ABB RELION REX640 Technical Manual page 881

1MRS759142 C
REX640
Technical Manual
transforms the fault into an ordinary single-phase earth fault by selectively tripping
one of the faulted feeders.
The cross-country detecting function can become active only if the network is either
isolated or high-impedance earthed (System grounding GFC = "High impedance" or
selection "From input" and binary input SYS_EARTHING is active).
The fault detection is based on monitoring two quantities.
• Residual current
• Phase-to-phase voltages
A cross-country fault is detected if:
• An earth fault is detected, EARTH_FLT = True
AND
• The calculated residual current exceeds the value of Gnd Op A XC GFC setting.
This setting value should exceed the highest expected single-phase earth-fault
current (calculated from phase currents). The condition is needed because the
cable current transformer saturates easily due to the high fault current.
AND
• The residual overcurrent condition must be valid at least for the duration defined
by the Cross-country Dl GFC setting. This shows that the detection is not misled
by the high-magnitude initial transients of a single-phase earth fault.
OR
• Any phase-to-phase voltage drops below the value of the PP voltage XC GFC
setting. The undervoltage condition is based on the fact that during a single-phase
earth fault, the phase-to-phase voltages are unaffected in isolated and high-
impedance earthed networks. The fulfilment of this criterion in combination with
the residual overcurrent condition avoids the need for any delays.
The detection of a cross-country fault is indicated by the XC_FLT output signal.
Section 4
Protection functions
875