1 2 3 4 A - SIPROTEC

Functions
2.2 Differential Protection
122
Figure 2-35 Example of an earth fault outside a single-phase transformer with current distribution
The matrix equation in this cases is as follows:
Where I SP is the current measured in the „Starpoint“ connection.
The zero sequence current is not eliminated. Instead of this, for each phase half of the starpoint current I SP is
added. The effect is that the zero sequence current is considered in case of an internal ground fault
(from I 0 =–/ 2 · I SP), whilst the zero sequence current is eliminated in case of an external fault because the
zero sequence current on the terminal side 2 · I 0 = (I L1 + I L3) compensates for the starpoint current I SP. Almost
full sensitivity (with zero sequence current) is thus achieved for internal earth faults and full elimination of the
zero sequence current in case of external earth faults.
Even higher earth fault sensitivity during internal earth fault is possible by means of the restricted earth fault
protection (section 2.3).
SIPROTEC, 7UT6x, Manual
C53000-G1176-C230-2, Release date 06.2012