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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 monitored data FAULT_LOOP as spe-
cified in Table 4.3-1.
Table 4.3-1 Fault types and corresponding fault loops
Output FAULT_LOOP Description Fault Type
1 Phase A-to-earth fault A-E
2 Phase B-to-earth fault B-E
3 Phase C-to-earth fault C-E
4 Phase A-to-B short circuit fault A-B
5 Phase B-to-C short circuit fault B-C
6 Phase C-to-S short circuit fault C-A
7 Three-phase short circuit A-B-C-(E)
In case of phase-to-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”) corres-
ponding to the faulty phase at the protected feeder is used for calculation.
Identification of the faulty phase is provided by the built-in Phase Selection Logic (PSL),
based on combined impedance and current criteria. Phase selection logic is virtually
setting-free and has only one parameter, Z Max phase load, for discriminating a large
symmetrical load from a three-phase fault. The parameter Z Max phase load can be cal-
culated using the equation:
equation_1.png
Figure 4.3-2 (Equation 1)
Uxy: the nominal phase-to-phase voltage
S
max
the maximum three-phase load
For example, if Uxy = 20 kV and S
max
= 1 MVA, then Z Max phase load = 320.0 ohm.
Fault 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
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COM600 series 5.1 1MRS758734
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