Siemens SIPROTEC 4 7UT6 Series Manual page 104

Functions
2.2 Differential Protection
[diff-trafo-erdkurzschuss-beisp1-020926-st, 1, en_GB]
Figure 2-29
The complete matrix equation for the earthed side (right) is in this case, including all in-flowing currents:
[diff-trafo-erdkurzschluss-021026-rei, 1, en_GB]
corresponds to –3 Ι
Ι
SP
internal fault (from Ι
line currents 3 · Ι
almost full 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 consideration 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
Starpoint Current not Available
In many cases, however, the starpoint current is not available. The total summation of the in-flowing currents
is thus not possible because Ι
sequence current must be eliminated from the line currents (–Ι
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 sequence 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 Ι
1 / · (2 Ι – 1 Ι
3 L1 L2
Zero sequence current elimination achieves that fault currents which flow via the transformer during earth
faults in the network in case of an earth point in the protected zone (transformer starpoint or starpoint former
by neutral earth reactor) are rendered harmless without any special external measures. Refer e.g. to
Figure 2-30: Because of the earthed starpoint, a zero sequence current occurs on the right side during a
network fault but not on the left side. Comparison of the phase currents, without zero sequence current elimi-
nation and without inclusion of the starpoint current, would cause a wrong result (current difference in spite
of an external fault).
104
Example for an earth fault outside a transformer with current distribution
in case of through-flowing current. The zero sequence current is included in case of an
0
= 1 / ), in case of an external earth fault, the zero sequence current component of the
Ι
0 3 SP
= (Ι + Ι + Ι ) (negative here) is compensated by the starpoint current Ι
0 L1 L2 L3
2.3 Restricted Earth Fault
is missing. In order to avoid false formation of the differential current, the zero
SP
– 1 Ι ) = 1 / · (3 Ι – Ι – Ι
L3 3 L1 L1
Protection).
= –3 · Ι
L3
:
A
– Ι ) = 1 / · (3 Ι – 3 Ι ) = (Ι
L2 L3 3 L1 0
. In this way,
SP
).
0
– Ι ).
L1 0
SIPROTEC 4, 7UT6x, Manual
C53000-G1176-C230-5, Edition 09.2016