Distance Backup/Supervision; Overview - GE L90 Instruction Manual

CHAPTER 8: APPLICATION OF SETTINGS

DISTANCE BACKUP/SUPERVISION

8.4 Distance backup/supervision

8.4.1 Overview

As an economical approach to supply customer load, many high voltage (HV) lines have transformers tapped to the lines.
The figure shows a typical configuration.
Figure 8-6: Typical high-voltage line configuration
Two distinct approaches are available (Distance Backup and Distance Supervision), depending on which concerns are
dominant. In either case, the distance function can provide a definite time backup feature to give a timed clearance for a
failure of the L90 communications. Additionally, a Permissive Over-reaching Transfer Trip (POTT) scheme can be selected
and activated after detection of an L90 communications failure, if an alternate lower bandwidth communications channel
is available.
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If Distance Backup is employed, dependability concerns usually relate to a failure of the communications. The distance
elements can then effectively provide a means of fault identification and clearance. However, for a line with tapped
transformers, a number of other issues need to be considered to ensure stability for the L90.
Any differential scheme has a potential problem when a low-voltage (LV) fault occurs at the tapped transformer location,
and the current at the tap is not measured. Because the transformer size can become quite large, the required increase in
the differential setting to avoid operation for the LV bus fault can result in a loss of sensitivity.
If the tapped transformer is a source of zero sequence infeed, then the L90 zero-sequence current removal has to be
enabled as described in the next section.
The zero sequence infeed creates an apparent impedance setting issue for the backup ground distance and the zero
sequence compensation term is also not accurate, so that the positive sequence reach setting must be increased to
compensate. The phase distance reach setting can also need to be increased to cope with a transfer across the two
transformers, but this is dependent on the termination and configuration of the parallel line.
L90 LINE CURRENT DIFFERENTIAL SYSTEM – INSTRUCTION MANUAL 8-13