Siemens SIPROTEC 7SD610 Manual page 84

Functions
2.5 Restricted Earth Fault Protection (optional)
Evaluation of Measurement Quantities
The earth fault differential protection compares the fundamental component of the current flowing in the star-
point connection, which is designated as 3I
the phase currents designated in the following as 3I
3I ' = I
0 SP
3I " = I + I
0 L1 L2
Only 3I ' acts as the tripping effect quantity. During a fault within the protected zone this current is always
0
present.
Figure 2-29
When an earth fault occurs outside the protected zone, a zero sequence current also flows though the phase
current transformers. This is, on the primary side, in counter-phase with the starpoint current and has equal
magnitude. Therefore, both the magnitude of the currents and their phase relationship are evaluated for re-
straint purposes. The following is defined:
A tripping current
I = |3I '|
Ref 0
and a stabilisation or restraining current
I = k · ( |3I
Rest
where k is a stabilisation factor which will be explained below, at first we assume k = 1. I
ping effect quantity, I
To illustrate the effect, have a look at the three important operating conditions with ideal and adapted measured
values:
84
+ I
L3
Principle of restricted earth fault protection
' – 3I "| – |3I ' + 3I "| )
0 0 0 0
counteracts this effect.
Rest
' in the following, with the fundamental component of the sum of
0
". Thus, the following applies (Figure 2-29):
0
C53000-G1176-C145-6, Release date 02.2011
produces the trip-
REF
SIPROTEC, 7SD610, Manual