3-24
Breaker Logic
Date Code 20010731
SEL-352-1, -2 Instruction Manual
T
HERMAL
P
ROTECTION OF
C
LOSE AND
T
RIP
R
ESISTORS
Application Description
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 model calculates the energy accumulated in the resistor and trips
the protected breaker or 86 lockout relay when resistor energy reaches a preset level. See
Figure 3.19
for a logic diagram of the thermal logic.
The output of the thermal protection is the assertion of the TTF (trip thermal failure) or CTF
(close thermal failure) Relay Word bit. These bits must be used in control logic for energizing
an external alarm retripping the breaker or asserting a lockout relay. Refer to
Lockout Relay
Control
in
Section 4: Close Logic
. Other useful Relay Word bits are 26TFA, 26TPA, 26CFA,
and 26CPA, which are the energy accumulation comparisons to the trip and close failure and
pending failure settings.
Operating Characteristic
The logic shown in Figure 3.19 is designed to protect the trip and close resistors in A-phase of
the circuit breaker. Logic for B-phase and C-phase is similar. The relay uses independent
thermal models to calculate the energy dissipated in each of the six sets of resistors.
Use the
HEAT
command to track or reset thermal model resistor energy.
The logic uses one input,
D
VA, calculated from other relay logic. The input
D
VA is the voltage
difference across the breaker after steady-state voltage differences have been removed. See
Figure 3.20 for the voltage nulling logic diagram.
Figure 3.19: Trip and Close Resistor Thermal Protection Logic
Содержание SEL-352-1
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