2 Functions
82
7SD610 Manual
C53000-G1176-C145-4
3I
0
' = I
St
3I
0
" = I
L1
+ I
L2
+ I
L3
Only 3I
0
' acts as the tripping effect quantity. During a fault within the protected zone
this current is always present.
Figure 2-29 Principle of restricted earth fault protection
When an earth fault occurs outside the protected zone, another earth current flows
though the phase current transformers. This is, on the primary side, in counter-phase
with the starpoint current and has equal magnitude. The maximum information of the
currents is evaluated for restraint: the magnitude of the currents and their phase posi-
tion. The following is defined:
A tripping current
I
Trip
=|3I
0
'|
and a stabilisation or restraining current
I
stab
= k · ( |3I
0
'–3I
0
"| – |3I
0
'+3I
0
"| )
where k is a stabilisation factor which will be explained below, at first we assume k =
1. I
REF
produces the tripping effect quantity, I
Rest
counteracts this effect.
To clarify the situation, three important operating conditions with ideal and matched
measurement quantities are considered:
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