Functions
100
7UM62 Manual
C53000-G1176-C149-3
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“).
Evaluation of
Measured
Quantities
The earth current differential protection compares the fundamental wave of the zero
currents on both sides (3I
01
and 3I
02
) and calculates from them the differential and the
restraint current.
I
0-Diff
= | 3I
01
+ 3I
02
|
I
0-Res
= | 3I
01
| + | 3I
02
|
Depending on the application, the current 3I
02
may be the calculated zero sequence
current of side 2 or the directly measured starpoint current I
St
.
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
stage.
Figure 2-48 Tripping and Restraint Characteristic
Tripping zone
Zone not considered
Restraint zone
1
1
I-EDP>
[I/InO]
I
0Res
with
[I/InO]
I
0Diff
with