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 112: Class 1, parallel line in service
The equivalent circuit of the lines can be simplified, see figure 113.
A
B
C
Z
0m
Z
0m
Z
0
-
Z
0m
Z
0
-
IEC09000253_1_en.vsd
IEC09000253 V1 EN-US
Figure 113: 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 110.
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 110)
By dividing equation 110 by equation 109 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 111)
Where:
KNm = Z0m/(3 · Z1L)
1MRK 506 369-UEN B Section 8
Impedance protection
Line distance protection REL670 2.2 IEC 219
Application manual
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