ABB RELION REX640 - 5.7.5.2 Fault impedance and distance calculation

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, Z Max phase load , for discriminating a heavy symmetrical load from
a three-phase fault. The setting Z Max phase load can be calculated using the
equation
Z Max phaseload
U
S
xy
= ⋅0 8
2
.
max
(Equation 292)
U
xy
Nominal phase-to-phase voltage
S
max
Maximum three-phase load
For example, if U
xy
= 20 kV and S
max
= 1 MVA, then Z Max phase load = 320.0 Ω.
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 .
Table 1149: Fault types and corresponding fault loops
Fault type Description Flt loop
- No fault No fault
A-E Phase A-to-earth fault AG Fault
B-E Phase B-to-earth fault BG Fault
C-E Phase C-to-earth fault CG Fault
A-B Phase A-to-B short circuit
fault
AB Fault
B-C Phase B-to-C short circuit
fault
BC Fault
C-A Phase C-to-A short circuit
fault
AC Fault
A-B-C-(E) Three-phase short circuit ABC Fault
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.
Protection related functions
1MRS759142 F
1196 REX640
Technical Manual