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Functions
Evaluation of
Measured
Quantities
100
When an external non-earthed fault causes a heavy current to flow through the pro-
tected zone, differences in the magnetic characteristics of the phase current trans-
formers under conditions of saturation may cause a significant summation current
which may simulate an earth current flowing into the protected zone. Measures must
be taken to prevent this current from causing a trip. The same may happen if, for ex-
ample, significant loads with a high inductive component (and thus high time con-
stants), such as motors or transformers, are switched on.
The earth current differential protection provides a number of restraint features which
differ significantly from conventional restraint methods (see margin heading „Restrain-
ing Measures").
The earth current differential protection compares the fundamental wave of the zero
currents on both sides (3I
restraint current.
I
= | 3I
+ 3I
|
0-Diff
01
02
I
= | 3I
| + | 3I
|
0-Res
01
02
Depending on the application, the current 3I
current of side 2 or the directly measured starpoint current I
Under no-fault conditions, and with ideal CTs, the zero sequence currents would be
zero, and consequently the differential and the restraint current would be zero as well.
To eliminate the influence of CT errors, the restraint is determined by the characteristic
(see Figure 2-48).
In case of an external earth fault, the differential current is zero or small, and the re-
straint current is twice the fault current. The measured quantities are inside the re-
straint zone. An internal earth fault, on the other hand, causes a fairly equal differential
and restraint current, which are both in the trip zone (along the dashed line).
The pickup threshold is set with the I-REF> stage.
I
0Diff
with
[I/InO]
1
I-EDP>
Figure 2-48 Tripping and Restraint Characteristic
and 3I
) and calculates from them the differential and the
01
02
may be the calculated zero sequence
02
1
.
St
Tripping zone
Restraint zone
Zone not considered
I
0Res
[I/InO]
with
7UM62 Manual
C53000-G1176-C149-3
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