2.5 Restricted Earth Fault Protection (optional)
85
7SD610 Manual
C53000-G1176-C145-4
This limit angle determines for which phase displacement between 3I
0
" and 3I
0
' the
pickup value for 3I
0
"=3I
0
' grows to ∞, i.e. no pickup occurs. In 7SD610 k = 4.
The restraint quantity I
Rest
in the above example a) is quadrupled once more; it
becomes thus 8 times the tripping effect quantity I
Ref
.
The threshold angle ϕ
Limit
=100°. That means no trip is possible for phase displace-
ment ϕ(3I
0
"; 3I
0
') ≥ 100°.
Figure 2-32 shows the operating characteristics of the restricted earth fault protection
dependent of the phase displacement between 3I
0
" and 3I
0
' for a constant infeed ratio
|3I
0
"| = |3I
0
'|.
Figure 2-32 Tripping characteristic of the restricted earth fault protection depending on the
phase displacement between 3I
0
” and 3I
0
' at 3I
0
” = 3I
0
' (180
o
= external fault)
It is also possible to increase the tripping value in the tripping area proportional to the
arithmetic sum of all currents, In this case the pickup value with the arithmetic sum of
all currents is stabilised, i.e. with„IrestREF=“ Σ | I |=|I
L1
|+|I
L2
|+|I
L3
|+|I
4
|
(Figure 2-33). The slope of this restraint characteristic can be set.
Pickup Normally, a differential protection does not need a pickup or „fault detection“function,
since the condition for a fault detection is identical to the trip condition. But the restrict-
ed earth fault protection provides like all protection functions a fault detection signal
which forms an additional precondition for tripping and defines the fault inception
instant for a number of further activities.
As soon as the fundamental wave of the differential current exceeds 85 % of the
pickup value, fault detection is indicated. In this aspect, the differential current is rep-
resented by the sum of all in-flowing currents.
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