transmission) to achieve the correct phase selective tripping during two simultaneous
single-phase ground-faults in different phases on different line sections.
Due to the resonance/high resistive grounding principle, the ground faults in the system
gives very low fault currents, typically below 25 A. At the same time, the occurring system
voltages on the healthy phases will increase to line voltage level as the neutral
displacement is equal to the phase voltage level at a fully developed ground fault. This
increase of the healthy phase voltage, together with slow tripping, gives a considerable
increase of the risk of a second fault in a healthy phase and the second fault can occur at
any location. When it occurs on another feeder, the fault is commonly called cross-country
fault.
Different practices for tripping is used by different utilities. The main use of this logic is
in systems where single phase-to-ground faults are not automatically cleared, only alarm
is given and the fault is left on until a suitable time to send people to track down and repair
the fault. When cross-country faults occur, the practice is to trip only one of the faulty
lines. In other cases, a sensitive, directional ground-fault protection is provided to trip, but
due to the low fault currents long tripping times are utilized.
Figure
243 shows an occurring cross-country fault. Figure 244 shows the achievement of
line voltage on healthy phases and an occurring cross-country fault.
AG
CG
Load
Load
Y
Y
ANSI06000550_2_en.vsd
ANSI06000550 V2 EN
Figure 243: An occurring cross-country fault on different feeders in a sub-
transmission network, high impedance (resistance, reactance) grounded
1MRK 506 369-UUS - Section 8
Impedance protection
Line distance protection REL670 2.2 ANSI 477
Application manual
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