Where:
Uph is phase to earth voltage at the relay point.
Iph is phase current in the faulty phase.
3I0 is earth fault current.
Z1 is positive sequence impedance.
Z0 is zero sequence impedance.
Z
0m
A B
Z< Z<
IEC09000250_1_en.vsd
IEC09000250 V1 EN-US
Figure 188: Class 1, parallel line in service
The equivalent circuit of the lines can be simplified, see figure 189.
A
B
C
Z
0m
Z
0m
Z
0
-
Z
0m
Z
0
-
IEC09000253_1_en.vsd
IEC09000253 V1 EN-US
Figure 189: 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 341.
0 0
0
0 1
1 3 3
3 1 3 1
m
L L
ph p
ph L
L L
Z
Z Z
U Z I I I
Z Z
æ ö
-
= × + × +
ç ÷
× ×
è ø
IECEQUATION1276 V3 EN-US (Equation 341)
By dividing equation 341 by equation 340 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 342)
Where:
KNm = Z0m/(3 · Z1L)
1MRK 506 369-UEN B Section 8
Impedance protection
Line distance protection REL670 2.2 IEC 367
Application manual
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