ABB RELION REX640 Technical Manual page 462

Section 4
Protection functions
456
Io
fault
Yo
=
Uo
−
fault
GUID-7259BC11-9B96-4F22-952D-5B6FEAE7D23B V1 EN
Admittance Clc mode = "Delta"
Io Io
−
fault prefault
Yo
=
( Uo Uo )
− −
fault prefault
GUID-9DEA8B5B-8A24-4622-A5F6-D9711912BC2C 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-A2619A53-EA42-4133-8A02-B8A91AC8B7DA V1 EN
Io
∆
=
Uo
− ∆
1MRS759142 C
(Equation 45)
(Equation 46)
(Equation 47)
REX640
Technical Manual