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10 APPLICATION OF SETTINGS
10.4 DISTANCE BACKUP/SUPERVISION
ation should also be given to a situation where the load impedance may enter into the relay characteristic for a time longer
than the chosen time delay, which could occur transiently during a system power swing. For this reason the power swing
blocking function should be used.
e) PHASE DISTANCE ZONE 4
A further contribution to remote backup, the reach of this element must be set to account for any infeed at the remote bus.
The time delay must coordinate with other time-delayed protections on the next line. The use of a lens characteristic or the
load encroachment element may be advantageous if load limits are a problem.
To avoid extremely large reach settings, the L90 has the ability to implement any element so that it is reverse looking, which
then can provide a back up for the longest line terminated on the local bus. This strategy can be beneficial if the reduced
reach helps discrimination between the load and fault conditions, but must be implemented at both ends of the protected
line.
f) PHASE DISTANCE ZONE 5
An additional fifth phase distance zone can be used in special applications. This zone may be start zone for arming tripping
from all other zones, in both forward and reverse directions of the of protected line. This zone may also be used as an addi-
tional reverse-looking zone for implementation of two reverse-looking zones, thus providing two zone backup reverse-look-
ing protection. The fifth distance zone can be used as an alarm zone, indicating that load impedance is approaching the
zone characteristic.
10.4.3 GROUND DISTANCE
a) NEUTRAL CURRENT SUPERVISION
The current supervision for the ground distance elements responds to an internally calculated neutral current (3 × I_0). The
setting for this element should be based on twice the zero-sequence line capacitance current or the maximum zero-
sequence unbalance under maximum load conditions. This element should not be used to prevent an output when the load
impedance is inside the distance characteristic on a steady state basis.
b) GROUND DISTANCE ZONE 1
The zone 1 reach must be set so that nominally instantaneous operation does not extend beyond the end of the protected
line. However this may be somewhat more complicated than for the phase elements, because of zero sequence mutual
induction with an adjacent parallel line, possibly carried on the same tower, which can be out of service and grounded at
multiple points. A fault beyond 100% of the protected line may cause overreach unless the reach is reduced significantly,
sometimes as low as 65% of the line length. If the line being protected does not have a significant interaction with an adja-
cent circuit, then the typical 80% setting may be used. If there is significant mutual coupling between the parallel lines, then
the mutual compensation feature of the ground distance elements can be used instead of a drastic reduction in the reach.
However, even in this case, there is more uncertainty as compared with the phase distance elements because the zero-
sequence impedance of the line and thus the zero-sequence-compensating factors may vary significantly due to weather
and other conditions.
c) GROUND DISTANCE ZONE 2
To ensure that the zone 2 can see 100% of the line, inter-circuit mutual effects must be considered, as they can contribute
to a significant under-reach. Typically this may occur on double circuit lines, when both lines may carry the same current.
An analytical study should be carried out to determine the appropriate reach setting.
The main purpose of this element is to operate for faults beyond the reach of the local zone 1 element, and therefore a time
delay must be used similar to the phase fault case.
d) GROUND DISTANCE ZONE 3
This remote back up function must have a reach which is set to account for any infeed at the remote bus, plus the imped-
ance of the longest line which terminates on this remote bus. Similar to the phase fault case, a zone 3 element must be
time coordinated with timed clearances on the next section.
10
GE Multilin
L90 Line Current Differential System
10-15
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