In electric power distribution, a recloser, or autorecloser, is a circuit breaker equipped with a mechanism that can automatically close the breaker after it has been opened due to a fault. Reclosers are used on overhead distribution systems to detect and interrupt momentary faults. Since many short-circuits on overhead lines clear themselves, a recloser improves service continuity by automatically restoring power to the line after a momentary fault.
In order to prevent damage, each station along the network is protected with circuit breakers or fuses which will turn off power in the event of a short circuit. This presents a major problem dealing with transient events. For instance, a tree branch that is blown off a tree during a windstorm and lands on the line may cause a short circuit that could cause damage. However, the fault will quickly clear itself as the branch falls to the ground. If the only protection system is the breakers at the distribution centres, large areas of the grid would be blacked out while the repair crew resets the breakers.
Reclosers address this problem by further dividing up the network into smaller sections. For instance, the city grid example above might be equipped with reclosers at every branch point on the network. Reclosers, because of their position in the network, handle much less power than the breakers at the feeder stations, and therefore can be set to trip at much lower power levels. This means that a single event on the grid will cut off only the section handled by the single recloser, long before the feeder station would notice a problem. A normal breaker could also be used for this role, but because they are distributed geographically throughout the grid, as opposed to being centralized at feeder stations, resetting a breaker might take considerable time. For this reason, reclosers are used to automatically re-connect after a brief interval. There is a strong likelihood that the fault will be gone when the power is restored. If the fault is still present, the recloser opens again.
The control system for a recloser allows a selected number of attempts to restore service after adjustable time delays. For example a recloser may have 2 or 3 "fast" reclose operations with a few seconds delay, then a longer delay and one reclose; if the last attempt is not successful, the recloser will lock out and require human intervention to reset. If the fault is a permanent fault (downed wires, tree branches lying on the wires, etc.) the autorecloser will exhaust its pre-programmed attempts to re-energize the line and remain tripped off until manually commanded to try again. About 80-90% of faults on overhead power lines are transient and can be cured by autoreclosing. The result is increased availability of supply.
Autoreclosers are made in single-phase and three-phase versions, and use either oil, vacuum, or SF6 interrupters. Controls for the reclosers range from the original electromechanical systems to digital electronics with metering and SCADA functions. The ratings of reclosers run from 2.4–38 kV for load currents from 10–1200 A and fault currents from 1–16 kA.
Autoreclosers in action
Residential customers in areas fed by affected overhead power lines can occasionally see the effects of an autorecloser in action. If the fault affects the customer's own distribution circuit, they may see one or several brief, complete outages followed by either normal operation (as the autorecloser succeeds in restoring power after a transient fault has cleared) or a complete outage of service (as the autorecloser exhausts its retries). If the fault is on an adjacent circuit, the customer may see several brief "dips" (sags) in voltage as the heavy fault current flows into the adjacent circuit and is interrupted one or more times. A typical manifestation would be the dip, or intermittent black-out, of domestic lighting during an electrical storm. Autorecloser action may result in electronic devices losing time settings, losing data in volatile memory, halting, restarting, or suffering damage due to power interruption. Owners of such equipment may need to protect electronic devices against the consequences of power interruptions.
Reclosers may cooperate with down-stream protective devices called sectionalizers, usually a disconnector or cutouts equipped with a tripping mechanism triggered by a counter or a timer. A sectionalizer is generally not rated to interrupt fault current and is therefore cheaper than a recloser. Each sectionalizer detects and counts fault current interruptions by the recloser (or circuit breaker). After a pre-determined number of interruptions, the sectionalizer will open, thereby isolating the faulty section of the circuit, allowing the recloser to restore supply to the other non-fault sections.
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