Ground Distance; Pott Signaling Scheme; Overview - GE L60 Instruction Manual

CHAPTER 8: APPLICATION OF SETTINGS

POTT SIGNALING SCHEME

8.2.9.4 Phase distance zone 3
If a remote backup philosophy is followed, then the reach of this element must be set to account for any infeed at the
remote bus, plus the impedance of the longest line which terminates on this remote bus. The time delay must coordinate
with other time-delayed protections on any remote line. Circuit loading limitations created by a long zone reach can be
overcome by using lens or quadrilateral characteristics and/or a load encroachment supervising characteristic. Consider
also a situation where the load impedance can enter into the relay characteristic for a time longer than the chosen time
delay, which can occur transiently during a system power swing. For this reason, use the power swing blocking function.

8.2.10 Ground distance

8.2.10.1 Neutral current supervision
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.
8.2.10.2 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 can be 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 can 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 adjacent circuit, then
the typical 80% setting can 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 compared with the phase distance elements because the zero-
sequence impedance of the line and thus the zero-sequence-compensating factors can vary significantly due to weather
and other conditions.
8.2.10.3 Ground distance zone 2
To ensure that 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 occurs on double circuit lines, when both lines carry the same current. Conduct an
analytical study 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.
8.2.10.4 Ground distance zone 3
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
8
be time coordinated with timed clearances on the next section.
8.3 POTT signaling scheme

8.3.1 Overview

This scheme is intended for two-terminal line applications.
This scheme uses an over-reaching Zone 2 distance element to essentially compare the direction to a fault at both the
ends of the line.
Ground directional overcurrent functions available in the relay can be used in conjunction with the Zone 2 distance
element to key the scheme and initiate its operation. This provides increased coverage for high-resistance faults.
L60 LINE PHASE COMPARISON SYSTEM – INSTRUCTION MANUAL 8-11