Z
0m
A
B
Z<
Z<
IEC09000250_1_en.vsd
IEC09000250 V1 EN-US
Figure 166: Class 1, parallel line in service.
The equivalent zero sequence circuit of the lines can be simplified, see figure
.
A
B
C
Z
0m
Z
0m
Z
0
-
Z
0m
Z
0
-
IEC09000253_1_en.vsd
IEC09000253 V1 EN-US
Figure 167: 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 relay point A will be changed according
to equation
L
L
m
L
ph
0
0p
L
L
Z0
Z1
Z0
Z1
I
3I
3I
3 Z1
3 Z1
ph
U
-
=
×
+
×
+
×
×
æ
ö
ç
÷
è
ø
EQUATION2312 V1 EN-US
(Equation 260)
by equation
and after some simplification we can write the
impedance present to the relay at A side as:
3 0
1 1
3 0
æ
ö
×
=
+
ç
÷
+
×
è
ø
L
I
KNm
Z
Z
I ph
I
KN
EQUATION1277 V3 EN-US
(Equation 261)
Where:
KNm
= Z0m/(3 · Z1L)
The second part in the parentheses is the error introduced to the measurement of the line
impedance.
If the current on the parallel line has negative sign compared to the current on the protected
line, that is, the current on the parallel line has an opposite direction compared to the current
on the protected line, the distance function will overreach. If the currents have the same
direction, the distance protection will underreach.
Maximum overreach will occur if the fault current infeed from remote line end is weak. If
considering a single phase-to-earth fault at 'p' unit of the line length from A to B on the parallel
line for the case when the fault current infeed from remote line end is zero, the voltage U
A
in
the faulty phase at A side as in equation
.
Section 7
1MRK 505 343-UEN B
Impedance protection
274
Application manual
Summary of Contents for Relion 670 series
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