2.3 Restricted Earth Fault Protection
143
7UT613/63x Manual
C53000-G1176-C160-2
Figure 2-59 Principle of Restricted Earth Fault Protection
For auto-transformers 3I
0
" is valid as the sum of all phase currents flowing to auto-con-
nected winding (full winding and tap(s)).
When an earth fault occurs outside the protected zone, another earth currents flows
through 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 effect current
I
from
=|3I
0
'|
and the stabilisation or restraining current
I
stab
= k · ( |3I
0
'–3I
0
"| – |3I
0
'+3I
0
"| )
k is a stabilisation factor which will be explained below, at first we assume k = 1. I
from
produces the tripping effect quantity, I
stab
counteracts this effect.
To clarify the situation, three important operating conditions with ideal and matched
measurement quantities are considered:
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