1 2 3 4 A - SIPROTEC

Functions
2.2 Differential Protection
106
All following considerations are based on the convention that all currents flowing into the protected zone are
defined as positive unless explicitly stated otherwise.
Basic Principle with more than Two Sides
Current Restraint
For protected objects with three or more sides or for busbars, the differential principle is expanded in that the
total of all currents flowing into the protected object is zero in healthy operation, whereas in case of a fault the
total in-flowing current is equal to the fault current.
See figure 2-18 as an example for four feeders. The three-winding transformer in figure 2-19 has 4 measuring
locations, so it is treated by the differential protection like a „4-winding“ transformer.
Figure 2-18 Basic principle of differential protection for four ends (single-phase illustration)
Figure 2-19 Basic principle of differential protection for 4 measuring locations — example of a three-winding
power transformer with 4 measuring locations (single-phase illustration)
When an external fault causes a heavy current to flow through the protected zone, differences in the magnetic
characteristics of the current transformers CT1 and CT2 (figure 2-17) under conditions of saturation may cause
a significant current flow through the measuring element M. If it is greater than the respective pickup threshold,
the device can trip even though no fault occurred in the protected zone. Current restraint (stabilisation) prevents
such erroneous operation.
In differential protection systems for protected objects with two terminals, a restraining quantity is normally
derived from the current difference |I 1 – I 2| or from the arithmetical sum |I 1| + |I 2|. Both methods are equal in the
relevant ranges of the stabilisation characteristics. For protected objects with more than two ends, such as
multi-winding transformers, busbars etc, only the arithmetical sum method is possible. The latter method is
used in 7UT6x for all protected objects. The following definitions apply for 2 measuring points:
a tripping or differential current
I diff = |I 1 + I 2|
and the stabilisation or restraining current
I stab = |I 1| + |I 2|
SIPROTEC, 7UT6x, Manual
C53000-G1176-C230-2, Release date 06.2012