Some observations from above equations for case 1, 2, and 3
GUID-6B411616-EB82-4447-BC2D-375D042BE110 v1
In case 1, the lowest impedance is measured, that is, the highest reach occurs due to the
parallel connection of the zero-sequence systems of both lines.
In case 3, the highest impedance is measured, which corresponds to the shortest reach.
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 earthed at both
ends. During an earth fault, a counteracting current is induced in the earthed 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
GUID-A881F22E-D827-4B20-9C0F-477B3A20B0EA v2
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-earth faults in the
following ways, by using:
•
Alternative 1: The possibility of different setting values that influence the earth-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
GUID-68709DD6-D5A7-4F04-ACE5-350B1F835364 v1
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
GUID-B0D8E294-C850-4776-B31A-2669D2684C4C v2
For case 1, parallel line having both ends earthed , zone 1 overreach caused by the earthed
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 earth fault
conditions. To eliminate the overreach caused by the earthed parallel line, set the K
N1
for the
zone 1 as:
0
1
0
1
1
0
3
m
N
Nm
K
Z
Z
Z
K
Z
Z
=
-
-
IECEQUATION14018 V1 EN-US
(Equation 196)
This K
N1
setting for zone 1 only affects the reach for earth faults while the reach for two and
three-phase faults are unaffected.
Section 7
1MRK 505 343-UEN B
Impedance protection
236
Application manual
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