Siemens SIPROTEC 4 7UT6 Series User Manual

a tripping effect current
Ι
Trip
= |3Ι
0
'|
and the stabilisation or restraining current
Ι
stab
= k · ( |3
Ι
0
' – 3Ι
0
"| – |3Ι
0
' + 3Ι
0
"| )
k is a stabilisation factor which will be explained below, at first we assume k = 1. Ι
Trip
produces the tripping
effect quantity, Ι
stab
counteracts this effect.
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
0
" is in phase opposition with 3Ι
0
', and of equal magnitude, i.e 3Ι
0
" = –3Ι
0
'
Ι
Trip
= |3Ι
0
'|
Ι
stab
= |3
Ι
0
' + 3Ι
0
'| – |3Ι
0
' – 3Ι
0
'| = 2 · |3Ι
0
'|
The tripping effect current (Ι
Trip
) equals the starpoint current; the restraining quantity (Ι
stab
) is double the
size.
•
Internal earth fault, fed only from the starpoint
In this case 3Ι
0
" = 0
Ι
Trip
= |3
Ι
0
'|
Ι
stab
= |3Ι
0
' – 0| – |3Ι
0
' + 0| = 0
The tripping effect current (Ι
Trip
) equals the starpoint current, the restraining quantity (Ι
stab
) is zero, i.e.
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 3
Ι
0
" = 3Ι
0
'
Ι
Trip
= |3Ι
0
'|
Ι
stab
= |3
Ι
0
' – 3Ι
0
'| – |3Ι
0
' + 3Ι
0
'| = –2 · |3Ι
0
'|
The tripping effect (Ι
Trip
) equals the starpoint current; the restraining quantity (Ι
stab
) is negative and there-
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. Figure 2-62 shows that the restraint is the strongest when the residual current from the
phase current transformers is high (area with negative 3Ι
0
"/3Ι
0
'). With ideal current transformers, 3Ι
0
" and 3Ι
0
'
opposite and equal, i.e. 3Ι
0
"/3Ι
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 3Ι
0
" (negative) is always higher than that of
3Ι
0
'.
Functions
2.3 Restricted Earth Fault Protection
130 SIPROTEC 4, 7UT6x, Manual
C53000-G1176-C230-5, Edition 09.2016

Siemens SIPROTEC 4 7UT6 Series Specifications

General

Brand	Siemens
Model	SIPROTEC 4 7UT6 Series
Category	Protection Device
Language	English