ABB 615 Series - Page 380

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 VG and IG 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 certain values during forward and reverse faults.
Fault in reverse direction, that is, outside the protected feeder.
Yo Y
Fdtot
= −
GUID-B6E3F720-1F9F-4C11-A5DC-722838E8CCDA V1 EN (Equation 26)
≈ − ⋅j
I
V
gFd
ph
GUID-135FBF62-1C0B-4327-9FA5-E8EC5F215B29 V1 EN (Equation 27)
Y
Fdtot
Sum of the phase-to-ground admittances (Y
FdA
, Y
FdB
, Y
FdC
) of the protected feeder
I
gFd
Magnitude of the ground-fault current of the protected feeder when the fault resistance is zero
ohm
V
ph
Magnitude of the nominal phase-to-ground voltage of the system
Equation 26 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 176.
The result of Equation 26 is valid regardless of the neutral grounding
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 1MAC059074-MB A
Protection functions
374 615 series ANSI
Technical Manual