection.
Neutral admittance calculation produces the following values during forward and
reverse faults:
Fault in reverse direction, that is, outside the protected feeder:
GUID-B6E3F720-1F9F-4C11-A5DC-722838E8CCDA V1 EN (Equation 18)
GUID-19AA418B-9A0A-4CEE-8772-0CD3F595E63F V1 EN (Equation 19)
Y
Fdtot
Sum of the phase-to-earth admittances (Y
FdA
, Y
FdB
, Y
FdC
) of the protected feeder
I
eFd
Magnitude of the earth-fault current of the protected feeder when the fault resistance is
zero ohm
U
ph
Magnitude of the nominal phase-to-earth voltage of the system
Equation 18 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 157.
The result of
Equation 18 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.
Section 4 1YHT530004D05 D
Protection functions
314 615 series
Technical Manual
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