ABB RELION REX640 Technical Manual page 960

Protection functions
Ph Pref logic Hi Imp
8 Acyc B-C-A
9 Acyc C-A-B
10 Acyc C-B-A
Faulted loop phase selection for low-impedance earthed networks
The faulted loop selection logic for low-impedance earthed networks serves as a
supplement to the faulted phase identification function and it provides a selective
filtering of measurement elements in case of a two-phase-to-earth fault in low-
impedance earthed networks. The earth faults can be located close to each other.
Figure 533: Two-phase-to-earth fault in low-impedance earthed system
Impedance can be measured during a two-phase-to-earth fault when the earth
faults are located in the same direction and they are in close proximity to each
other. Generally, too large an impedance is measured for the lagging phase-to-earth
loop, for example phase B in an AB-E fault. Similarly, low impedance is measured
for the leading phase-to-earth loop, for example phase A in an AB-E fault. The
corresponding phase-to-phase loop is measured correctly, for example phase-to-
phase loop AB in an AB-E fault.
X
Figure 534: Impedance measured during a two-phase-to-earth fault
The available faulted loop selection logic schemes allow blocking the phase-earth
loop of the leading phase to prevent overreaching
leading PE".
960
A & B XC_FLT B & C XC_FLT
RELEASE_PE
AB Fault BC Fault
AB Fault BC Fault
AB Fault BC Fault
R1
Z<
S
CB1
A
B
C
Lagging phase- to -earth loop
Phase-to-phase loop
Faulted loops
C & A XC_FLT A & B XC_FLT
RELEASE_PP
CA Fault AB Fault
CA Fault AB Fault
CA Fault AB Fault
Leading phase-to-earth loop
R
1MRS759142 F
B & C XC_FLT C & A XC_FLT
BC Fault CA Fault
BC Fault CA Fault
BC Fault CA Fault
R2
Z<
R
CB2
Ph Prf mode Lo Z GFC = "BLK
Technical Manual
REX640