Simplification of equation 121, solving it for 3I0p and substitution of the result into
equation 120 gives that the voltage can be drawn as:
0
0
3 p
p 3
2 p
1
A L ph N Nm
I
V Z I K I K
×
= × + × + ×
-
æ ö
ç ÷
è ø
EQUATION1280 V2 EN-US (Equation 122)
If we finally divide equation 122 with equation 117 we can draw the impedance
present to the IED as
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-US (Equation 123)
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 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
M17048-477 v3M17048-479 v7
Z0m
A B
21
21
en05000222_ansi.vsd
CLOSED
CLOSED
OPEN OPEN
ANSI05000222 V1 EN-US
Figure 127: The parallel line is out of service and grounded
1MRK 504 163-UUS A Section 8
Impedance protection
Transformer protection RET670 2.2 ANSI 269
Application manual
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