Siemens SIPROTEC 4 7UT6 Series Manual page 107

Use on Single-phase Auto-transformers
Single-phase transformers can be designed with one or two windings per side; in the latter case, the winding
phases can be wound on one or two iron cores. In order to ensure that optimum matching of the currents
would be possible, always two measured current inputs shall be used even if only one current transformer is
installed on one phase. The currents are to be connected to the inputs Ι
nated Ι and Ι
L1 L3
If two winding phases are available, they may be connected either in series (which corresponds to a wye-
winding) or in parallel (which corresponds to a delta-winding). The phase displacement between the windings
can only be 0° or 180°.
per side with the definition of the direction of the currents.
[diff-trafo-einpasen-020926-rei, 1, en_GB]
Figure 2-34
Like with 3-phase power transformers, the currents are matched by programmed coefficient matrices which
simulate the difference currents in the transformer windings. The common form of these equations is:
(Ι ) = k· (K)· (Ι
m
with
(Ι ) - matrix of the matched currents Ι
m
k - constant factor for magnitude matching,
(K) - coefficient matrix,
(Ι ) - matrix of the phase currents Ι
n
Since the phase displacement between the windings can only be 0° or 180°, matching is relevant only with
respect to the treatment of the zero sequence current (besides magnitude matching). If a "Starpoint" of the
protected transformer winding is not earthed (left in
If the "starpoint" is earthed (figure
unless it can be compensated by considering the "Starpoint current". By eliminating the zero sequence current,
fault currents which flow through the transformer during earth faults in the network in case of an earth point
in the protected zone (transformer starpoint) are rendered harmless without any special external measures.
The matrices for the left and the right winding as per
[diff-trafo-gleich-einph-nullstromelim-021026-rei, 1, en_GB]
The disadvantage of elimination of the zero sequence current is that the differential protection becomes less
sensitive (by factor
protected zone). Higher earth fault sensitivity can be achieved if the "starpoint" current is available, i.e. if a CT
is installed in the "starpoint" connection to earth and this current is fed to the device
eration of the earth fault current, the advanced parameter diff protection with measured earth current, side x
must be switched on (addresses 1211DIFFw.IE1-MEAS bis 1215DIFFw.IE5-MEAS = YES).
SIPROTEC 4, 7UT6x, Manual
C53000-G1176-C230-5, Edition 09.2016
in the following.
Figure 2-34 shows an example of a single-phase power transformer with two phases
Example of a single-phase transformer with current definition
)
n
Figure 2-34
1 / because the zero sequence current amounts to
2
, Ι ,
A C
, Ι .
L1 L3
Figure 2-34), the phase currents can directly be used.
right side), the zero sequence current must be eliminated
Figure 2-34 are
1 /
2.2 Differential Protection
and Ι of the device, they are desig-
L1 L3
in case of an earth faults in the
2
(Figure 2-35). For consid-
Functions
107