Siemens SIPROTEC 7UT613 series Manual page 116

2 Functions
Earthed Starpoint
Starpoint Current
not Available
116
Differential protection makes use of the fact that the total of all currents flowing into the
protected object is zero in healthy operation. If the starpoint of a power transformer
winding is connected to earth, a current can flow into the protected zone across this
earth connection in case of earth faults. Consequently, this current should be included
in the current processing in order to obtain a complete image of the in-flowing quanti-
ties. Figure 2-29 shows an external earth fault which produces an out-flowing zero se-
= –3 · I
quence current (–I
L3
= 3 · I
(I ). As a result, these currents cancel each other out.
St 0
Figure 2-29 Example for an earth fault outside a transformer with current distribution
The complete matrix equation for the earthed side (right) is in this case, including all
in-flowing currents:
I corresponds to –3 I
SP 0
is included in case of an internal fault (from I
fault, the zero sequence current component of the line currents 3 · I
(negative here) is compensated by the starpoint current I
sensitivity (with zero sequence current) is achieved for internal earth faults and full
elimination of the zero sequence current in case of external earth faults. For consider-
ation of the earth fault current, the advanced parameter diff protection with measured
earth current, side x must be switched on (addresses 1211 DIFFw.IE1-MEAS to
1215 DIFFw.IE5-MEAS = YES).
Even higher earth fault sensitivity during internal earth fault is possible by means of
the restricted earth fault protection (section 2.3).
In many cases, however, the starpoint current is not available. The total summation of
the in-flowing currents is thus not possible because I
false formation of the differential current, the zero sequence current must be eliminat-
ed from the line currents (–I
Figure 2-30 shows an example of a YNd5 vector group with earthed starpoint on the
Y-side.
In figure 2-30 on the left side, the zero sequence currents cancel each other because
of the calculation of the current differences. This complies with the fact that zero se-
quence current is not possible outside the delta winding. On the right side, the zero
sequence current must be eliminated if the starpoint current cannot be included. This
results from the matrix equation, e.g. for I
), which corresponds with the in-flowing starpoint current
0
in case of through-flowing current. The zero sequence current
=
0
= –3 · I
).
L3 0
:
A
I
1
/ ); in case of an external earth
3 SP
= (I
0
. In this way, almost full
SP
is missing. In order to avoid
SP
7UT613/63x Manual
C53000-G1176-C160-2
+ I + I
)
L1 L2 L3