Functions
2.10 Restricted Earth Fault Protection (optional)
SIPROTEC, 7SD5, Manual
C53000-G1176-C169-5, Release date 02.2011
268
1. Through current on an external earth fault:
3
I
0
" is in phase opposition with 3
I
0
', and of equal magnitude, i.e. 3
I
0
" = –3
I
0
'
I
Ref
= |3
I
0
'|
I
Rest
= |3
I
0
' + 3
I
0
'| – |3
I
0
' – 3
I
0
'| = 2 · |3
I
0
'|
The tripping effect current (
I
Ref
) equals the starpoint current; restraint (
I
Rest
) corresponds to twice the trip-
ping effect current.
2. Internal earth fault, fed only from the starpoint
In this case 3
I
0
" = 0
I
Ref
= |3
I
0
'|
I
Rest
= |3
I
0
' – 0| – |3
I
0
' + 0| = 0
The tripping effect current (
I
Ref
) equals the starpoint current; restraint (
I
Rest
) is zero, i.e. full sensitivity during
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 3
I
0
" = 3
I
0
'
I
Ref
= |3
I
0
'|
I
Rest
= |3
I
0
' – 3
I
0
'| – |3
I
0
' + 3
I
0
'| = –2 · |3
I
0
'|
The tripping effect current (
I
Ref
) equals the starpoint current; the restraining quantity (
I
Rest
) 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, even small earth currents can cause tripping. In contrast, strong restraint becomes effective for
external earth faults. Figure 2-135 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 3
I
0
"/3
I
0
' negative).
With ideal transmission behaviour, the currents 3
I
0
" and 3
I
0
' would be inversely equal, i.e. 3
I
0
"/3
I
0
' = –1.
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 3
I
0
" is always higher than that of 3
I
0
'.
Figure 2-135
Tripping characteristic of the restricted earth fault protection depending on the earth current
ratio 3
I
0
”/3
I
0
' (both currents in phase + or counter-phase –);
I
REF
> = setting;
I
REF
= tripping
current
