what is STUB protection in transmission system and switchyard area?
STUB protection is an enhanced form of Overcurrent protector and is used significantly in transmission system networks especially, the OAHS which is the one and a half breaker scheme.
Specifically, it is intended to shield areas of the power system for which regular distance protection is not effective when the line isolators are switched off.
Purpose of STUB Protection
STUB protection is necessary when the input is disconnected by the opening of the line isolator, which cannot send required voltage and current to the distance protection.
In such cases the region between the circuit breakers and the open isolator can be prone to the faults and hence the STUB protection is provided as one form of back up protection.
Working Principle of STUB Protection
The fact is the functioning of STUB protection is based on the status of the line isolator.
When the isolator is open, STUB protection system is operative, the current is covered by the distance relay.
The protection is normally set to snap if the current of fault is above a certain level, often above the full load of current on the line.
It allows the equipment to operate normally without the flowing current tripping and yet provides quick fault clearance when the isolator breaker is open.
Key Features of STUB Protection
Activation Condition
Here, one can note that STUB protection is only activated when the isolator of the line is open.
Fault Current Setting
The current threshold is chosen to be higher than the full load current so as to prevent the circuit breaker from tripping during normal operations.
Zero Delay
The time delay of tripping is most preferably set to zero or less than the Zone-2 time delay for tripping the connected lines to counter faults.
Application in One-and-a-Half Breaker Scheme
- In one and a half breaker bus configuration two lines connect to two buses through three circuit breakers. This situation means that, while a single line is out of service with the isolator opened, it is only possible for the remaining line to loop through a single breaker.
- STUB protection facilitates protection of the stub bus area that remains unprotected due to opening of the circuit breakers by offering the same when the latter is open.