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
121
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 122: 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 186.
2
1 0
0 1 0
1 0
1
(2
0
3
1
1
(2 3
(2 )
3
E f
m
U
f
f
Z Z R
Z
K
Z Z Z R
Z Z R
× × + +
= = -
× × + +
× × + +
EQUATION2008 V1 EN
(Equation 186)
This means that the reach is reduced in reactive and resistive directions.
Ensure that the underreaching zones from both line ends will overlap a sufficient
amount (at least 10%) in the middle of the protected circuit.
Tapped line application
Section 3 1MRK504116-UUS C
IED application
272
Application manual
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