ABB RELION REX640 Technical Manual page 957

1MRS759142 F
• The calculated residual current exceeds the value of
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
by the high-magnitude initial transients of a single-phase earth fault.
OR
• Any phase-to-phase voltage drops below the value of the
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.
System grounding GFC
System grounding GFC
SYS_EARTHING = TRUE
Figure 531: Cross-country fault detection logic for high-impedance earthed
networks
Phase preference logic for high-impedance earthed networks
The phase preference logic is applicable only in isolated and high-impedance
earthed networks. The phase preference logic for high-impedance earthed networks
is a supplement to the faulted phase identification function. It provides selective
filtering of phase-to-earth measurement elements in case of a cross-country fault or
double earth-fault in isolated and high-impedance earthed networks.
The logic releases the preferred phase-to-earth loop for measurement when a cross-
country fault with two faulted phase-to-earth loops is detected. If only one faulted
phase-to-earth loop is detected by phase selection logic (GFC), the other faulted
phase-to-earth loop is determined by analysing voltages. The phase preference
scheme must be set in the same way for the entire system so that all distance relays
measure the same preferred phase-to-earth loop. As a result, the cross-country fault
is changed into an ordinary single-phase earth-fault by selectively tripping one of
the faulted feeders.
REX640
Technical Manual
Cross-country Dl GFC setting. This shows that the detection is not misled
= "High impedance"
= "From input"
EARTH_FLT = TRUE
PP voltage XC GFC
U<
U
AB
U<
U
BC
U U<
CA
Gnd Op A XC GFC
I>
I +I +I
A B C
OR
AND
AND
OR
AND
Cross-country Dl GFC
t
Protection functions
Gnd Op A XC GFC setting.
PP voltage XC GFC
XC_FLT
AND
OR
957