ABB RELION 620 Series Technical Manual page 422

Section 4
Protection functions
416
Io Io
−
fault prefault
Yo
=
( Uo Uo
− −
fault prefault
GUID-B0611FF1-46FD-4E81-A11D-4721F0AF7BF8 V1 EN
Yo Calculated neutral admittance [Siemens]
Io Residual current during the fault [Amperes]
fault
Uo Residual voltage during the fault [Volts]
fault
Io Prefault residual current [Amperes]
prefault
Uo Prefault residual voltage [Volts]
prefault
ΔIo Change in the residual current due to fault [Amperes]
ΔUo 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
GUID-B6E3F720-1F9F-4C11-A5DC-722838E8CCDA V1 EN
I
eFd
≈ − ⋅ j
U
ph
GUID-19AA418B-9A0A-4CEE-8772-0CD3F595E63F V1 EN
Y Sum of the phase-to-earth admittances (Y
Fdtot
I Magnitude of the earth-fault current of the protected feeder when the fault resistance is
eFd
zero ohm
U Magnitude of the nominal phase-to-earth voltage of the system
ph
Io
∆
=
) Uo
− ∆
, Y
FdA
1MRS757644 F
(Equation 26)
(Equation 27)
(Equation 28)
, Y ) of the protected feeder
FdB FdC
Technical Manual
620 series