In electric power distribution, a recloser, also known as autorecloser or automatic circuit recloser (ACR), is a switchgear designed for use on overhead electricity distribution networks to detect and interrupt transient faults. Reclosers are essentially rated circuit breakers with integrated current and voltage sensors and a protection relay, optimized for use as a protection asset. Reclosers are governed by the IEC 62271-111/IEEE Std C37.60 and IEC 62271-200 standards. The four major classes of operating maximum voltage are 15.5 kV, 27 kV, 38 kV and 72 kV. For overhead electric power distribution networks, up to 80-87% of faults are transient. Transient faults can occur due to various causes, such as lightning strikes, voltage surges, or foreign objects coming into contact with exposed distribution lines. When a transient fault occurs, the resulting arc will ionize the air. The ionized air will maintain the arc even after the material that caused the short circuit is removed. Consequently, these transient faults can be resolved by a simple reclose operation. The minimum reclose time allowed for any operation is .3 seconds. This is the minimum amount of time required for the ionization to dissipate from the arc path. Reclosers are designed to handle a rapid open-close duty cycle, where electrical engineers can optionally configure the number and timing of attempted close operations prior to transitioning to a lockout stage. The number of reclose attempts is limited to a maximum of four by recloser standards noted above. At two multiples of the rated current, the recloser's rapid trip curve can cause a trip (off circuit) in as little as 1.5 cycles (or 30 milliseconds). During those 1.5 cycles, other separate circuits can see voltage dips or blinks until the affected circuit opens to stop the fault current. Automatically closing the breaker after it has tripped and stayed open for a brief amount of time, usually after 1 to 5 seconds, is a standard procedure. Reclosers are often used as a key component in a smart grid, as they are effectively computer controlled switchgear which can be remotely operated and interrogated using supervisory control and data acquisition (SCADA) or other communications. Interrogation and remote operation capabilities allow utilities to aggregate data about their network performance, and develop automation schemes for power restoration. Automation schemes can either be distributed (executed at the remote recloser level) or centralized (close and open commands issued by a central utility control room to be executed by remotely controlled closes).
Description Reclosers are made in single-phase and three-phase versions, using oil, vacuum, or sulfur hexafluoride (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. On a 3-phase circuit, a recloser is more beneficial than three separate fuse cutouts. For example, on a wye to delta conversion, when cutouts are used on the wye side and only 1 out of 3 of the cutout fuses open, some customers on the delta side have a low voltage condition, due to voltage transfer through the transformer windings. Low voltage can cause severe damage to electronic equipment. But when a recloser is used, all three phases open, thereby eliminating the problem.
History Reclosers were invented in the mid 1900s in the USA with the earliest reclosers introduced by Kyle Corporation in the early 1940s. Reclosers were originally oil-filled hydraulic devices with rudimentary mechanical-protection-relaying capabilities. Modern automatic circuit reclosers are significantly more advanced than the original hydraulic units. The advent of semiconductor based electronic protective relays in the 1980s resulted in increased recloser sophistication, allowing for differing responses to the various cases of abnormal operation or fault on an electric power distribution network. The high-voltage insulation and interrupting devices in modern reclosers typically consist of solid dielectric insulation with vacuum interrupters for current interruption and arc quenching.
Purposes
Protection during fault conditions To prevent electric power distribution network damage, each station along the network is protected with circuit breakers or fuse cutouts which turn off power in the event of a short circuit. These protection solutions present a major problem when restoring power immediately following transient events, because repair crews need to manually reset the circuit breakers or replace fuses. Alternatively, reclosers are programmed to automate the reset process remotely after a short circuit and allow a more granular approach to service restoration, resulting in increased availability of supply. Using reclosers during a transient fault, for instance, a tree limb blown off a tree during a windstorm that lands on the power line and quickly clear itself as the limb falls to the ground, allows power to be remotely restored.
Remote restoration Reclosers can save significant operational expenditure when operated remotely, as they can reduce the need of field crews to travel to site to reset devices which have transitioned to lockout.
Division Reclosers can also address electric power distribution network damage by dividing up the network into smaller sections, possibly at every electric power distribution downstream branch point, which handle much less power than the breakers at the feeder stations, and can be set to trip at much lower power levels. Consequently, a single event on the grid will cut off only the section handled by a single recloser, long before the feeder station would notice a problem and cut power.
Reconfiguration and load flow resolution Reclosers can resolve load flow issues by reconfiguring the electric power distribution network.
Typical fault conditions and reclosing principles The basic philosophy of reclosing is to actively consider the fault types and provide an effective response based on probabilities of the detected fault type. Fault currents are sensed by current sensing transformers.
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