A special type of medium-voltage circuit breaker used to quickly interrupt and re-establish power in distribution lines is called a recloser.
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Reclosers are designed to trip if ever a distribution line suffers a “transient” (momentary) fault due to some natural event such as a lightning strike causing an insulator to “flash over” to ground or a tree branch touching one or more line conductors, then automatically re-close moments later to test whether or not the fault still persists. If the fault clears on its own – a common occurrence with tree branches, as the branch may break off or burn away following the initial arc – then the recloser remains closed and continues providing power to customers. By some estimations transient faults account for 70% to 90% of all faults occurring on overhead power lines. If non-reclosing fault protection were applied to all distribution and transmission lines, extended interruptions of electric power service would be far more common than they are now.
If you have ever experienced momentary cessations of electrical power service to your home or business where the power “blinks” off and on in rapid succession, you have experienced a recloser at work. The recloser opens as soon as an overcurrent condition is detected, then recloses briefly to “test” for continued presence of the fault. If the fault persists, the recloser trips again and then recloses once more to “re-test” for the fault. If the fault has cleared by then, the recloser remains closed and restores normal power service to customers. Only if the fault persists after multiple “shots” does the recloser remain in the tripped (open) state and waits for line crews to repair the fault.
Reclosers are typically located some distance “downstream” of the substation, to isolate certain remote portions of the distribution network. Circuit breakers at the substation provide protection for each distribution line as a whole:
A typical recloser resembles a “circuit breaker on a stick,” located near the top of a distribution power pole near the line conductors. Modern reclosers use SF6 gas quenching to extinguish arcing resulting from the interruption of high-magnitude fault currents. Legacy reclosers typically employed oil quenching. This photograph shows a modern (SF6 -quenched) recloser, with its three phase current-interrupting contact units clearly visible:
The grey enclosure located near ground level on this pole contains the protective relay responsible for issuing the “trip” and “close” signals to the recloser’s coils. Current transformers located within the recloser provide isolated sensing of line current to the reclosing relay, necessary for detecting any overcurrent conditions that might result from a transient fault. Each attempt by the reclosing relay to re-close the breaker is called a shot. If the reclosing relay fails to clear the fault by a certain number of shots, it enters the lockout state whereby the recloser remains open and must be re-closed by human intervention.
Distribution line reclosers, unlike circuit breakers located in substations, cannot rely on an auxiliary “station power” energy source for opening and closing its line-interrupting contacts. Therefore these small units utilize AC line voltage as the actuating power for the contacts. Low- voltage “trip” and “close” circuits still exist for control purposes, but the actual energy source for rapid tripping/reclosing cycles comes from the AC line itself.
The principle of automatic reclosing may be applied to transmission lines as well as distribution
lines, but new challenges exist at this level of a power grid. When transmission lines serve to interconnect distributed generating stations, interruption of that line for any significant time invites generator de-synchronization. Recall that AC generators, once synchronized with each other and connected in parallel on a common grid circuit, tend to remain synchronized with each other as though their mechanical shafts had become coupled. If a circuit breaker opens along a transmission line system and de-couples generators from each other, those generators will be free to fall out of synchronization. Reclosing that circuit breaker when those generators are out of sync with each other can be disastrous. Automatic reclosing at the distribution line level of a power grid, therefore, must either be fast enough that generators will not have enough time to fall out of sync with each other, or blocked by other protective relay logic to prevent reclosure in an out-of-sync situation.