Siemens SIPROTEC 4 7UT6 Series Manual page 130

Functions
2.3 Restricted Earth Fault Protection
a tripping effect current
= |3Ι '|
Ι
Trip 0
and the stabilisation or restraining current
= k · ( |3Ι ' – 3Ι
Ι
stab 0
k is a stabilisation factor which will be explained below, at first we assume k = 1. Ι
effect quantity, Ι
To clarify the situation, three important operating conditions with ideal and matched measurement quantities
are considered:
•
Through-fault current on an external earth fault:
3Ι " is in phase opposition with 3Ι
0
= |3Ι '|
Ι
Trip 0
= |3Ι ' + 3Ι
Ι
stab 0
The tripping effect current (Ι
size.
•
Internal earth fault, fed only from the starpoint
In this case 3Ι
= |3Ι '|
Ι
Trip 0
= |3Ι ' – 0| – |3Ι
Ι
stab 0
The tripping effect current (Ι
full sensitivity during internal earth fault.
•
Internal earth fault, fed from the starpoint and from the system, e.g. with equal earth current magnitude:
In this case 3Ι
= |3Ι '|
Ι
Trip 0
= |3Ι ' – 3Ι
Ι
stab 0
The tripping effect (Ι
fore 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 contrast, strong restraint becomes effective for
external earth faults.
phase current transformers is high (area with negative 3Ι
opposite and equal, i.e. 3Ι
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 3Ι
3Ι '.
0
130
"| – |3Ι ' + 3Ι "| )
0 0 0
counteracts this effect.
stab
', and of equal magnitude, i.e 3Ι
0
'| – |3Ι ' – 3Ι '| = 2 · |3Ι
0 0 0 0
) equals the starpoint current; the restraining quantity (Ι
Trip
" = 0
0
' + 0| = 0
0
) equals the starpoint current, the restraining quantity (Ι
Trip
" = 3Ι '
0 0
'| – |3Ι ' + 3Ι '| = –2 · |3Ι
0 0 0
) equals the starpoint current; the restraining quantity (Ι
Trip
Figure 2-62 shows that the restraint is the strongest when the residual current from the
"/3Ι ' = –1.
0 0
'|
'|
0
"/3Ι '). With ideal current transformers, 3Ι
0 0
" (negative) is always higher than that of
0
produces the tripping
Trip
" = –3Ι '
0 0
) is double the
stab
) is zero, i.e.
stab
) is negative and there-
stab
SIPROTEC 4, 7UT6x, Manual
C53000-G1176-C230-5, Edition 09.2016
" and 3Ι '
0 0