Recloser

In electric power distribution, automatic circuit reclosers (ACRs) are a class of switchgear which is designed for use on overhead electricity distribution networks to detect and interrupt momentary faults. Also known as reclosers or autoreclosers, ACRs are essentially high voltage rated circuit breakers with integrated current and voltage sensors and a protection relay, optimized for use as an overhead network distribution protection asset. Commercial ACRs are governed by the ANSI/IEEE C37.60, IEC 62271-111 and IEC 62271-200 standards. The three major classes of operating voltage are 15.5 kV, 27 kV and 38 kV.
For overhead distribution networks, the majority of faults are transient, such as lightning strike, surges or foreign objects coming into contact with the exposed distribution lines. By this logic, 80% of outages can be resolved by a simple close operation. Reclosers are designed to handle a short close-open duty cycle, where electrical engineers can optionally configure the number of attempted close operations prior to transitioning to a lockout stage.
Reclosers were invented in the mid 1900s in the USA. Some of the earliest reclosers were introduced by Kyle Corporation (which was acquired by Cooper Power Systems - part of the Eaton family) in the early 1940s. The brand was the industry leader in reclosers, sectionalizers and switchgear until the 2000s when many other manufacturers entered the market. 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 sophistication, allowing for differing responses to the various cases of abnormal operation or fault on a distribution networks. The high voltage insulation and interrupting device in modern reclosers typically consist of solid dielectric insulation with vacuum interrupters for current interruption and arc quenching.
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 SCADA or other communications. This capability allows utilities to aggregate data about their network performance, and develop automation schemes for power restoration. This automation can either be distributed (executed at the remote recloser level) or centralized (close and open commands issued by a central control room to be executed by remotely controlled ACRs).
Description.
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 when dealing with transient events. For instance, a tree limb blown off a tree during a windstorm that lands on the power line may cause a short circuit that could cause damage. However, the fault could quickly clear itself as the limb falls to the ground. If the only protection system is provided by breakers at distribution substations, large areas of the distribution network could be blacked out while repair crews reset the breakers. Reclosers are programmed to automate the reset process and allow a more granular approach to service restoration. The result is increased availability of supply.
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 upstream 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 a single recloser, long before the feeder station would notice a problem.
Modern recloser installations are often equipped with SCADA communications, allowing the majority of reclosers to be remotely operated by staff in the utility control room. This allows for re-switching the network, as operators can use information provided by the reclosers in the field to reconfigure the distribution network if a fault is detected in the field, or to fix load flow issues. Remote control of reclosers also saves significant operational expenditure, as it can reduce the necessity of field crews to travel to site to reset devices which have transitioned to lockout.
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.
On a 3-phase circuit, a recloser is more beneficial than three separate fuse cutouts. For example, on a wye to delta conversion if cutouts are used on the wye side, and only 1 out of 3 of the cutout fuses open, some customers on the delta side would have a low voltage condition, due to voltage transfer through the transformer windings. Low voltage can cause severe damage to electronic equipment. But if a recloser were used, all three phases will open, eliminating the problem.
Reclosing principles.
Whilst original hydraulic recloser designs had rudimentary protection capabilities, modern semiconductor controlled devices exhibit sophisticated control systems which allow for the configuration of varying responses to different classes of faults on the distribution network. The number of reclose attempts is limited to a maximum of four by recloser Standards. The basic philosophy of reclosing is to actively consider the fault cases and provide an effective response based on the fault type, this is done on a probabilistic methodology in conjunction with the detection of fault type.
The most common fault type on an overhead distribution network is lightning strike. Lightning surges cause an increase in voltage which can cause localised breakdown of insulation, allow arcing over insulators. Reclosers can detect this as an overcurrent or earth fault (depending on the asymmetry of the fault). Lightning surges pass very quickly (reduce in 50ms), so the first reclose operation of a recloser can be configured to both trip and reclose quickly. This first reclose allows for interruption of the arcing caused by lightning, but restores the power quickly.
If the recloser closes onto a fault, it is likely that the fault is a secondary class of fault, vegetation contact or equipment failure. An overcurrent fault would indicate a line to line class fault, which can be confirmed by negative phase sequence overcurrent protection, whereas an earth fault can indicate a Line to Ground or Double Line to Ground fault. Reclosers can then apply a fuse burning policy, where they remain closed for a short period to allow fuses on lateral lines to burn, isolating the fault. If the fault is not cleared, the recloser trips open again. This same policy can be used to deliver energy to fault sites to burn the fault off the line. This could be a branch crossing between multiple lines, or fauna (birds, snakes, etc.) coming into contact with the conductors.
Sensitive earth fault protection in reclosers is typically set to immediate lockout. This detection of small leakage currents (less than 1 ampere) on a medium voltage line can indicate insulator failure, broken cables or lines coming into contact with trees. There is no merit in applying reclosing to this scenario, and the industry best practice is not to reclose on sensitive earth fault. Reclosers with sensitive earth fault protection capable of detecting 500mA and below are used as a fire mitigation technique, as they provide an 80% risk reduction in fire starts, however they are never to be used as reclosers in this application, only as single shot distributed circuit breakers which allow for sensitivity to verify the existence of these faults.
Applications.
Traditional reclosers were designed simply to automate the action of a line crew visiting a remote distribution site to close a tripped circuit breaker and attempt to restore power. With the advanced protection functionality of modern reclosers, these devices are used in a multitude of additional applications
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 and also power surges.
Sectionalizers.
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 however it often has a larger Basic Insulation Level, allowing some sectionalizers to be used as a point of isolation. 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. Some modern recloser controllers can be configured to have reclosers operate in sectionalizer mode. This is used in applications where protection grading margins are too small to provide effective protection co-ordination between electrical assets.
Fire safety and wildfires.
Fire risk is an innate risk of an overhead distribution network. Regardless of the choice of distribution protection switchgear, the fire risk is always higher with overhead conductors than with underground reticulation.
The Victorian Royal Commission into the 2009 bushfires indicated that reclosing must be disabled on high bushfire risk days, however on low risk days it should be applied for reliability of supply.
Incorrectly configured or old model reclosers have been implicated in the starting or spread of wildfires. Research into the Australian 2009 Black Saturday Bushfires indicated that reclosers operating as single shot circuit breakers with Sensitive Ground Fault protection configured at 500mA would reduce fire start risk by 80%. Any form of reclosing should be removed on high fire risk days, and reclosing in general should not be applied to detected Sensitive Earth Fault faults.
Victorian utilities responded to the Royal Commission by converting some of their overhead network in high risk areas to underground cable, replacing exposed overhead conductors with insulated cables, and replacing old reclosers with modern ACRs with remote communications to ensure that settings can be adjusted on high bushfire risk days.