0 1 0
1
1 0
3
m
N Nm
K
Z Z Z
K
Z Z
=
-
-
IECEQUATION14018 V1 EN (Equation 176)
This K
N1
setting for zone 1 only affects the reach for ground faults while the reach for two
and three-phase faults are unaffected.
For case 2, when the parallel line is out of operation but not grounded, the zone 1 nominal
reach for ground faults is reduced. The measured impedance can be calculated:
1
0 1
1
1
1
1
3
N
Z x Z
K
Z Z
Z
+
= ×
+
-
×
IECEQUATION14015 V1 EN (Equation 177)
The reduced reach must be taken into account when using a permissive underreaching
scheme.
For case 3, when the parallel line is in service, the reach for ground faults can further
reduce because of the mutual coupling. The measured impedance can be calculated:
1
0 1
1
1
1
1
3 2
N
Nm
Z x Z
K
Z Z x
K
Z x
+
= ×
+
-
+
-
×
IECEQUATION14014 V1 EN (Equation 178)
This reduction of the reach is most pronounced with no in-feed in the line terminal nearest
fault. When the reach is reduced at one line end, it is extended in the opposite one.
Therefore, this reach reduction does not affect a permissive underreaching scheme.
Setting of overreaching zone
Zone 1 is not allowed to overreach under any condition. Overreaching zones 2 and 3 are
not allowed to underreach the whole line under any condition.
Setting of zone 2
The overreaching zone 2 for backup or permissive overreaching distance protection
schemes must at least safely cover 100% of the line with a safety margin of about 20%.
This must be guaranteed for the most unfavorable condition of case 3.
The ground fault compensation factor must be adjusted to the case of parallel line in
service (case 3) where the highest ground fault impedance occurs.
Section 8 1MRK 506 369-UUS -
Impedance protection
272 Line distance protection REL670 2.2 ANSI
Application manual
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