Lines With Tapped Transformers - GE L60 Instructions Manual

CHAPTER 8: APPLICATION OF SETTINGS DISTANCE/BACKUP SUPERVISION
Figure 8-2: 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 L60 communications. Additionally, a Permissive Over-reaching Transfer Trip (POTT) scheme can be selected
and activated after detection of an L60 communications failure, if an alternate lower bandwidth communications channel
is available.
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 L60.
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 L60 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.
Three terminal line applications generally result in larger reach settings for the distance backup and require a calculation
of the apparent impedance for a remote fault. Carry this out for each of the three terminals, as the calculated apparent
impedance differs at each terminal.
Distance Supervision essentially offers a solution for the LV fault condition, but the differential setting must still be
increased to avoid operation for an external L-g or L-L-g fault external ground fault. In addition, the distance element
reach setting must still see all faults within the protected line and be less than the impedance for a LV bus fault.
The effective source impedance ratio (SIR) for the LV fault generally is not high, so that CVT transients do not contribute to
measuring errors.
If the distance supervision can be set to avoid operation for a transformer LV fault, then generally the filtering associated
8
with the distance measuring algorithm ensures no operation under magnetizing inrush conditions. The distance element
can be safely set up to 2.5 × V / I , where V is the system nominal voltage and I is the peak value of the
nom peak nom peak
magnetizing inrush current.
For applications where the tapped station is close to one terminal, then it can be difficult to set the distance supervision to
reach the end of the line, and at the same time avoid operation for an LV fault. For this system configuration, use a three-
terminal L60; the third terminal is then fed from CT on the high side of the tapped transformer.

8.2.2 Lines with tapped transformers

If a protected line has a tapped transformer, it is preferable to apply the L60 in a three-terminal configuration. This
provides the most secure and reliable solution. However, if current measurements or the channel between the tapped
line(s) and the two other terminals are not available, then the measures outlined in the following sections must be taken.
L60 LINE PHASE COMPARISON SYSTEM – INSTRUCTION MANUAL 8-5