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 CBF provided for each phase. For high-speed operation of the BFP, a high-speed reset overcurrent element (less than 20ms) is used. The CBF element resets when the current falls below 80% of the operating value as shown in Figure 2.1.44.
Figure 2.1.44 CBF element Characteristic
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 initiation is performed on per-phase basis. Figure 2.1.45 shows the scheme logic for the BFP. The BFP is started by per-phase base trip signals EXT TRIP-APH to EXT TRIP-CPH or three-phase base trip signal EXT TRIP-3PH by binary inputs. 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 2.1.46 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
0 I
Pick-up Drop-off
Drop-off/Pick-up=0.8
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.
The BFP protection can be disabled by the scheme switches [BTC] and [RTC] or the binary input signal CBF BLOCK.
Figure 2.1.45 Breaker Failure Protection Scheme Logic
+ "ON"
Figure 2.1.46 Sequence Diagram
Setting
The setting elements necessary for the breaker failure protection and their setting ranges are as follows:
Element Range Step Default Remarks
CBF 0.5 – 10.0 A 0.1 A 2.5 A Overcurrent setting (0.10 - 2.00 A)(*) (0.01 A) (0.50 A)
TRTC 0.00 – 300.00 s 0.01 s 0.40 s Retrip time setting TBTC 0.00 – 300.00 s 0.01 s 0.50 s Back trip time setting [RTC] Off / DIR / OC Off Retrip control
[BTC] Off / On Off Back trip control
(*) Current values shown in the parentheses are in the case of 1 A rating. Other current values are in the case of 5 A rating.
The overcurrent element CBF checks that the circuit breaker has opened and that the current has disappeared. Therefore, since it is allowed to respond to load current, it can be set to 10 to 200% of the rated current.
The settings of TRTC and TBTC are determined by the opening time of the original circuit breaker (Tcb in Figure 2.1.46) and the reset time of the overcurrent element (Toc in Figure 2.1.46). The timer setting example when using retrip can be obtained as follows.
Setting of TRTC = Breaker opening time + CBF reset time + Margin = 40ms + 10ms + 20ms
= 70ms
Setting of TBTC = TCBF1 + Output relay operating time + Breaker opening time + CBF reset time + Margin
= 70ms + 10ms + 40ms + 10ms + 10ms = 140ms
If retrip is not used, the setting of the TBTC can be the same as the setting of the TRTC.
Fault
CBF TRIP TBTC CBF RETRIP TRTC OCBF Original breakers Adjacent breakers
TRIP
Retrip
Toc Toc
Tcb Tcb
TBF1
TBF2 Normal trip
Open Closed
Start CBFP
Open Closed Open
Trip