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2.2 Differential Protection
SIPROTEC, 7UT6x, Manual
C53000-G1176-C230-2, Release date 06.2012
Functions
2.2 Differential Protection
The differential protection represents the main protection feature of the device. It is based on current
comparison under consideration of the transformation ratio of the transformer.7UT6x is suitable for unit protection
of transformers, generators, motors, reactors, short lines, also with feeders, and (under observance of the
available number of current inputs) for busbar arrangements. Protection of generator/transformer units, transformer/winding
combinations or transformer/starpoint former, can also be realised. 7UT612 allows up to two,
7UT613 and 7UT633 allow up to 3, 7UT635 allows up to 5 three-phase measuring locations.
7UT6x can be used as a single-phase differential protection relay. In this case, 7UT612 allows up to 7, 7UT613
and 7UT633 allow up to 9, and 7UT635 up to 12 measuring locations, e.g. currents from a busbar with up to 7
or 9 or 12 feeders.
The protected zone is selectively limited by the CTs at its ends.
2.2.1 Functional Description of the Differential Protection
Processing of the measured values depends on the way the differential protection is used. This section
discusses first the differential protection function in general, regardless of the type of protected object. A singlephase
system is referred to. Particulars with regard to the individual protected objects follow thereafter.
Basic Principle with Two Sides
Differential protection is based on current comparison. It makes use of the fact that a protected object carries
always the same current i (dashed line in 2-17 below) at its two sides in healthy operation. This current flows
into one side of the considered zone and leaves it again on the other side. A difference in currents is a clear
indication of a fault within this section. If the actual current transformation ratio is the same, the secondary windings
of the current transformers CT1 and CT2 at the sides of the protected object can be connected to form a
closed electric circuit with a secondary current I; a measuring element M, which is connected to the electrical
balance point, remains at zero current in healthy operation.
Figure 2-17 Basic principle of differential protection for two sides (single-phase illustration)
When a fault occurs in the zone limited by the transformers, a current i 1 + i 2, which is proportional to the fault
currents I 1 + I 2 flowing in from both sides is fed to the measuring element. As a result, the simple circuit shown
in Figure 2-17 ensures a reliable tripping of the protection if the fault current flowing into the protected zone
during a fault is high enough for the measuring element M to respond.
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