ABB RET670 Applications Manual page 176

Section 3
IED application
170
-
Z
Z
0
0m
A
-
Z
Z
0 0m
B
IEC09000253 V1 EN
Figure 52: Equivalent zero sequence impedance circuit of the double-circuit,
parallel, operating line with a single phase-to-ground fault at the
remote busbar
When mutual coupling is introduced, the voltage at the relay point A will be changed
according to equation 60.
æ
= × +
V Z 1 I 3 I
ç
ph 0
ph L
è
EQUATION1276 V3 EN
By dividing equation 60
impedance present to the relay at A side as:
æ
×
3 0 I K Nm
= +
Z Z 1
ç
t
+
I ph 3 0
è
EQUATION1277 V2 EN
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-ground 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 V in the faulty phase at A side as in equation 62.
A
Z
0m
IEC09000253_1_en.vsd
-
Z 0 Z 1 Z 0
× ×
L L 3 I
0 p
× ×
3 1 Z 3 1 Z
L
by equation 59 and after some simplification we can write the
ö
÷
×
I K N
ø
1MRK504116-UUS C
C
ö
m
÷
ø
L
Application manual
(Equation 60)
(Equation 61)