Siemens SIPROTEC 7SD610 Manual page 85

1. Through current on an external earth fault:
3I " is in phase opposition with 3I
0
I = |3I
Ref 0
= |3I
I
Rest
The tripping effect current (I
ping effect current.
2. Internal earth fault, fed only from the starpoint
In this case 3I
I = |3I
Ref 0
I = |3I
Rest
The tripping effect current (I
internal earth fault.
3. Internal earth fault, fed from the starpoint and from the system, e.g. with equal earth current magnitude:
In this case 3I
I = |3I
Ref 0
I = |3I
Rest
The tripping effect current (I
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, even small earth currents can cause tripping. In contrast, strong restraint becomes effective for
external earth faults. Figure 2-30 shows that the higher the zero-sequence current transmitted by the phase
current transformers, the stronger results the restraint in case of external earth faults (area 3I
With ideal transmission behaviour, the currents 3I
If the starpoint current transformer is designed weaker than the phase current transformers (e.g. by selection
of a smaller accuracy limit factor or by higher secondary burden), no trip will be possible under through-fault
condition even in case of severe saturation as the magnitude of 3I
Figure 2-30
SIPROTEC, 7SD610, Manual
C53000-G1176-C145-6, Release date 02.2011
', and of equal magnitude, i.e. 3I
0
'|
' + 3I '| – |3I ' – 3I '| = 2 · |3I
0 0 0 0
) equals the starpoint current; restraint (I
Ref
" = 0
0
'|
' – 0| – |3I ' + 0| = 0
0 0
) equals the starpoint current; restraint (I
Ref
" = 3I '
0 0
'|
' – 3I '| – |3I ' + 3I '| = –2 · |3I
0 0 0 0
) equals the starpoint current; the restraining quantity (I
Ref
Tripping characteristic of the restricted earth fault protection depending on the earth current
ratio 3I "/3I ' (both currents in phase + or counter-phase –); I
0 0
current
2.5 Restricted Earth Fault Protection (optional)
'|
0
'|
0
" and 3I ' would be inversely equal, i.e. 3I
0 0
" is always higher than that of 3I
0
" = –3I '
0 0
) corresponds to twice the trip-
Rest
) is zero, i.e. full sensitivity during
Rest
) is negative and,
Rest
"/3I
0
"/3I
0 0
> = setting; I = tripping
REF REF
Functions
' negative).
0
' = –1.
'.
0
85