A
B
C
Z0
m
Z0
Z0
m
-
Z0
Z0
m
-
99000038.vsd
IEC99000038 V1 EN
Figure 140: Equivalent zero sequence impedance circuit of the double-circuit,
parallel, operating line with a single phase-to-ground fault at the remote
busbar
When mutual coupling is introduced, the voltage at the IED point A will be changed.
If the current on the parallel line have negative sign compare to the current on the protected
line that is, the current on the parallel line has an opposite direction compare to the current
on the protected line, the distance function will overreach. If the currents have the same
direction, the distance protection will underreach.
Calculation for a 400 kV line, where we for simplicity have excluded the resistance, gives
with X1L=0.303 Ω/km, X0L=0.88 Ω/km, zone 1 reach is set to 90% of the line reactance
p=71% that is, the protection is underreaching with approximately 20%.
The zero-sequence mutual coupling can reduce the reach of distance protection on the
protected circuit when the parallel line is in normal operation. The reduction of the reach
is most pronounced with no infeed in the line IED closest to the fault. This reach reduction
is normally less than 15%. But when the reach is reduced at one line end, it is
proportionally increased at the opposite line end. So this 15% reach reduction does not
significantly affect the operation of a permissive under-reach scheme.
Parallel line out of service and grounded
Z0m
A B
Z< Z<
en05000222.vsd
DOCUMENT11520-IMG867 V1 EN
Figure 141: The parallel line is out of service and grounded.
Section 8 1MRK 506 369-UUS -
Impedance protection
288 Line distance protection REL670 2.2 ANSI
Application manual
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