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 809: 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 individual earth faults. When the faults are located at
the same feeder, the corresponding phase-to-phase loop (either “AB Fault” or “BC
Fault” or “CA Fault”) is used for calculation. When the faults are located at different
feeders, the phase-to-earth loop (either “AG Fault” or “BG Fault” or “CG Fault”)
corresponding to the faulty phase at the protected feeder is used for calculation.
5.7.4.2
Fault impedance and distance calculation
As soon as a fault condition is recognized by the phase selection logic, the
fault distance calculation is started with one of the seven impedance-measuring
elements, that is, the fault loops. SCEFRFLO employs independent algorithms for
each fault type to achieve optimal performance.
The inherent result from the fault distance calculation is the ohmic fault loop
impedance value.
Table 810: The calculated impedance values available in the recorded data
Impedance valule
Description
Flt phase reactance
Estimated positive sequence reactance from the substation to the
fault location in primary ohms.
Flt point resistance
Fault resistance value in the fault spot in primary ohms. The compo-
sition of this term depends on the fault loop as described in the
following subsections.
Table continues on the next page
1MRS757644 H
Protection related functions
620 series
Technical Manual
837
