Intruduction: Breaker Failure Protection (BFP) is also known as BFR (Breaker Failure Relay) or LBB (Local Breaker Backup) protection.
The idea behind BFP is that in case of any protection operation the Circuit Breaker (CB) should interrupt the fault current within specified time (Normally < 60ms, depending on voltage level). There may be cases where CB is unable to interrupt the current doe to any of the following causes:
In the first two cases, the interrupter is not at all operated. Therefore there is no risk of prolonged arc inside interrupter, Only risk is continuous feeding of fault current may lead to disturbance in grid operation.
In the third case, due to prolonged arcing inside interrupter, temperature may go high and it may explode due to high pressure buid-up. Normally porcelain enclosures are used for SF6 filled interrupters. Failure may lead to cracking of porceline into pieces, which are dangerous for men working the vicinity of CB. Further SF6 gas will be released to atmosphere, which is not good for environment. In addition, risk of disturbance in grid operation is already there.
Working Principle: The BFP protection is activated when tripping command is issued to CB. If after a pre-set time (typical 200ms), current is still flowing through CB, BFP will operate. Operation of BFP will isolate the concerned CB from all sides by tripping all the CBs connecting to it. For example in the image shown below, following will be the isolation logic:
There are two methods used for operation of BFP:
The idea behind BFP is that in case of any protection operation the Circuit Breaker (CB) should interrupt the fault current within specified time (Normally < 60ms, depending on voltage level). There may be cases where CB is unable to interrupt the current doe to any of the following causes:
- CB tripping circuit problems
- CB operating mechanism problems
- CB interrupter problems
In the first two cases, the interrupter is not at all operated. Therefore there is no risk of prolonged arc inside interrupter, Only risk is continuous feeding of fault current may lead to disturbance in grid operation.
In the third case, due to prolonged arcing inside interrupter, temperature may go high and it may explode due to high pressure buid-up. Normally porcelain enclosures are used for SF6 filled interrupters. Failure may lead to cracking of porceline into pieces, which are dangerous for men working the vicinity of CB. Further SF6 gas will be released to atmosphere, which is not good for environment. In addition, risk of disturbance in grid operation is already there.
Working Principle: The BFP protection is activated when tripping command is issued to CB. If after a pre-set time (typical 200ms), current is still flowing through CB, BFP will operate. Operation of BFP will isolate the concerned CB from all sides by tripping all the CBs connecting to it. For example in the image shown below, following will be the isolation logic:
| BFP operated for | CB to be opened |
| CB-1 | CB-2, CB-4, CB-11 |
| CB-2 | CB-1, CB-3, CB-11, CB-21 |
| CB-3 | CB-2, CB-6, CB-21 |
- Current level detection: If current through CT is above set value (typical value 100A) after pre-set time, relay interprets that CB has failed to interrupt the current.
- CB status from auxiliary contacts: If CB status as per auxiliary contacts (52a) is still closed, relay interprets that CB has failed to open.
First method is more reliable and works for all the three cases mentioned in introduction, therefore used in most of the cases. Only drawback is there must be some significant amount of current flowing for operation of BFP.
Second method does not work for third case (CB interrupter problems). It is used where lower level of current, which is not significant of operation of relay, is also dangerous for health of equipment / person working in the vicinity,
The working principle can be understood from image below:
The working principle can be understood from image below:
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