1 2 3 4 A - SIPROTEC

SIPROTEC, 7UT6x, Manual
C53000-G1176-C230-2, Release date 06.2012
Functions
2.2 Differential Protection
Figure 2-27 shows an example of a three-winding power transformer. Winding 1 (S1) and 2 (S2) are rated for
72 MVA; The settings recommended are the same as in figure 2-26. But the third winding (S3) has 16 MVA
rating (e.g. for auxiliary supply). The rated current of this winding (= side of the protected object) results in
924 A. On the other hand, the differential protection has to process comparable currents. Therefore, the
currents of this winding must be referred to the rated power of the protected object, i.e. 72 MVA. This results in
a rated current (i.e. the current under nominal conditions of the protected object, 72 MVA) of 4157 A. This is
the base value for the third winding: These currents must be multiplied by the factor k3.
Figure 2-27 Magnitude matching — example of a three-winding power transformer (phase relation not
considered)
The device carries out this magnitude matching internally, based on the nominal values set according to Subsection
„General Power System Data“ under margin heading „Object Data with Transformers“, and „Current
Transformer Data for 3-phase Measuring Locations“). Once the vector group has been entered, the protective
device is capable of performing the current comparison according to fixed formulae.
Conversion of the currents is performed by programmed coefficient matrices which simulate the difference
currents in the transformer windings. All conceivable vector groups (including phase exchange) are possible.
In this regard, the conditioning of the starpoint(s) of the power transformer is also essential.
115