Siemens SIPROTEC 4 7UT6 Series Manual page 102

Functions
2.2 Differential Protection
[betraganpassung-bsp-3wick-trans280503-st, 1, en_GB]
Figure 2-27
The device carries out this magnitude matching internally, based on the nominal values set according to
Section "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.
Non-earthed Transformer Starpoint
Figure 2-28 illustrates an example for a power transformer Yd5 (wye-delta with 150° phase displacement)
without any earthed starpoint. The figure shows the windings (above) and the vector diagrams of symmetrical
currents (below). The general form of the matrix equation is:
[diff-trafo-gleich-yd5-021026-rei, 1, en_GB]
(I )
m
k
(K)
Ι
n
On the left (delta) winding, the matched currents Ι
currents Ι , Ι , Ι
L1 L2
matching not considered).
102
Magnitude matching — example of a three-winding power transformer (phase relation not
considered)
Matrix of the matched currents Ι
Constant factor for magnitude matching,
Coefficient matrix, dependent on the vector group,
Matrix of the phase currents Ι
, On the right (wye) side, the matched currents are equal to the phase currents (magnitude
L3
, Ι , Ι
A B C,
, Ι , Ι .
L1 L2 L3
, Ι , Ι are derived from the difference of the phase
A B C
C53000-G1176-C230-5, Edition 09.2016
SIPROTEC 4, 7UT6x, Manual