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Section 8
Impedance protection
222
particular line section and use them for calculating the reach for the underreaching
zone.
X
0
m
R
=
R
⋅
1
+
0
E
0
2
R
+
0
DOCUMENT11520-IMG3502 V2 EN-US
X
0
=
⋅
1
−
X
X
0
E
0
2
+
R
0
DOCUMENT11520-IMG3503 V2 EN-US
Parallel line out of service and not earthed
A
Z
0m
Z<
Z<
IEC09000254_1_en.vsd
IEC09000254 V1 EN-US
Figure 116:
Parallel line is out of service and not earthed
When the parallel line is out of service and not earthed, the zero sequence on that
line can only flow through the line admittance to the earth. 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
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 earthed at both ends.
-
Z
Z
0
0m
I
0
A
-
Z
Z
0
0m
B
IEC09000255 V1 EN-US
Figure 117:
Equivalent zero-sequence impedance circuit for a double-circuit
line with one circuit disconnected and not earthed
The reduction of the reach is equal to equation 119.
2
2
X
0
2
m
2
X
0
B
Z
0m
I
0
C
IEC09000255_1_en.vsd
1MRK 506 369-UEN B
(Equation 117)
(Equation 118)
116
Line distance protection REL670 2.2 IEC
Application manual
M17048-537 v5
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