Basler Electric BE1-87T - Manual (Page 37)

9171300990 Rev V 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 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.
Percentage Restraint
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.
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.)
Harmonic Restraints
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.