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.