Z p ZI
I KN I KN
I p
p
I I KN
L
ph m
ph
= ⋅
+ ⋅ + ⋅
⋅
−
+ ⋅
3
3
2
3
0
0
0
EQUATION1379 V3 EN (Equation 36)
Calculation for a 400 kV line, where we for simplicity have excluded the resistance, gives
with X1L=0.48 Ohm/Mile, X0L=1.4Ohms/Mile, zone 1 reach is set to 90% of the line
reactance p=71% that is, the protection is underreaching with approximately 20%.
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 infeed in the line 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 under-reach scheme.
Parallel line out of service and grounded
Z0m
A B
21
21
en05000222_ansi.vsd
CLOSED
CLOSED
OPEN OPEN
ANSI05000222 V1 EN
Figure 63: The parallel line is out of service and grounded
When the parallel line is out of service and grounded at both ends on the bus bar side of
the line CT 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 63.
1MRK 506 369-UUS - Section 8
Impedance protection
Line distance protection REL670 2.2 ANSI 167
Application manual
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