A
B
C
Z0m
Z0
Z0
m
-
Z0
Z0
m
-
99000038.vsd
IEC99000038 V1 EN-US
Figure 154: 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 v2
SEMOD154680-228 v1
Z0m
A
B
Z<
Z<
en05000222.vsd
DOCUMENT11520-IMG867 V1 EN-US
Figure 155: 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 equivalent zero sequence
circuit of the parallel lines will be according to figure
Section 7
1MRK 505 343-UEN B
Impedance protection
250
Application manual
Содержание Relion 670 series
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