Functions
2.15 Earth Current Differential Protection (ANSI 87GN,TN)
SIPROTEC, 7UM62, Manual
C53000-G1176-C149-7, Release date 03.2010
130
Figure 2-49 Example of an external fault
When an external non-earthed fault causes heavy currents to flow through the protected zone, differences in
the magnetic characteristics of the phase current transformers under conditions of saturation may cause a sig-
nificant summation current which resembles 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 example, significant loads
with a high inductive component (and thus large time constants), such as motors or transformers, are switched
in.
For these reasons the earth current differential protection provides a number of restraining features which differ
significantly from conventional restraining methods (see margin heading "Restraining Measures").
Evaluation of Measured Values
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 (stabilizing) current.
I
0-Diff
= | 3I
01
+ 3I
02
|
I
0-Stab
= | 3I
01
3I
02
|
Depending on the application, 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 zero also. To eliminate the influence of CT errors, the restraint is determined
by the characteristic (see following figure).
In case of an external earth fault, the differential current is zero or minimal, and the restraint current is twice the
fault current. The measured quantities are within the restraint zone. An internal earth fault, on the other hand,
causes a fairly equal differential and restraint current. This is now in the trip zone (along the dashed line).
The pickup threshold is set with the I-REF> stage.
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