Reclosers: fundamentals of reclosers

A recloser is an automatic, high-voltage electric switch. Like a circuit breaker on household electric lines, it shuts off electric power when trouble occurs, such as a short circuit. Where a household circuit breaker remains shut off until it is manually reset, a recloser automatically tests the electrical line to determine whether the trouble has been removed. If the problem was only temporary, then the recloser automatically resets itself and restores the electric power.

Reclosers are used throughout the power distribution system, from the substation to residential utility poles. They range from small reclosers for use on single-phase power lines, to larger three-phase reclosers used in substations and on high-voltage power lines up to 38,000 volts.

Standards for reclosers are defined by ANSI/IEEE C37.60.

Automatic circuit reclosers are recognized by electric utilities throughout the world as an essential device for achieving their prime goal: providing maximum continuity of electric service to their customers simply and economically. Reclosers sense and interrupt fault currents and automatically restore service after a momentary outage has occurred. The automatic circuit recloser is essentially a self-contained device with the necessary intelligence to sense overcurrents to time and interrupt fault currents and to re-energize the line by reclosing automatically. If a fault is permanent, the recloser locks open after a preset number of operations (usually three or four), isolating the faulted section from the main part of the system.

Reclosers save the electric companies considerable time and expense, since they permit power to be restored automatically, after only a flicker or two. For outages that require a repair crew, reclosers minimize the outage area and help the crews to quickly locate the problem and restore power. Consumers of electric power – residential, business, industrial and institutional – are saved from the expense and inconvenience frequent power outages would cause.

Without this high level of power reliability, many critical process power-use devices that are commonplace today, such as computers, pumps, and assembly lines, would not have been practical.

The recloser senses when trouble occurs and automatically shuts off the power. An instant later (the length of time may be noticeable only as a lightbulb flicker), the recloser turns the power back on, but if the trouble is still present, it shuts it off again. If the trouble is still present after three attempts, the recloser is programmed to consider the problem permanent and it remains off. A power company crew must then repair the problem on the line and reset the recloser to restore power.

Examples of permanent problems include: power lines or other equipment damaged by lightning strikes, fallen tree limbs or vehicle crashes.

On high-voltage lines, most trouble occurrences (faults) are temporary – such as lightning, windblown tree branches or wires, birds, or rodents – and will, by their very nature, remove themselves from the electric line if the power is shut off before permanent damage occurs to the lines.  Examples of temporary faults include the following:

• Windblown conductors touching one another
• Lightning surges flashing over an insulator
• Birds, reptiles or small animals bridging between an energized line and grounded surface
• Tree branches touching energized lines
• Switching surges that flash over an insulator

Based on statistics and observations, the need for a trip-and-reclose function was readily recognized.  If the line could be tripped open momentarily, then a subsequent reclosure very likely would be successful because, by then, the cause of the fault would be gone.  Thus, the automatic circuit recloser, in providing this trip-and-reclose function, virtually eliminates prolonged outages on distribution systems due to temporary faults or transient overcurrent conditions. 

Single-phase reclosers are used to protect single-phase lines such as branches or taps of a three-phase feeder. They can also be used on three-phase circuits where the load is predominantly single-phase. When a permanent phase-to-ground fault occurs, one phase can be locked-out while service is maintained to the remaining two-thirds of the system. 

Due to the lighter weight of the single-phase recloser compared to larger three-phase reclosers, single-phase reclosers are typically mounted directly to the pole or substation steel structure individually by the built-in mounting hanger bracket which eliminates the need for an additional mounting frame.

Single-phase reclosers can be controlled with a hydraulic control (integrated within the recloser tank), or an electronic control (housed in a separate enclosure) based upon the recloser design.

Three-phase reclosers are used on three-phase circuits to improve system reliability and where lockout of all three phases is required for any permanent fault, to prevent single phasing of three-phase loads such as large three-phase motors. The recloser selection is based upon electrical ratings required, interrupting and insulation medium, and the selection of hydraulic or electronic control.

The mode of operation is as follows:

• Three-phase trip and three-phase lockout: Larger reclosers use this mode. For any fault (single-phase-to-ground, phase-to-phase or three-phase), all contacts open simultaneously for each trip operation. The three phases, mechanically linked together for tripping and reclosing, are operated by a common mechanism.

Several mounting options are available for three-phase reclosers including pole mounting frame and substation frame.  

Triple-single reclosers are electronically controlled and have three modes of operation:

• Three-phase trip and three-phase lockout: All three phases simultaneously trip on an overcurrent, reclose and sequence together.
• Single-phase trip and three-phase lockout: Each phase operates independently for overcurrent tripping and reclosing.  If any phase sequences to lockout condition (due to permanent fault), or if “lockout” is locally or remotely asserted, the other two phases trip open and lock out. Extended single-phase energization of three-phase loads is prevented.
• Single-phase trip and single-phase lockout: Each individual phase trips and sequences to lockout independent of each other. This is primarily for residential loads and/or where single-phasing of three-phase loads is protected by other means.

Triple-single reclosers can be mounted to a pole with use of pole mount frame or in the substation with substation frame (or directly to steel substation structure).

Hydraulic recloser control is used in most single-phase reclosers and in some three-phase reclosers. It is built as an integral part of the recloser. With this type of control, an overcurrent is sensed by a trip coil that is connected in series with the line.

When the overcurrent flows through the coil, a plunger is drawn into the coil to trip open the recloser contacts. Timing and sequencing are accomplished by pumping oil through separate hydraulic chambers or ducts.

For smaller reclosers, the reclosing energy is provided by springs that are charged by the series trip-coil plunger during overcurrent protection.

Larger reclosers are closed from a separate closing solenoid that is energized by line potential from the source side of the recloser.

Microprocessor-based and/or electronic controls are housed in a cabinet separate from the recloser and conveniently permits changes to operational settings. A wide range of accessories is available to customize the basic operation, solving many application problems.

Compared with the hydraulic control, they are more flexible, more easily customized and programmed, and have advanced protection, metering, and automation functionality.

Microprocessor controls typically utilize PC-based interface software to configure control settings, record metering information and establish communication parameters. It also provides analysis tools that include fault locating, event recording, and oscillography functions.

Electronic controls have been used in most three-phase reclosers (since mid-1980’s) and may still be in operation today.

Oil interrupters

Reclosers using oil for current interruption use the same oil for basic insulation. Some reclosers with hydraulic control also utilize the same oil for timing and counting functions

Vacuum interrupters

Vacuum interrupters provide fast, low-energy arc interruption with long contact and interrupter life, low mechanical stress, and maximum operating safety. With arc interruption taking place in a vacuum, contact and interrupter life far exceeds other interrupting media.

Depending on type, a vacuum recloser may use oil, air, or epoxy as the insulating medium.