Date Code 20020129
Differential, Restricted Earth Fault,
3-5
Overcurrent, Voltage, and Frequency Elements
SEL-387E Instruction Manual
condition then is satisfied, and Relay Word bit 87Rn asserts, indicating operation of the restrained
differential element, n. This does not, as yet, produce a trip. The relay still needs the results of
the harmonic and dc blocking decision logic, which is described later.
I,57Q
,23Q
83
VHWWLQJ
,57Q
23
VHWWLQJ
8Q
5Q
2Q
Q RU
B
B
B
5HOD\
:RUG
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':*0
8
8
8
8
S
F
,+57Q
+5675 <
Figure 3.4: Differential Element Decision Logic
Harmonic Restraint
Consider the harmonic restraint feature (HRSTR=Y) if your practices require independent
harmonic restraint. This feature disables common harmonic blocking (IHBL=Y). It also disables
second- and fourth-harmonic blocking since it adds the second- and fourth-harmonic quantities to
the differential characteristic restraint quantity. Blocking features are discussed in more detail
later in this section.
For harmonic blocking, the harmonic content of the differential current must exceed the
individual (PCT2 or PCT4) threshold values, i.e., the thresholds are independent measurements of
each harmonic value. For harmonic restraint, the values of the second- and fourth-harmonic
currents are summed, and that value is used in the relay characteristic. Consider, for example, the
simple case of Slope 1, i.e., a straight line through the origin. The general equation for a line is:
F
[
P
\
+
=
More specifically, in the SEL-387E Relay:
F
,57
6/3
,23
+
=
Because the line starts at the origin, the value of c is normally zero. The sum of the second- and
fourth-harmonic currents now forms the constant c in the equation, raising the relay characteristic
proportionally to the harmonic values.
Blocking
While the restrained differential elements are making decisions, a parallel blocking decision
process occurs regarding the magnitudes of specific harmonics in the IOP quantities.
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