The extra high voltage transmission lines transmit huge amount of electric power. Hence, it is always desirable that the continuation of power flow through the lines should not be interrupted for a long time. There may be a temporary or permanent fault in the lines. Temporary faults get automatically cleared, and these do not require any attempt for fault rectification. It is normal practice by the operators that after each initial faulty tripping of the line, they close the line. If the fault is transient, the line holds after the second attempt of closing the circuit breaker, but if the fault persists, the protection system again trips the line and then it is declared as permanent fault.
But as the extra high voltage transmission lines carry huge power, if any delay occurs due to manual operation for reclosing the circuit, there will be a big loss of system in the view of cost and stability. By introducing the auto reclosing scheme in the extra high voltage transmission systems, we can avoid the unwanted delay due to human operation. We categorize the faults in electrical transmission system in three ways,
- Transient Fault
- Semi Permanent Fault
- Permanent Fault
The transient faults are those which automatically removed momentarily. Semi permanent faults are also transient in nature but there take few moments to remove. Semi-permanent faults may get occurred due to the falling of things on the live conductors. Semi-permanent faults get removed after the cause of faults is burnt away. During both of the above mentioned faults, line is tripped but the line can be restored if the circuit breakers associated with the line are closed.
Auto-recloser or auto-reclosing scheme exactly does this. In an overhead transmission system, 80% of the faults are transient, and 12% of faults are semi-permanent. In auto-reclosing scheme if the fault is not cleared at first attempt, there will be double or triple shorts of reclosing until the fault is cleared. It the fault still persists, this scheme permanently opens the circuit breaker. A prescribed time delay may be imposed on the auto-reclosing system to permit the semi-permanent fault to remove from the circuit.