Siemens SIPROTEC 7UT613 series Manual page 145

7UT613/63x Manual
C53000-G1176-C160-2
It was assumed in the above examples that the currents 3I
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
"; 3I ') = 90° then the restraint quantity is zero. This corresponds to the
0 0
conventional method of direction determination by use of the vectorial sum and differ-
ence comparison.
Figure 2-61 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
This limit angle determines for which phase displacement between 3I
pickup value for 3I " = 3I
0
to 4.
The restraint quantity I
becomes thus 8 times the tripping effect quantity I
The limit angle is ϕ
Limit
ϕ(3I ')| ≥ +100°.
";°3I
0 0
Figure 2-62 shows the operating characteristics of the restricted earth fault protection
dependent of the phase displacement between 3I
|3I "| = |3I '|.
0 0
.
Limit
' grows to ∞, i.e. no pickup occurs. In 7UT613/63x k is equal
0
in the above example a) is quadrupled once more; it
stab
= 100°. That means no trip is possible for phase displacement
2.3 Restricted Earth Fault Protection
" and 3I ' are in counter-
0 0
" and 3I
0
.
from
" and 3I ', for a constant infeed ratio
0 0
' the
0
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