[erddiff-ausloesekennlinie-020926-rei, 1, en_GB]
Figure 2-62 Tripping characteristic of the restricted earth fault protection depending on the earth current
ratio 3Ι
0
”/3Ι
0
' (both currents in phase + or counter-phase –); Ι
REF
≥ Einstellwert; Ι
Trip
= tripping
current
It was assumed in the above examples that the currents 3Ι
0
" and 3Ι
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 displacement φ(3
Ι
0
"; 3Ι
0
') = 90° then the restraint quantity is zero. This corresponds to the conventional
method of direction determination by use of the vectorial sum and difference comparison.
[erddiff-stabgroesse-020926-rei, 1, en_GB]
Figure 2-63 Phasor diagram of the restraint quantity during internal 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
.
This limit angle determines for which phase displacement between 3Ι
0
" and 3Ι
0
' the pickup value for 3Ι
0
" = 3Ι
0
'
grows to ∞, i.e. no pickup occurs. In7UT6x is equal to 4.
The restraint quantity Ι
stab
in the first example above is quadrupled once more; it becomes thus 8 times the
tripping effect quantity Ι
Trip
.
Functions
2.3 Restricted Earth Fault Protection
SIPROTEC 4, 7UT6x, Manual 131
C53000-G1176-C230-5, Edition 09.2016
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