Siemens SIPROTEC 4 7UT6 Series, Manual

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Differential Protection for Mini-Busbars and Short Lines
A mini-busbar or branch-point is defined here as a three-phase, coherent piece of conductor which is limited
by sets of current transformers. Examples are short stubs or mini-busbars. The differential protection in this
operation mode is not suited to transformers; use the function “Differential Protection for Transformers” for
this application. Even for other inductors, like series or shunt reactors, the busbar differential protection
should not be used because of its lower sensitivity.
This operation mode is also suitable for short lines or cables. “Short” means in this context that the current
transformer connections from the CTs to the device do not cause impermissible load to the current trans-
formers. On the other hand, capacitive charging currents do not harm this operation because the differential
protection is normally less sensitive with this application.
Since the current direction is normally defined as positive in the direction of the protected object, this results
in the definitions as illustrated in
Figure 2-39 and Figure 2-40 .
Model 7UT612 allows 3-phase branch points or mini-busbars with 2 terminals to be protected, 7UT613 and
7UT633 allow 3 terminals and 7UT635 up to 5 terminals.
Figure 2-41 shows the example of a busbar with 4
feeders.
[diff-ss-stromdefinition-020926-rei, 1, en_GB]
Figure 2-39 Definition of current direction at a branch-point (busbar with 2 feeders)
[diff-leitung-stromdefinition-020926-rei, 1, en_GB]
Figure 2-40 Definition of current direction at short lines
[diff-ss-stromdefinition-4-abzweige-020926-st, 1, en_GB]
Figure 2-41 Definition of current direction at busbar with 4 feeders
The differential protection feature of the 7UT6x refers all currents to the rated current of the protected object.
The device is informed during setting about the rated current of the protected object (in this case the busbar
or line), and about the primary rated CT currents. Measured value matching is therefore reduced to magnitude
factors. The basis for current comparison is the rated busbar current (address 371
I PRIMARY OP.
). If the
feeders or ends have different rated currents, the largest of the three rated currents is used as the basis for the
current comparison, and all other currents are converted accordingly. As a rule, no external matching devices
are necessary.
2.2.5
Functions
2.2 Differential Protection
110 SIPROTEC 4, 7UT6x, Manual
C53000-G1176-C230-5, Edition 09.2016