Parallel line out of service and not grounded
Z0m
A
OPEN
21
21
en05000223_ansi.vsd
CLOSED
OPEN
CLOSED
B
ANSI05000223 V1 EN
Figure 65: Parallel line is out of service and not grounded
When the parallel line is out of service and not grounded, the zero sequence on that line
can only flow through the line admittance to the ground. The line admittance is high which
limits the zero sequence current on the parallel line to very low values. In practice, the
equivalent zero sequence impedance circuit for faults at the remote bus bar can be
simplified to the circuit shown in figure 65.
The line zero-sequence mutual impedance does not influence the measurement of the
distance protection in a faulty circuit. This means that the reach of the underreaching
distance protection zone is reduced if, due to operating conditions, the equivalent zero
sequence impedance is set according to the conditions when the parallel system is out of
operation and grounded at both ends.
Z Z0
0 m
Z Z0
0
m
Z0
m
A
B
C
99000040.vsd
I
0
I
0
IEC99000040 V1 EN
Figure 66: Equivalent zero-sequence impedance circuit for a double-circuit line with
one circuit disconnected and not grounded
The reduction of the reach is equal to equation 40.
1MRK 506 369-UUS - Section 8
Impedance protection
Line distance protection REL670 2.2 ANSI 169
Application manual
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