GE D90 Plus Instruction Manual page 235

CHAPTER 7: PROTECTION
PLUS
D90 LINE DISTANCE PROTECTION SYSTEM – INSTRUCTION MANUAL
Phase distance zone 4 guidelines for stepped distance
As 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 protection schemes on the next line. The use of a lens characteristic or the load
encroachment element can be advantageous if load limits are a problem.
To avoid extremely large reach settings, the D90
element so that it is reverse looking. This can provide a back up for the longest line
terminated on the local bus. This strategy is beneficial if the reduced reach helps
discrimination between the load and fault conditions, but it must be implemented at both
ends of the protected line.
Phase distance zone 5 guidelines for stepped distance
An additional fifth phase 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.
Ground distance neutral current supervision guidelines for stepped distance
The current supervision for the ground distance elements responds to an internally
calculated neutral current (3 × I_0). Base the setting for this element on twice the zero-
sequence line capacitance current or the maximum zero-sequence unbalance under
maximum load conditions. Do not use this element to prevent an output when the load
impedance is inside the distance characteristic on a steady-state basis.
Polarizing current and non-homogeneity correction angle of quadrilateral
characteristic for stepped distance
An ideal reactance line for single-line-to-ground faults is polarized from the fault current
flowing through the fault resistance. Such a line defines constant reach and can be
implemented by the following angle comparator.
The relay can only approximate the unknown fault current by the zero-sequence or the
negative-sequence currents measured at the relaying point. Depending on system
parameters, either the zero-sequence or the negative-sequence current presents better
approximation of the fault current angle.
Figure 199: Understanding system homogeneity
GROUPED PROTECTION ELEMENTS
Plus
has the ability to implement any
Eq. 8
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