ABB RELION REX640 Technical Manual page 1196

Protection related functions
5.7.5.1 Phase selection logic
Phase selection logic identifies the faulty phases based on a combined impedance
and current criterion. Phase selection logic is virtually setting-free and has only one
parameter,
a three-phase fault. The setting
equation
Z Max phase load
U
xy
S
max
For example, if U
The identification of the faulty phases is compulsory for the correct operation of
SCEFRFLO. This is because only one of the impedance-measuring elements (fault
loops) provides the correct result for a specific fault type. A three-phase fault is an
exception and theoretically it can be calculated with any of the fault loops. The fault
loop used in the fault distance calculation is indicated in the recorded data Flt loop
as specified in
Table 1149: Fault types and corresponding fault loops
Fault type
-
A-E
B-E
C-E
A-B
B-C
C-A
A-B-C-(E)
In case of two-phase-to-earth-faults (A-B-E, B-C-E or C-A-E), the selected fault loop
depends on the location of the earth faults. When the faults are located at the
same feeder, the corresponding phase-to-phase loop ("AB Fault", "BC Fault" or "CA
Fault") is used for calculation. When the faults are located at different feeders, the
phase-to-earth loop ("AG Fault", "BG Fault" or "CG Fault") corresponding to the
faulty phase at the protected feeder is used for calculation.
5.7.5.2 Fault impedance and distance calculation
As soon as a fault condition is recognized by Phase selection logic, the fault
distance calculation is started with one of the seven impedance-measuring
elements, that is, the fault loops. SCEFRFLO uses independent algorithms for each
fault type to achieve optimal performance.
1196
Z Max phase load , for discriminating a heavy symmetrical load from
Z Max phase load can be calculated using the
2
U
xy
0 8 .
= ⋅
S
max
Nominal phase-to-phase voltage
Maximum three-phase load
= 20 kV and S
xy
Table 1149
.
Description
No fault
Phase A-to-earth fault
Phase B-to-earth fault
Phase C-to-earth fault
Phase A-to-B short circuit
fault
Phase B-to-C short circuit
fault
Phase C-to-A short circuit
fault
Three-phase short circuit
= 1 MVA, then Z Max phase load = 320.0 Ω.
max
1MRS759142 F
(Equation 292)
Flt loop
No fault
AG Fault
BG Fault
CG Fault
AB Fault
BC Fault
AC Fault
ABC Fault
REX640
Technical Manual