STUB Protection is simple overcurrent protection it is used to protect the power system transmission line when the line isolator is in open condition.
STUB protection is mainly required in one and half breaker busbar schemes. In one and half breaker scheme, 2 nos. lines are connected to 2nos. Bus through the 3 nos. Circuit breakers as shown in the diagram. When 1 no. line gets shut down or switched off, the other line can connect only through one breaker to the single bus. For the system stability, line should be connected to both the buses.
In this case, when 1 no. line gets shutdown, its Main and Tie Circuit Breaker will close after opening the line isolator as shown in the diagram. In this process, the other line is allowed to connect to both the buses.
Why STUB Protection?
The distance protection needs line voltage and current inputs to operate. As the line is under shutdown and the line isolator is in open condition, there is no voltage in the distance relay. In this case, to protect the area as marked in the diagram, we need STUB Protection.
STUB protection is the function of distance relay. When the distance relay gets the open status from the line isolator, it will block the distance protection and enable the STUB protection to protect the area. Under STUB protection, the minimum fault current limit is set above the full load current of the line, and delay time is set as zero.
The fault current of STUB protection should not be set below the full load current of the line because during the service condition of the line, due to any reason, if wrong isolator status goes to the relay, the STUB protection will activate, and it will trip the line.
The delay time is also set to zero or less than the Zone-2 time delay of the lines connected to the bus because if the fault is not isolated instantly, the fault will detect by other connected lines under zone 2 and all the connected lines will trip.