ABB RELION REX640, Technical Manual, Page: 960 / 1960

Ph Pref logic Hi Imp Faulted loops
A & B XC_FLT B & C XC_FLT C & A XC_FLT A & B XC_FLT B & C XC_FLT C & A XC_FLT
RELEASE_PE RELEASE_PP
8 Acyc B-C-A AB Fault BC Fault CA Fault AB Fault BC Fault CA Fault
9 Acyc C-A-B AB Fault BC Fault CA Fault AB Fault BC Fault CA Fault
10 Acyc C-B-A AB Fault BC Fault CA Fault AB Fault BC Fault CA Fault
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.
A
B
C
S R
Z<
CB1 CB2
Z<
R1 R2
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.
Lagging phase- to -earth loop
Leading phase-to-earth loop
Phase-to-phase loop
X
R
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
Ph Prf mode Lo Z GFC = "BLK
leading PE".
Protection functions 1MRS759142 F
960 REX640
Technical Manual