Siemens SIPROTEC 7UT613 series Manual page 118

2 Functions
Use on Auto-Trans-
formers
118
Figure 2-31 Example of an earth fault outside the protected transformer with a neutral earth-
ing reactor within the protected zone
The disadvantage of elimination of the zero sequence current is that the protection
becomes less sensitive (factor
in case of an earth fault in the protected area. Therefore, elimination is suppressed in
case the starpoint is not earthed (see figure 2-28), or the starpoint current can be in-
cluded (figure 2-29). If, for example, a surge voltage arrester is connected to the star-
point, one should do without the advantage of that option in order to avoid recognition
of a breakdown of the surge voltage arrester as an internal fault. For this purpose, the
starpoint of the respective side must be set to Earthed (addresses 313 STARPNT
SIDE 1, 323 STARPNT SIDE 2, 333 STARPNT SIDE 3, 343 STARPNT SIDE 4,
353 STARPNT SIDE 5).
In order to achieve comparable currents for the differential protection, all currents are
referred to the winding (= side) with the highest power rating. This apparent power is
named the rated power of the protected object. If this rated apparent power occurs
several times, the side with the higher nominal current is selected as reference side.
Auto-connected windings in auto-transformers can only be connected Y(N)y0 (figure
2-32). If the starpoint is earthed, all auto-connected windings connected to the system
parts (higher and lower voltage system) are affected. The zero sequence system of
both system parts is coupled because of the common starpoint.
Figure 2-32 Auto-transformer with earthed starpoint
In this case, too, the starpoint current I
of all currents flowing into the protected zone. If it is not accessible, the zero sequence
current from the phase currents must be eliminated. This is achieved by the applica-
tion of the matrices with zero sequence current elimination. As for separate windings,
2
/ because the zero sequence current amounts to
3
would be required for a complete treatment
SP
1
/
3
7UT613/63x Manual
C53000-G1176-C160-2