A
B
C
Z0
m
Z0
Z0
m
-
Z0
Z0
m
-
99000038.vsd
IEC99000038 V1 EN-US
Figure 138: Equivalent zero sequence impedance circuit of the double-circuit,
parallel, operating line with a single phase-to-earth 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 earthed
SEMOD154680-226 v2SEMOD154680-228 v1
Z0m
A B
Z< Z<
en05000222.vsd
DOCUMENT11520-IMG867 V1 EN-US
Figure 139: The parallel line is out of service and earthed.
When the parallel line is out of service and earthed at both ends on the bus bar side
of the line CT so that zero sequence current can flow on the parallel line, the
Section 8 1MRK 506 369-UEN B
Impedance protection
270 Line distance protection REL670 2.2 IEC
Application manual
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