Local Breaker Back-up Protection or LBB protection is provided to ensure the healthiness Power System by isolating the faulty section in case of failure of tripping of Breaker intended for tripping on sensing fault. This protection is also called Breaker Failure Protection.
Master Trip Relay is one which gives the tripping command to the Breaker after receiving trip signal from individual protection element. Let us say, Earth Fault occurred in the Feeder-1, so in this case Earth Fault sensing Relay will not directly give the tripping command to the Breakers rather it will give tripping command to Master Trip Relay which in turn will trip the associated Breakers.
Now coming back to our discussion, let us assume that after getting the trip command from Master Trip Relay, Breaker CB-1C could not open but CB-1A opened. What will happen then?
As CB-1C failed to open, the fault will continue to be fed by Bus-2 vial CB-1B which is not a healthy condition for Power System. Thus in this case we need to isolate the fault which is achieved by Local Breaker Back-up Protection / Breaker Failure Protection.
So, LBB protection, shall isolate the faulty section by opening Breaker CB-1B. Thus LBB or Local Breaker Back-up Protection opens the CB-1A and CB-1B so that the faulty section is completely isolated.
Logic of Operation of LBB Protection:
As clear from the foregoing discussion, LBB Protection shall operate in the event of failure of a particular Breaker to trip. But the question arises “How will LBB Relay sense Breaker Failure?”
The LBB Relay is given an Input signal of Master Trip Relay. So in case LBB Relay that Master Trip Relay has operated but still overcurrent as measured by LBB Relay is existing then it implies that Breaker has failed to Open. In such case LBB Relay will operate.
Suppose a fault as shown in the figure above occurred. If Breaker CB-1A opens and CB-1C fails to open the LBB Protection Relay of CB-1C will see an overcurrent even after the operation of Master Trip Relay. Thus the LBB Relay confirms the failure of Breaker to trip.
There are two thing which are important to understand for LBB Protection. One is LBB initiation and another one is LBB actuation.
The concept of LBB initiation arises because of some inherent time delay in between the operation of concerned Fault Sending Relay & Master Trip Relay and Breaker Opening. Let the time delay in the sensing of fault and actuation of Master Trip Relay be 30ms and 40ms be the time for tripping of Breaker after getting trip command from Master Trip Relay. Thus for this 40ms, LBB Relay will sense overcurrent even after the actuation of Master Trip Relay. So in this 40ms, the LBB Relay is not supposed to actuate. Therefore an intentional time delay is provided in LBB Relay. Normally this time delay is 200 ms.
To avoid such a situation, LBB initiation and actuation logic is provided.
LBB Initiation Logic:
LBB Relay will initiate when
Master Trip Relay actuated AND over current is persisting
So the timer of LBB Relay will start on.
LBB Actuation Logic:
As an intentional time delay (say 200ms) is provided, hence after the initiation of LBB logic, say the condition for LBB Initiation is persisting for 200 ms then LBB Relay shall actuate to trip CB-1A and CB-1B.
You might wonder why LBB is giving trip command to CB-1A as it has already open. Since it is taking us to safer side, therefore it is normal practice to give trip command to CB-1A and CB-1B.
Next, assume the same case of fault in Feeder-1 but this time CB-1A failed to open. In this case LBB Protection shall trip CB-1C and CB-2A.Check out Power System by C.L Wadhwa. Must solve objective given at the end of the book.
Hello sir,
I understand clearly. But whats my doubt is if CB-1B also get failed to opened means at that time what will happen
How to reset Lbb 50BF relay in pick up indication..? I’m trying and I can’t do that.. still now I’m not given any current and voltage to the relay.
Go to view records, then find Reset Indications and reset the relay.
See, if CB-1B fails to open then LBB protection of CB-1B will appear which will actuate Bus Bar Differential protection to isolate the BUS.thank you!
Thank you
WHy should 2A trip in 2nd case when 1A fails to operate?How is feeder-1 being fed by 2A??
Ok i got my mistake,2A can feed the feeder1 through 1A..hence LBB relay will isolate 2A also..got it.
Wonderful explanation
Thank you Ramkumar!
kindly explain me bus bar protection in 400 kv system
I will soon write a post on Bus Bar Protection in 400 kV System. Please subscribe to get the notification. Thank you!
under the LBB actuation logic, I feel there is some correction required.
“Next, assume the same case of fault in Feeder-1 but this time CB-1A failed to open. In this case LBB Protection shall trip CB-1C and CB-2A.Check out Power System by C.L Wadhwa. Must solve objective given at the end of the book”
in the above text CB-2A should be replaced by CB-1A
No, No…see CB-1A failed to open. This means that it won’t open event when second trip command is issued. Agreed? So what we need to do? We actually need to operate busbar differential protection of Bus-1. This will trip all the main breakers connected to Bus-1 to isolate the fault. Did you understand? Please revert if you still have doubt. Thank you for asking.
In the first case if 1A and 1B operates, then we loss the supply for the feeder 2. But it is a healthy feeder. How can it be justified?
If tie breaker 1C fails to open, the fault will be fed through the healthy feeder. Therefore to isolate the fault, it is required to isolate the fault. Hence tripping of both the main breakers (1A and 1B) are tripped in case of LBB actuation of Tie Breaker.
I shall add to the above discussion, if CB-1c fails to open , cb-1A and cb-1b also open to isolate the fault . and also feeder-1 other end is also tripped by carrier protection.if tie lbb i.e CB-1 C fails to operate other end of feeder-1 also trips.
Thank you for adding carrier trip. Yes, carrier trip will be sent to the remote end of both feeders 1 & 2.
Can you please add something about re trip and back trip?
And how external and internal lbb works for a 1 and half breaker system?
Please explain with SLD.
LBB protection for a feeder in one half breaker system, LBB protection usually have 200mA and 200-500ms setting. when a fault occurs main protection has acted and given trip command to breaker . Now breaker has got stuck up and LBB Relay sees the current and gives trip command to the same relay that is RETRIP. If the breaker still doesnt open , it gives BACK TRIP and all the feeders connected to the faulty feeder bus trips and associated tie breaker also trips. RETRIP is usually 100ms and BACK TRIP is 200ms.
Thank you for defining Back Trip and Re-Trip.
there is text correction in the fourth line it should be corrected as to the same breaker
Could not understand, what correction is required. It will be kind of you if you can elaborate. Thank you for pointing out.
Nice explanation
In case:1 after tripping CB-1B, still feeder1 get supply from feeder2 through CB-1C. To stop supply to feeder1 fault feeder2 other end CB also must trip, which will be achieved by DT(direct trip) teleprotection. That completes the LBB operation. Is that right..?
The intent is to isolate the faulty section.
can you explain re-trip and back-trip logic in LBB protection
I will publish a post on re-trip and back trip in LBB logic.
please post protection related articals
Ok…any specific topic if you want?