Overhead Line Recloser

Overview

An overhead line recloser is a self-resetting circuit breaker mounted on a distribution pole. Most faults on an overhead feeder are temporary: a branch brushes a conductor, a bird bridges a phase, lightning flashes over an insulator. The arc clears itself once the line is de-energised for a moment. A recloser exploits this by tripping on a fault, waiting a fraction of a second, and then closing back in. If the fault has cleared, service is restored and few customers notice. If the fault is permanent, the recloser repeats this a set number of times and then locks out, isolating the faulted section.

This unit is a three-phase, vacuum-interrupting recloser with solid-dielectric insulation and a magnetic actuator. It pairs a sealed tank holding the switching parts with a pole-mounted control cubicle that runs the protection relay. Together they replace the older oil reclosers that needed periodic oil testing and the SF6 designs that carry a greenhouse-gas burden.

How it works

Three vacuum interrupter poles, one per phase, sit inside the stainless tank. Each pole encapsulates a vacuum interrupter bottle in a cycloaliphatic epoxy body that provides the external insulation. The three push rods are tied to a common cross-bar so all phases open and close together, driven by a single magnetic actuator. Power enters and leaves through six epoxy bushings, a source and a load bushing on each phase, each fitted with a terminal lug and a surge arrester.

Vacuum interruption

Inside each bottle a moving contact and a fixed contact meet in a high vacuum. When the contacts part under load, an arc forms in metal vapour boiled off the copper-chromium contact faces. Because there is almost nothing to ionise in a vacuum, the arc cannot sustain itself; at the next natural current zero, every 8 to 10 ms at 50 or 60 Hz, it extinguishes and does not restrike. The contact gap is only a few millimetres, so the stem bellows that seals the moving stem has a short stroke and survives the full 10,000-operation mechanical life. Vacuum interruption needs no arc-quenching medium and produces no exhaust, which is why the tank can be sealed for life.

Magnetic actuator

The magnetic actuator is bistable. A permanent magnet latches the plunger in the closed position with no holding current, so the recloser stays closed even with the control fully de-energised. To trip, the coil is pulsed from a charged capacitor; the field overcomes the magnet, and the opening spring drives the contacts open in roughly 30 to 40 ms. To close, the coil is pulsed the other way and the plunger is pulled back to the magnet, taking about 50 to 60 ms. A position sensor confirms the real contact state, and a manual trip lever lets a lineworker trip and lock out the unit from the ground with a hookstick before working the line. With one moving assembly and no gear train, the mechanism has very few wear parts.

Sensing and control

Protection runs on integrated low-power sensors rather than conventional instrument transformers. Three current sensors, Rogowski coils or low-power CTs, measure phase current across the full range from load to fault. Six capacitive voltage sensors, one on each side of each phase, give live-line indication and the polarising voltage for directional protection. Their signals run down the control cable to the controller board, where an MCU digitises them and runs the relay algorithms. The operator reads settings and event records on the HMI display, a GPS receiver time-stamps events for feeder-wide fault analysis, and a communications modem reports to SCADA over DNP3 or IEC 61850. A backup power supply with a battery and charger keeps the control alive and the actuator capacitors charged through an outage.

Reclose logic

When the relay sees current above the pickup threshold, it trips. After a dead time, typically 1 to 5 seconds, it commands a close. If the fault is gone, the recloser stays closed and resets after a set time. If the fault persists, it trips again and repeats, usually up to four operations, before going to lockout. The early trips are often "fast" curves that beat downstream fuses, giving transient faults a chance to clear without blowing a fuse; later trips use slower curves that coordinate with those fuses so only the smallest faulted section is finally isolated.

Variants and use

Reclosers are rated for 15, 27, and 38 kV class feeders, with continuous ratings of 630 or 800 A and interrupting ratings of 12.5 or 16 kA. They are placed at substation getaways, at mid-feeder points to sectionalise long rural lines, and at the head of laterals. Networked together over the communications link, a group of reclosers can run loop automation that detects a fault, isolates the faulted span, and back-feeds the healthy sections, cutting outage times for everyone downstream.