3.6
SEL-387-0, -5, -6 Relay
Instruction Manual
Date Code 20170601
Differential, Restricted Earth Fault, Thermal, and Overcurrent Elements
Differential Element
Because the line starts at the origin, the value of c normally is zero. When
harmonic restraint is used, the sum of the scaled second- and fourth-harmonic
currents from the enabled windings of the relay now forms the constant c in
the equation, raising the relay restraint slope characteristic proportionally to
the harmonic values.
For this reason, it is important not to set the PCT2 and PCT4 settings to OFF
when using the harmonic restraint function (HRSTR = Y). Doing so will
effectively remove all harmonic restraint from the relay.
Increasing the values used for PCT2 and PCT4 settings decreases the
respective harmonic content that is added to the IOP characteristic of the relay.
This is a result of the inverse relationship created by the 100/PCT2 and
100/PCT4 multipliers on the respective harmonic content. Use the default
values for these settings to provide secure differential element operation for
inrush conditions.
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.
Common (Cross) or Independent Blocking
Use common or independent blocking elements (87BL1, 87BL2, and 87BL3)
to supervise the restrained differential elements. Common blocking disables
all restrained elements if any blocking element is picked up.
shows
how independent blocking disables the restrained element associated with the
blocking element.
If IHBL is set to N (No), the logic shown in
in the top drawing,
IHBL = N, is enabled. In this case all 87R
n
elements enter one OR gate, and
all 87BL
n
elements enter another OR gate, whose output is negated at the
upper AND gate. If the 87R
n
OR output asserts but the 87BL
n
OR output does
not, the 87R Relay Word bit asserts and tripping can take place. In other
words, with IHBL = N, blocking within ANY differential element will prevent
operation and tripping of ALL the restrained differential elements.
Summary of Contents for SEL-387-0
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