2 Functions
84
7SD610 Manual
C53000-G1176-C145-4
Figure 2-30 Tripping characteristic of the restricted earth fault protection depending on the
earth current ratio 3I
0
”/3I
0
' (both currents in phase + or counter-phase –);
I
REF
> = setting; I
Trip
= tripping current
It was assumed in the above examples that the currents 3I
0
" and 3I
0
' are in counter-
phase for external earth faults which is only true for the primary measured quantities.
Current transformer saturation may cause phase shifting between the fundamental
waves of the secondary currents which reduces the restraint quantity. If the phase dis-
placement ϕ(3I
0
"; 3I
0
') = 90° then the restraint quantity is zero. This corresponds to the
conventional method of direction determination by use of the vectorial sum and differ-
ence comparison.
Figure 2-31 Phasor diagram of the restraint quantity during external fault
The restraint quantity can be influenced by means of a factor k. This factor has a
certain relationship to the limit angle ϕ
Limit
.
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