ABB RET670 Applications Manual page 334

Section 3
IED application
328
The zero sequence mutual coupling can reduce the reach of distance protection on the
protected circuit when the parallel line is in normal operation. The reduction of the
reach is most pronounced with no current infeed in the IED closest to the fault. This
reach reduction is normally less than 15%. But when the reach is reduced at one line
end, it is proportionally increased at the opposite line end. So this 15% reach reduction
does not significantly affect the operation of a permissive underreaching scheme.
Parallel line out of service and grounded
A
OPEN
CLOSED
21
ANSI05000222 V1 EN
Figure 145: The parallel line is out of service and grounded.
When the parallel line is out of service and grounded at both line ends on the bus bar
side of the line CTs so that zero sequence current can flow on the parallel line, the
equivalent zero sequence circuit of the parallel lines will be according to figure 54.
-
Z Z
0 0m
I
A
0
-
Z
Z
0
0m
B
IEC09000252 V1 EN
Figure 146: Equivalent zero sequence impedance circuit for the double-circuit line
that operates with one circuit disconnected and grounded at both ends.
Here the equivalent zero sequence impedance is equal to Z
Z )/Z -Z +Z which is equal to equation 66.
0m 0 0m 0m
2 2
-
Z Z
0 om
=
Z
E
Z
0
EQUATION2002 V4 EN
B
OPEN
Z0m
CLOSED
21
en05000222_ansi.vsd
Z
0m
I
0
C
IEC09000252_1_en.vsd
1MRK504116-UUS C
-Z m in parallel with (Z
0 0
(Equation 257)
Application manual
-
0