The mutual impedance will influence the distance measurement of ground faults and
cause either an extension or a reduction of the reach relative to the set reach.
The maximum overreach will occur when the parallel line is out of service and grounded
at both ends. During a ground fault, a counteracting current is induced in the grounded
line. The counter current will reduce the apparent zero sequence impedance seen by the
relay in the faulty line.
When both lines are in service under normal generating conditions, the distance relay will
underreach, but in the case of an extremely weak source behind the relay, an overreach
may occur. However, this overreach will not cause the relay to operate for faults beyond
the end of the protected line.
From the operational point-of-view, an extension of the zone 1 beyond the end of the line
is not acceptable.
Alternatives for underreaching and overreaching zones
As stated earlier, the distance protection within the IED can compensate for the influence
of a zero sequence mutual coupling on the measurement at single phase-to-ground faults
in the following ways, by using:
• Alternative 1: The possibility of different setting values that influence the ground-
return compensation for different distance zones within the same group of setting
parameters
• Alternative 2: Different groups of setting parameters for different operating
conditions of a protected multi circuit line.
Alternative 1: Different kN setting values within the same setting group
Setting of underreaching zone
The set zone should fulfill two criteria:
• It should avoid overreach beyond the remote line terminal in case 1
• It should cover as much of the line as possible, at least 50% plus a safety margin in the
most unfavorable case 3.
A possible setting strategy is to set the zone to for example 85% of line length for case 1
and check afterwards if sufficient reach exists in the cases 2 and 3.
Setting of zone 1
For case 1, parallel line having both ends grounded, zone 1 overreach caused by the
grounded parallel line can be avoided by the K
N
factor setting for zone 1 suitably.
The compensation factor K
N
compensates for the additional loop impedance under
ground fault conditions. To eliminate the overreach caused by the grounded parallel line,
set the K
N1
for the zone 1 as:
1MRK 506 369-UUS - Section 8
Impedance protection
Line distance protection REL670 2.2 ANSI 271
Application manual
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