Breaker Failure Protection - Toshiba GRE110 Instruction Manual

6 F 2 T 0 1 7 2

2.8 Breaker Failure Protection

When fault clearance fails due to a breaker failure, the breaker failure protection (BFP) clears the
fault by backtripping adjacent circuit breakers.
If the current continues to flow even after a trip command is output, the BFP judges it as a breaker
failure. The existence of the current is detected by an overcurrent element provided for each phase.
For high-speed operation of the BFP, a high-speed reset overcurrent element (less than 20ms) is
used. The element resets when the current falls below 80% of the operating value.
In order to prevent the BFP from starting by accident during maintenance work and testing, and
thus tripping adjacent breakers, the BFP has the optional function of retripping the original breaker.
To make sure that the breaker has actually failed, a trip command is made to the original breaker
again before tripping the adjacent breakers to prevent unnecessary tripping of the adjacent breakers
following the erroneous start-up of the BFP. It is possible to choose not to use retripping at all, or
use retripping with trip command plus delayed pick-up timer, or retripping with trip command plus
overcurrent detection plus delayed pick-up timer.
An overcurrent element and delayed pick-up timer are provided for each phase which also operate
correctly during the breaker failure routine in the event of an evolving fault.
Scheme logic
The BFP is performed on per-phase basis. Figure 2.8.1 shows the scheme logic for the BFP. The
BFP is started by per-phase base trip signals EXT TRIP-A to -C or three-phase base trip signal
EXT TRIP3PH of the external line protection or an internal trip signal CBF INIT. These trip
signals must continuously exist as long as the fault is present.
The backtripping signal to the adjacent breakers CBF TRIP is output if the overcurrent element
CBF operates continuously for the setting time of the delayed pick-up timer TBTC after initiation.
Tripping of adjacent breakers can be blocked with the scheme switch [BTC].
There are two kinds of modes of the retrip signal to the original breaker CBF RETRIP, the mode in
which retrip is controlled by the overcurrent element CBF, and the direct trip mode in which retrip
is not controlled. The retrip mode together with the trip block can be selected with the scheme
switch [RTC]. In the scheme switch [RTC], "DIR" is the direct trip mode, and "OC" is the trip
mode controlled by the overcurrent element CBF.
Figure 2.8.2 shows a sequence diagram for the BFP when a retrip and backup trip are used. If the
circuit breaker trips normally, the CBF is reset before timer TRTC or TBTC is picked up and the
BFP is reset. As TRTC and TBTC start at the same time, the setting value of TBTC should include
that of TRTC.
If the CBF continues to operate, a retrip command is given to the original breaker after the setting
time of TRTC. Unless the breaker fails, the CBF is reset by retrip. TBTC does not time-out and the
BFP is reset. This sequence of events may happen if the BFP is initiated by mistake and
unnecessary tripping of the original breaker is unavoidable.
If the original breaker fails, retrip has no effect and the CBF continues operating and the TBTC
finally picks up. A trip command CBF TRIP is given to the adjacent breakers and the BFP is
completed.
 43 