ABB RELION REX640, Technical Manual, Page: 493 / 1960

Yo Calculated neutral admittance [Siemens]
Io
fault
Residual current during the fault [Amperes]
Uo
fault
Residual voltage during the fault [Volts]
Io
prefault
Prefault residual current [Amperes]
Uo
prefault
Prefault residual voltage [Volts]
Δ Io Change in the residual current due to fault [Amperes]
Δ Uo Change in the residual voltage due to fault [Volts]
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 certain values during forward and reverse
faults.
Fault in reverse direction, that is, outside the protected feeder.
Yo Y
Fdtot
= −
(Equation 59)
≈ − ⋅
j
I
U
eFd
ph
(Equation 60)
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 59
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 280
.
The result of
Equation 59
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
1MRS759142 F Protection functions
REX640
Technical Manual
493