Differential Protection (ANSI 87G/87M/87T)
77
7UM62 Manual
C53000-G1176-C149-3
This result shows that for internal fault and under ideal conditions I
diff
= I
stab
.
Consequently, the characteristic of internal faults is a straight line with a upward slope
of 45° (dot-and-dash line in Figure 2-30).
The currents I
diff
and I
stab
are compared by the differential protection with the
operating characteristic according to Figure 2-30. If the quantities result into a locus in
the tripping area, a trip signal is given.
Figure 2-30
Operation Characteristics of the Differential Protection with Fault Characteristic
Matching the
Values of Measured
Quantities
The rated CT currents are matched to the rated current of the protected object,
regardless of what that object is. As a result, all currents are referred to the protected
object. To match the currents, the characteristic values of the protected object
(apparent power, rated voltage) and the rated primary currents of the CTs are entered
in the protective device for each side of the protected object.
2.12.1.1 Protected Object Generator or Motor: Particularities
Definition and
Matching of
Measured
Quantities
The differential protection function of the 7UM62 can be used as longitudinal or as
transverse differential protection. The operation modes differ from each other only by
the definition of the measured current and the limits of the protected zone.
Since the current direction is normally defined as positive in the direction of the
protected object, the definitions as illustrated in Figure 2-31 result. The protected zone
is limited by the CTs in the neutral point of generator and the CTs at the terminal side.
The differential protection feature of the 7UM62 refers all currents to the rated current
of the protected object. The characteristic values of the protected object (apparent
power, rated voltage) and the primary rated currents of the CTs are entered in the
protective device for each side of the protected object. The measured quantity
matching is thus limited to the factors for the absolute current values.
1
2
3
4
5
6
7
8
9
10 11 12
13 14 15 16 17 18
1
2
3
4
5
6
7
8
9
10
2031
I DIFF >>
2021
I DIFF>
Trip area
I
diff
I
N
---------
I
stab
I
N
------------
Fault characteristic
Block area
