Siemens SIPROTEC 7UT613 series Manual page 144

2 Functions
144
1. Through-fault current on an external earth fault:
3I " is in phase opposition with 3I
0
I
= |3I '|
from 0
I
= |3I ' + 3I '| – |3I
stab 0 0
The tripping effect current (I
tity (I ) is double the size.
stab
2. Internal earth fault, fed only from the starpoint
In this case 3I " = 0
0
I
= |3I '|
from 0
I
= |3I ' – 0| – |3I
stab 0
The tripping effect current (I
tity (I ) is zero, i.e. full sensitivity during internal earth fault.
stab
3. Internal earth fault, fed from the starpoint and from the system, e.g. with equal
earth current magnitude:
In this case 3I " = 3I
0
I
= |3I '|
from 0
I
= |3I ' – 3I '| – |3I
stab 0 0
The tripping effect (I
is negative and therefore set to zero, i.e. full sensitivity during internal earth fault.
This result shows that for an internal fault no restraint is effective since the restraining
quantity is either zero or negative. Thus, small earth current can cause tripping. In con-
trast, strong restraint becomes effective for external earth faults. Figure 2-60 shows
that the restraint is the strongest when the residual current from the phase current
transformers is high (area with negative 3I
and 3I ' opposite and equal, i.e. 3I
0
If the starpoint current transformer is designed weaker than the phase current trans-
formers (e.g. by selection of a smaller accuracy limit factor or by higher secondary bur-
den), no trip will be possible under through-fault condition even in case of severe sat-
uration as the magnitude of 3I
Figure 2-60 Tripping characteristic of the restricted earth fault protection depending on the
earth current ratio 3I
I > = setting; I
REF
' and of equal magnitude, i.e 3I
0
' – 3I '| = 2 · |3I '|
0 0 0
) equals the starpoint current; the restraining quan-
from
' + 0| = 0
0
) equals the starpoint current, the restraining quan-
from
'
0
' + 3I '| = –2 · |3I '|
0 0 0
) equals the starpoint current; the restraining quantity (I
REF
"/3I
0 0
"/3I = –1.
0 '0
" (negative) is always higher than that of 3I
0
"/3I ' (both currents in phase + or counter-phase –);
0 0
= tripping current
from
'). With ideal current transformers, 3I
7UT613/63x Manual
C53000-G1176-C160-2
" = –3I '
0 0
)
stab
"
0
'.
0