Siemens SIPROTEC 7UT613 series Manual page 119

Auto-transformer
Bank with Current-
sum Comparison
Use on Single-
phase Auto-trans-
formers
7UT613/63x Manual
C53000-G1176-C160-2
the differential protection in case of earth faults in the protected zone is less sensitive
2
by the factor / , because the zero sequence current is
3
If, however, the starpoint current is accessible and connected to the device, then all
currents flowing into the protected zone are available. The zero sequence currents in
the phases will then be cancelled at the externally located earth faults by the sum of
the starpoint current. In case of internally located earth fault, the full sensitivity of the
differential protection is ensured. For consideration of the earth fault current, the ad-
vanced parameter diff protection with measured earth current, side x must be switched
on (addresses 1211 DIFFw.IE1-MEAS to 1215 DIFFw.IE5-MEAS = YES).
Increased earth fault sensitivity during internal fault can be achieved by using the re-
stricted earth fault protection or the high-impedance differential protection.
A further possibility to increase the earth fault sensitivity is useful for auto-transformer
banks where 1 single-phase auto-transformers are arranged to a transformer bank. In
this arrangement, single-phase earth faults are the most probable whereas inter-
winding faults (between two windings) can be excluded because of the physical sep-
aration of the three transformers. A current comparison protection can be built up over
each of the auto-connected windings which compares the currents flowing into the
„total winding". However, a further galvanically separated winding (usually delta wind-
ing), can not be protected by means of this protection method. A further requirement
is that during configuration of the functional scope PROT. OBJECT = Autotr. node
is set and the protection topology is determined accordingly (section 2.1.4, sub-
section „Topology of the Protected Object" under margin heading „Auto-transformer
Banks").
Figure 2-33 Auto-transformer bank with current transformer in starpoint connection
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 mea-
sured 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 I
they are designated I
L1
If two winding phases are available, they may be connected either in series (which cor-
responds 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°. Figure 2-34 shows
an example of a single-phase power transformer with two phases per side with the def-
inition of the direction of the currents.
and I
in the following.
L3
2.2 Differential Protection
1
/ of the fault current.
3
and I
of the device,
L1 L3
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