Auto-Recloser in feeders

Introduction: Electricity is transferred on higher voltage for long distances. Transmission / distribution feeders pass through forests, hills, fields etc befor reaching destination. 90% of the faults in feeders are transient in nature. These are caused by lightning, birds, falling trees, Forest growth, swinging of conductor, High velocity winds etc. Transient faults disappear after a short interval of time. In this manner, If line is tripped, 90% probability is that line will charge immediatly without any rectification required. For reducing the outage time, auto-reclose is used. Which means automatic closing of circuit breaker (CB) after small gap of time, called dead time. Minimum dead time is selected such that ionized air due to arc is de-ionized within dead time. It is generally between 0.5 s to 1.0 s.

Different approaches in auto recose: Auto reclose selection has four options:

1. No Auto reclose: Auto reclose disabled, no auto reclose will be attempted.
2. 1 phase auto reclose: 1-ph auto reclose will be attempted for 1-ph faults. If the fault is 2-ph or 3-ph, 3-ph trip will be initiated and no auto reclose will be attempted. It is used without synchronism check as remaining two phases will be connected from both ends. Typical time for auto-reclose attempt (Dead time) is 1 sec.
3. 1+3ph auto reclose: 1-ph auto reclosed will be attempted for 1-ph faults and 3-ph auto reclose will be attmpted for 2-ph or 3-ph faults. 
4. 3 ph auto reclose: 3-ph auto reclose will be attmpted for 1-ph, 2-ph and 3-ph faults 

Multi-cycle 3-ph auto reclose: In some cases multiple attampt may be required for auto reclose. First cycle is fast one having dead time less than 1 minute. remaing cycles can have dead time upto 1 hour. It is mostly used on radial type of feeders.

Circuit Breaker requirements: CB is key equipment during auto-recloser. Its operating mechanism should have sufficient energy for simultaneous Open- Close -Open operation to take care of persistant fault during auto-reclose. When CB is ready for this operation, CB ready signal is provided to Auto-recloser relay. AR will happen only in case of CB ready signal.

Another requirement is arc quenching capability of interrupting chamber. In case of SF6 CBs, SF6 gas retains its dielectric properties after ~25 sec. Therefore, after autoreclose operation, next auto reclose can happen only after 25 sec. This delay is called reclaim time. Any fault during reclaim time will lead to 3-ph trip and no AR attampt will be taken.