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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
Z0
0
m
Z
Z0
0
m
Z0
m
A
B
C
99000040.vsd
I
0
I
0
IEC99000040 V1 EN-US
Figure 158: 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
.
(
)
(
)
(
)
2
1
0
0
1
0
0
1
0
1
2
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
× × +
+
=
= -
× ×
+
+
× × +
+
EQUATION1284 V1 EN-US
(Equation 219)
This means that the reach is reduced in reactive and resistive directions. If the real and
imaginary components of the constant A are equal to equation
.
Re( )
0 (2 1
0 3
)
0 ( 0 2
1)
A
R
R
R
Rf
X
X
X
=
× ×
+
+ ×
-
×
+ ×
EQUATION1285 V1 EN-US
(Equation 220)
0
1
0
1
0
1
0
Im( )
(2
3
)
(2
)
A
X
R
R
R
R
X
X
=
× × +
+ ×
+
× ×
+
EQUATION1286 V1 EN-US
(Equation 221)
The real component of the KU factor is equal to equation
.
Re
Re
Re
Im
K
A
X
A
A
u
m
( )
= +
( )
⋅
( )
+
( )
1
0
2
2
2
EQUATION1287 V3 EN-US
(Equation 222)
The imaginary component of the same factor is equal to equation
.
Section 7
1MRK 505 343-UEN B
Impedance protection
252
Application manual
Содержание Relion 670 series
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