LBB stands for Local Breaker backup, and it is used as backup protection of the circuit breaker to isolate the fault in case the circuit breaker is failed to trip. It is also called Breaker failure relay (BFR) or 50BF. ANSI code of LBB protection is 50Z.
LBB protection plays important role in power system protection. The protection relay is responsible to detect the fault of a transmission line or other elements and isolate the part by opening the connected circuit breakers.
The Protection relay sends the trip command to the circuit breaker, it is observed that due to failure of the trip coil or any mechanical failure, the circuit breaker is failed to trip and isolate the fault. It is very much essential to isolate the fault immediately for the stable power system and failure of the connecting equipment.
To prevent the above situation, a Local Breaker Backup (LBB) or Breaker failure relay is used for all the circuit breakers to isolate the fault by opening the other connecting breakers.
How LBB protection works?
If any trip relay operates, it will send the trip command to the circuit breaker and also send the command as LBB initiation to the LBB relay to active. After getting the initiation from the trip relay, the LBB relay is waiting for isolation of the fault. After a certain duration, if the fault current is persisting, LBB relay will operate, and it will send the trip command to all the connecting Circuit breakers to isolate the fault.
Setting of LBB relay:
Following settings are required to set in the LBB relay.
- Current setting
- Time setting
Under current setting, we need to set the minimum current value for sensing the relay as fault current to operate. Generally, the current setting provided in the relay around 200A primary current.
Under time setting, we need to set a minimum waiting time after getting the initiation from the trip relay. Generally, the time setting provided in the relay is around 200 ms.
We know that the LBB will trip all the connecting circuit breakers to isolate the fault. Based on the substation scheme, the logic is prepared for tripping the circuit breakers for LBB operation. As shown in the picture, for one and half breaker scheme, following logic will be followed in case of LBB operation.
- LBB operation of Main CB
If the LBB operates in the Main circuit breaker of the one and a half breaker scheme, LBB will initiate to trip Tie CB and operate busbar protection of the connecting bus to trip all the connecting CBs with the bus.
- LBB operation of Tie CB
If the LBB operates in the Tie Circuit Breaker, it will initiate trip for both the connecting main circuit breaker to isolate the fault.