Siemens SIPROTEC 7SD610 - Page 86

Functions
2.5 Restricted Earth Fault Protection (optional)
SIPROTEC, 7SD610, Manual
C53000-G1176-C145-6, Release date 02.2011
86
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, is in fact, true for the primary measured quantities. Current transformer saturation may, however,
feign a phase displacement between the starpoint current and the sum of the phase currents which reduces
the restaint quantity. With ϕ(3I
0
"; 3I
0
') = 90° the restraint quantity is zero. This corresponds to the conventional
direction determination using the method of summation and and difference comparison.
Figure 2-31 Phasor diagram of the restraint quantity during external fault
The restraint quantity can be influenced by a factor k. This factor has a certain relationship to the limit angle
ϕ
limit
.
This limit angle determines for which phase displacement between 3I
0
" and 3I
0
' the pickup value for 3I
0
"=3I
0
'
grows to ∞, i.e. no pickup occurs. In 7SD610 k = 4.
In the aforementioned example 1), the restraint quantity I
Rest
is quadrupled once more; thus, it becomes 8 times
the tripping quantity I
REF
.
The threshold angle ϕ
Limit
= 100°. That means no trip is possible for phase displacement ϕ(3I
0
"; 3I
0
') ≥ 100°.
Figure 2-32 shows the operating characteristics of the restricted earth fault protection dependent of the phase
displacement between 3I
0
" and 3I
0
' for a constant infeed ratio |3I
0
"| = |3I
0
'|.