Date Code 20010515
Specifications
2-19
SEL-2BFR, -2/BFR Instruction Manual
Thermal Protection of Close and Trip Resistors
The logic shown in Figure 2.9 is designed to protect the trip and close resistors in the A-phase of
the circuit breaker. B- and C-phase logic is similar. The relay calculates the energy dissipated in
each of the six resistors using independent thermal models.
The relay determines which model to heat based on the trip and close inputs. When a trip input is
asserted, the relay closes switch T and opens switch C. When the close input is asserted, the relay
closes switch C and opens switch T.
The 47Q negative-sequence overvoltage element supervises heating of the close resistor thermal
model. If the breaker is closed and the 47Q element picks up, the relay inhibits heating of the
close resistor thermal model. Pickup of the 47Q element indicates a blown potential fuse.
The thermal models and 37OP overpower element use current through the breaker and voltage
across it. When power dissipated in the breaker exceeds the 37OP setting value, the 62OP pickup
timer starts. If 37OP remains asserted for 62OP time, the H1 and H2 switches close. The close
or trip resistor thermal model (CRTM or TRTM) heats, depending on the state of the switches T
and C. If the energy accumulated by the thermal model reaches the 26CP (or 26TP) level before
the 37OP element drops out, the CRP_ (or TRP_) (close or trip resistor pending failure) bit
asserts in the Relay Word. If 37OP remains asserted until the resistor energy reaches the 26CF
(or 26TF) value, the CRF_ or TRF_ (close or trip resistor failure) bit asserts in the Relay Word.
The Relay Word bits CTF (Close resistor Thermal Failure) and TTF (Trip resistor Thermal
Failure) assert when any Close or Trip resistor thermal model has reached the failure energy level
and at least 0.1 amp secondary of current is flowing in the hot resistor phase. If you set the CTF
and TTF bits in the M86T logic mask, the relay asserts the 86BF TRIP outputs when a resistor
thermal failure occurs.
The relay models cooling of the breaker resistors using a settable time constant. The thermal
failure and pending failure elements do not drop out until the resistor thermal models have cooled
below the element thresholds. The cooling function helps prevent hot resistors from being put
back in service.
Thermal model cooling is performed at discrete time intervals. Tests reflect this interval as a
uniformly distributed measurement error with bounds of ±T minutes where RTC equals the
resistor cooling time constant and:
T = RTC (minutes) / 383
No initiating input is required with this protection scheme. If the protected breaker is not
equipped with trip or close resistors, do not enable this function for 86BF tripping. Three-phase
potential inputs from both sides of the protected breaker are required when you use the thermal
protection logic, see
Section 5: Applications
.
Use the
HEAT
command to track or reset thermal model resistor energy.
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