What is a Local Breaker Backup (LBB) Protection?
As the name suggests Local Breaker Backup (LBB) protection is used as backup protection in case of failure of circuit breaker , it is also called breaker failure relay. Normally when a fault occurs ,different protective relays sense the fault and send tripping command to master trip relay (Relay-86) and then 86 relay operates which sends command (+Ve supply) to breaker and then breaker trips thus isolating the faulty part. Basically a circuit breaker trips :
- Once the tripping mechanism is activated, it causes the contacts inside the circuit breaker to open.
- Opening the contacts interrupts the flow of electrical current in the circuit.
If due to some reason 86 relay has operated and still the breaker fails to trip and the fault current is persisting then Local Breaker Backup(LBBs) come into action. There may be a case when the tripping coil of breaker is defective or there may be some mechanical problem in the circuit breaker and due to these reasons the breaker will not trip despite the existing fault. Here comes the role of LBB relay.
How Local Breaker Backup (LBB relay) operates?
Lets understand this in these easy steps:
1.As soon as any fault is detected by the other protective relays ,the timer of LBB relay also initiates and it waits for a certain time for clearing the fault and waits for opening of the circuit breaker and if for some reason the breaker doesn’t open even after passage of set time limit in the LBB relay which is usually 200 ms then LBB relay sends tripping command to all other circuit breaker in the substation to isolate the fault current so that the fault current doesn’t flow ahead.
2.Thus a LBB protection scheme consists of two relays: One which has built-in timer (50LBB/2T) in which time setting is done and other is just like master trip relay (Relay -86) ,and nomenclatured as Relay 96 which sends trip command to trip coils of breakers .
3.LBB wiring scheme in relay panel is done in such a way that if any breaker (At one voltage level-220kv,132kv or 33kv) in a substation doesn’t trip on fault, then all other breakers are tripped by LBB protection.Also coordination between relays is crucial in power systems to ensure that the circuit breaker nearest to the fault operates first while minimizing the impact on the rest of the system.
The time delays and settings of the local breaker backup relays are configured to ensure that the backup protection operates only when the primary protection fails, preventing unnecessary tripping during transient conditions.
Caution during testing and commissioning: During testing and commissioning of new panels ,C.Ts, C.Bs we often create a fault using current injection kit , also single phase current can be injected by using a 200/100w test lamps if we don’t have the kit , and if for some reason we keep injecting the fault current even after the breaker has tripped or the duration of injection of fault current is more , then the LBB relay will sense it as breaker failure fault and it will initiate and all circuit breakers will trip which may lead to major power failure.
So, it is advisable that while testing we should disable the LBB relay and then conduct the testing and commissioning related works.Proper coordination between primary protection and the LBB scheme is critical to prevent unnecessary tripping and ensure the rapid and selective operation of protective devices.
It’s worth noting that modern power systems often employ redundancy and diverse protection schemes to ensure that faults are detected and cleared promptly, and LBB is an important part of this protection strategy.
You can watch this youtube video for more clarity :
FAQs:
- When is LBB Protection activated?
LBB Protection is activated when the breaker fails to trip in case of a fault or abnormal condition. It provides a backup mechanism to ensure that the fault is cleared, preventing potential damage or system instability.LBB is designed to protect against various faults, including short circuits, overcurrent conditions, and other abnormal events that may occur in the power system. It acts as a secondary defense mechanism to ensure a swift response to faults. - What are the advantages of using LBB Protection?
- Redundancy: LBB Protection acts as a secondary layer of defense, ensuring system reliability.
- Equipment Protection: It prevents damage to expensive electrical equipment.
- System Stability: LBB Protection helps maintain the stability of the power system.
- Are there any disadvantages to LBB Protection?
One potential drawback is that LBB Protection relies on preset time and current settings, which may not be optimized for all fault conditions. Careful coordination with the primary protection system is necessary to minimize the risk of unnecessary tripping. Caution is required while testing and commissioning also. - How is LBB Protection coordinated with primary protection?
Coordination between LBB Protection and primary protection systems involves setting time-delay and current settings to ensure the primary system operates first, allowing LBB Protection to act as a backup only when needed. - Can LBB Protection be applied to all types of circuit breakers?
LBB Protection can be applied to a wide range of circuit breakers, including air, oil, gas, and vacuum circuit breakers. The specific implementation may vary based on the breaker type and the characteristics of the power system. - How often should LBB Protection be tested and maintained?
Regular testing and maintenance are essential to ensure the reliability of LBB Protection. Industry standards and manufacturer recommendations should be followed to establish a maintenance schedule. - What measures are taken for maintenance of Local Breaker Backup relays?
Regular maintenance and testing are crucial for the effectiveness of LBB. Periodic testing of the backup relays, including functional and calibration checks, helps ensure that the backup protection is ready to operate when needed. - Is Local Breaker Backup used in all types of power systems?
LBB is commonly employed in high-voltage substations and transmission systems. Its application may vary depending on the specific requirements and characteristics of the power system. - How does Local Breaker Backup contribute to overall system reliability?
By providing an additional layer of protection, LBB contributes to the overall reliability of the power system. It helps prevent cascading failures and minimizes the impact of faults, ultimately improving the stability and security of the electrical grid.
You may read about differential protection of transformer here.