9171300990 Rev R
BE1-87T Functional Description
3-3
Restrained Trip Output
Restrained trip output contacts are subject to three types of restraint (i.e., inhibit) signals:
•
Percentage
restraint
•
Second-harmonic
restraint
•
Fifth-harmonic
restraint
These signals are developed within the relay in response to external conditions and block the restrained
output contacts from closing.
Percentage Restraint
Percentage restraint developed from the maximum through current and the slope setting determines the
minimum operating current
I
OP
(Figure 3-1) in a comparator where
I
OP
must be greater than %
I
MAX
to
produce a Restraint Trip output. The
I
OP
desired trip level is adjustable on the front panel
RESTRAINED
PICKUP LEVEL
switches shown in Figures 2-1 through 2-3.
Comparators in the
Calculate Max. Individual Current
circuit determine which input (of a particular phase)
is receiving the greatest current. That input is chosen and then called the
I
MAX
signal. The
I
MAX
output is
then scaled by the front panel
RESTRAINED PICKUP LEVEL
switches (shown as the
Restrained Pickup
Control
in Figure 3-1). The resulting signal (
%I
MAX
), that represents the percentage of through-current is
extended to the
Then Trip
comparator and the
I
OP
Divided By
%I
MAX
function.
The
Then Trip
circuitry compares the operating current (
I
OP
) to
%I
MAX
. If the operating current is greater
than
%I
MAX
(and there is no
5th
or
2nd
harmonic restraint to cause an inhibit as described below), a
Restrained Trip
output is produced.
I
OP
Divided By
%I
MAX
contains eight comparators and compares
I
OP
to the preset percentage levels of
%I
MAX
. If
I
OP
is greater than the preset percent of
%I
MAX
for a specific comparator, the LED associated with
that comparator lights. The eight LEDs represent the following approximate percentages:
First
LED:
3%
Second
LED:
7%
Third
LED:
11%
Fourth
LED:
20%
Fifth
LED:
40%
Sixth
LED:
60%
Seventh
LED:
80%
Eighth
LED:
100%
NOTE
As each successive LED lights, all previous or lesser percentage value LEDs will also
light.
Harmonic Restraints
Development of a restrained trip output may be inhibited by either of two harmonic restraints. These are
generated by bandpass filters tuned to the second and fifth harmonics of the operating current.
Comparators monitor these signals. When the fifth-harmonic content exceeds 35% of the operate current
(indicating overexcitation of the transformer) or when the second-harmonic content exceeds 12% (single-
phase) or 18% (three-phase) of the operate current (indicating a magnetic inrush condition), an inhibit
signal is developed that blocks operation of the
Restrained Trip
output contact. (Stated percentages
represent the factory setting.)
Unrestrained Trip Output
I
OP
is also compared against a reference established by the front panel
UNRESTRAINED PICKUP
LEVEL
switch is shown in Figures 2-1 through 2-5. When this reference is exceeded, the
Unrestrained
Trip
output relay is energized. An unrestrained trip is not affected by through-current or harmonic inhibits.
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