5-14
Applications
Date Code 20010515
SEL-2BFR, -2/BFR Instruction Manual
operate on the retrip signal. If the RTTD time delay is too long and the initial trip does not
operate the breaker, the breaker may not have time to fully open on the retrip signal before the
SEL-2BFR Relay operates the bus lockout relay. Time delays between two and six cycles are
typical.
In our example application, the time-delayed retrip feature is not used. The DRTE setting is N.
With this setting, the relay sets the Relay Word trip bits without time delay or overcurrent
supervision, allowing instantaneous retrip to be used, if desired.
Select Trip and Close Resistor Thermal Element Settings
If the protected breaker is equipped with trip and close resistors and three-phase potentials are
available on both sides of the breaker, you can use the relay thermal protective elements to
protect breaker resistors. Occasionally, a trip or close resistor can be left in service following a
breaker operation. The relay can detect that condition, model the energy accumulated in the
resistor, and trip the protected breaker or 86 lockout relay when resistor energy reaches a preset
level.
Operation of the thermal protection logic is described in
Section 2: Specifications
. Setting
selection is described below.
37OP Setting
Select a 37OP element setting such that the element asserts when the minimum line current is
flowing through the minimum breaker resistance possible while resistors are in circuit. The 37OP
element setting is based on single-phase power dissipated in any breaker pole.
The minimum breaker resistance with resistors in circuit is found when the trip and close resistors
are inserted in parallel.
For the example breaker:
primary
Ω
53
primary
Ω
70
220
70
•
220
R
70
Ω
35
•
2
R
220
Ω
110
•
2
R
MIN
CLOSE
TRIP
=
+
=
Ω
=
=
Ω
=
=
Line-charging current is 350 A primary, thus the minimum resistor in circuit voltage is:
secondary
V
4.33
4300
Ω
53
•
350A
V
MIN
=
=
Line-charging current times minimum resistor in circuit voltage across the breaker yields the
minimum power for which 37OP should be required to assert.
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