ABB RELION REX640 Technical Manual page 882

Section 4
Protection functions
876
System grounding GFC
= "High impedance"
System grounding GFC
= "From input"
SYS_EARTHING = TRUE
PP voltage XC GFC
U
AB
U
BC
U
CA
Gnd Op A XC GFC
I +I +I
A B C
GUID-A6D0B9DE-B5F2-4981-BE65-2C22BD7888EB V2 EN
Figure 472: 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.
The phase preference logic influences only the selection of the phase-earth loops and
has no effect on the selection of the phase-to-phase loops. The influence is seen in the
RELEASE_PE monitored data. The logic does not affect the RELEASE_PP
monitored data or START_GFC/STARTS_GFC outputs.
All schemes, except "No filter", require a cross-country fault detection XC_FLT =
TRUE; otherwise all the phase-to-earth measuring elements are blocked resulting in
RELEASE_PE value "No fault". This guarantees that tripping at a single-phase earth
fault in the high-impedance earthed network is blocked.
If a cross-country fault is detected, XC_FLT = TRUE and only a single fault loop can
be identified as faulty by the faulted phase selection function, then that loop is directly
OR
AND
EARTH_FLT = TRUE
U<
U< OR
U<
AND
I>
Cross-country Dl GFC
t
1MRS759142 C
AND
AND
OR
REX640
Technical Manual
XC_FLT