ABB REL670 2.2 IEC, Applications Manual

Page: 260 / 908

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 173)
Section 8
1MRK 506 369-UEN B
Impedance protection
254 Line distance protection REL670 2.2 IEC
Application manual