GE D90 Plus Instruction Manual page 237

CHAPTER 7: PROTECTION
Series compensated line protection application guidelines
PLUS
D90 LINE DISTANCE PROTECTION SYSTEM – INSTRUCTION MANUAL
Ground distance zone 3 guidelines for stepped distance
This remote back up function must have a reach that is set to account for any infeed at the
remote bus, plus the impedance of the longest line that 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.
Ground distance zone 4 guidelines for stepped distance
As a further contribution to a remote backup philosophy, 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 load
encroachment element is advantageous when load limits are a problem. To avoid
extremely large reach settings, the D90
that it is reverse-looking. This strategy can be beneficial if the reduced reach enhances the
discrimination between the load and fault conditions. If adopted, this approach must be
implemented at both ends of the protected line.
Ground distance zone 5 guidelines for stepped distance
An additional fifth ground distance zone can be used in special applications. This zone can
be the start zone for arming tripping from all other zones, in both forward and reverse
directions of the protected line. This zone can also be used as an additional reverse-
looking zone for implementation of two reverse-looking zones, thus providing two-zone
backup reverse-looking protection. The fifth distance zone can be used as an alarm zone,
indicating that load impedance is approaching the zone characteristic.
This section provides general application guidelines for implementing protection for series
compensated lines with the D90
It is recommended to apply a combination of distance, ground directional overcurrent, and
high-set overcurrent functions for protection of series compensated lines. The setting rules
described here must take into account a variety of system configurations, particularly a
status of series capacitors (in-service, by-passed). Either consider the worst-case topology
or, if possible, apply adaptive settings through the multiple settings groups mechanism.
A line compensating capacitor is a bank of three physical capacitors and their overvoltage
protecting devices (air gaps, MOVs, or both). If none of the MOV or gaps conducts any
significant current, the positive-sequence, negative-sequence, and zero-sequence
reactance of the three-phase bank equal the reactance of the actual (phase) capacitors.
However, under asymmetrical conditions such as a single-line-to-ground fault, when only
one MOV or gap may operate, the series capacitor bank creates extra (series) asymmetry
in addition to the fault (shunt) asymmetry. The positive-sequence, negative-sequence, and
zero-sequence impedances differ from each other and do not equal the impedance of the
phase capacitors. Moreover, there can be mutual coupling between the sequence
networks representing the series capacitor bank. This makes analytical analysis of fault
conditions very burdensome. For setting calculations, however, it is justified to assume that
the zero-sequence, positive-sequence, and negative-sequence reactance of the capacitor
bank equal the reactance of the actual (phase) capacitors. This represents a worst-case
low-current fault scenario, when the steady-state effects of series compensation are most
weighty.
Distance setting guidelines for protecting series compensated lines
Traditionally, the reach setting of an underreaching distance function is specified based on
the net inductive impedance between the potential source of the relay and the far-end
busbar, or location for which the zone must not overreach. Faults behind series capacitors
on the protected and adjacent lines need to be considered for this purpose. For further
illustration, a sample system shown in the figure is considered.
GROUPED PROTECTION ELEMENTS
Plus
has the ability to implement any element so
Plus
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