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Computation method
2.26.1
The FL calculation requires more than three cycles of power system information to determine
the location of the fault. The FL calculation uses a reference current (I f ') obtained by measuring
the change in the current before and after the occurrence of a fault. The reference current (I f ')
removes the influence of load current (I L ) and arc voltage.
The accuracy† of the FL calculation is influenced by both the line impedance (Z) and the
fault current (I f ). Thus, the user should note that the FL computation maintains an accuracy
of better than ± 2.5 km (1.6 mile) for the distance GF shown in Figure 1.1-1 for up to 100km
(62 mile)‡ of the length of the line regardless of the setting of the length of the line GH ( 0 to
500 km (310 mile)). The accuracy of the FL calculation is better than ± 2.5% when the fault is
located 100 km to 500 km from the FL function, provided that the length of the line GH is
between 100 km to 500 km.
The FL calculation runs separately for phase-to-phase faults and earth-faults. Hence, the
selection of algorithm depends upon the type of fault that occurs.
†Note: Both line impedance and rated-current affect the accuracy of the FL computation.
If the VCT is set for 1 ampere rated-current operation, the accuracy of the FL
computation can be maintained provided that the ratio of the reactance of the
series-impedance per unit-length is greater than 0.05, provided that the fault
current that flows is greater than 2 ampere on the secondary side of the CT
circuit. For the VCT being set for 5 ampere rated-current, the same FL accuracy
can be kept when the ratio of the reactance of the series-impedance per-unit-
length is greater than 0.01, provided that the fault current that flows is greater
than 10 ampere. For more information on the rated-current of the VCT, see
chapter
‡Note: In order to maintain the accuracy of the FL computation, the user can use
compensation factors when unbalanced impedances are occurred in the
transmission line.
Equations for earth faults
(i)
Figure 2.26-1 shows an earth fault occurring in phase-a on transmission line GH. The fault
distance (GF = χ) can be determined using the following equations:
Technical description: Transformer module
- 294 -
6F2T0207 (0.01)
(→p. 628).
GRE200 (5,6)
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