Automatic Reclosing 79 (RREC): How It Works

What 79 automatic reclosing is

Most faults on overhead lines are transient: a lightning strike, a branch touching the line or an insulator flashover. When the breaker opens, the arc extinguishes and the insulation recovers. If the breaker closes again after a short time, service is restored without operator action. That is what function 79 does.

The 79 is not a protection: it does not detect faults. It receives the initiate signal from the protections (distance, overcurrent, line differential, teleprotection), controls the timers and issues the close command.

• ANSI/IEEE C37.2: 79, AC reclosing relay.
• IEC 61850: logical node RREC.

In distribution the function lives in the recloser, a breaker with its control built in. In transmission it lives in the line relay or in a separate IED.

Where it is applied

• Overhead transmission lines, with single-pole or three-pole reclosing, high-speed and almost always single-shot.
• Overhead distribution feeders, with several shots and sometimes fuse-saving schemes.
• Rural and mining networks with reclosers on the main line and on laterals.
• Lines connecting solar or wind plants, where reclosing must be coordinated with the anti-islanding protection of the generation.
• It is not applied to underground cables, transformers, buses or generators, where faults are usually permanent.

How it works

A typical cycle is:

1. The protection trips and initiates reclosing.
2. The dead time runs: the breaker stays open so the arc extinguishes and the air deionizes.
3. The 79 checks the conditions (breaker ready, no blocking, synchronism or dead line where applicable) and closes.
4. The reclaim time runs. If there is no new trip, the cycle is considered successful and the 79 is ready again.
5. If there is a new trip during that time, it moves to the next shot or, if all shots are used, the 79 goes to lockout and waits for a manual close.

Other elements of the logic:

• Single-pole reclosing: only the faulted phase opens. The dead time must be longer than for three-pole reclosing, because the healthy phases sustain, through capacitive and inductive coupling, a secondary arc that takes longer to extinguish.
• Synchronism and dead-line check: in transmission, one terminal closes on dead line and live bus, and the other with synchronism check 25.
• Switch onto fault: after a reclose or a manual close, the protection usually trips without delay if the fault persists.
• Fuse saving: in distribution, the first trip is fast so the recloser clears before the lateral fuse melts; the following trips are time-delayed so the fuse operates if the fault is permanent.

The rated operating sequence of the breaker, for example O – 0.3 s – CO – 3 min – CO under IEC 62271-100 for breakers intended for rapid auto-reclosing, limits what the 79 can ask of it.

Setting criteria

• Dead time: it must exceed the arc deionization time, which grows with voltage, and the clearing time of the remote terminal. For single-pole reclosing the secondary arc is taken into account. IEEE C37.104 gives the guidance.
• Number of shots: one is common in transmission; several in distribution, according to the utility’s philosophy.
• Reclaim time: longer than the recharge time of the breaker mechanism and than the time of the slowest protection that could trip again.
• Which protections initiate and which block: instantaneous and teleprotection trips usually initiate; time-delayed zones, 50BF, 87T and transfer trips usually block.
• Stability of the system and of nearby generation: a high-speed reclose onto a permanent fault close to a generator can be very severe for the shaft.
• Coordination with fuses, other reclosers and the remote terminal, according to the protection coordination study.

How each manufacturer implements it

• Siemens SIPROTEC separates reclosing into programs for single-phase and multi-phase faults, with settings per cycle. It is configured in DIGSI.
• SEL controls it with logic equations that define initiation, blocking and close condition, plus the timers for each shot. It is set in AcSELerator QuickSet.
• Relion (Hitachi Energy 670/650; ABB 615/620) names the function after the IEC 61850 logical node and configures it in PCM600, with the graphical application logic.
• GE Vernova (Multilin) offers multi-shot reclosing with several modes in its line and feeder relays, configured in EnerVista.
• Schneider MiCOM / Easergy includes it in line and feeder relays, configured in MiCOM S1 or Easergy Studio.
• NOJA Power integrates reclosing with the fast and delayed curves in the controller of its reclosers, configured with its CMS software.

What changes is how the initiating functions are defined, the behavior on a trip during the dead time, the way shots are counted and the state after a manual close. The same scheme is written very differently on each brand.

Why it trips or blocks

Common field situations:

• Does not reclose: function out of service; a protection that does not initiate; breaker without a ready signal; blocking by 50BF, 87T or transfer trip; synchronism conditions not met.
• Recloses when it should not: it was initiated by a time-delayed zone or by a fault on an underground section.
• Goes to lockout after a transient fault: reclaim time too short, or a protection picked up again during it on the inrush of the feeder transformers.
• Pole discrepancy in single-pole reclosing: the dead time exceeds the pole discrepancy setting, which opens all three phases. See the pole discrepancy test.
• The remote terminal closes first onto the fault: dead-line criteria assigned incorrectly.

Start with the sequence of events: who tripped, whether reclosing was initiated, the breaker status and which condition was missing when the dead time expired.

How it is tested

With an Omicron CMC 356 the fault is simulated, the trip is read and the breaker is emulated with its auxiliary contacts, or the actual breaker is used. The checks are:

• dead time and reclaim time of each shot;
• number of shots and transition to lockout;
• initiation from the intended protections and blocking from the others;
• synchronism and dead-line conditions;
• behavior on a manual close and on a permanent fault.

On lines, testing is completed with end-to-end tests to confirm coordinated reclosing at both terminals.

Did your relay trip or block on this function and the cause is not clear? At Tiza Group we review the events and settings remotely. See remote relay configuration or contact us.

References

• IEEE C37.2, device function numbers.
• IEEE C37.104-2022, Guide for Automatic Reclosing on AC Distribution and Transmission Lines.
• IEC 62271-100, high-voltage alternating-current circuit-breakers.
• IEC 61850-7-4, logical node models.
• J. L. Blackburn and T. J. Domin, Protective Relaying: Principles and Applications.
• S. H. Horowitz and A. G. Phadke, Power System Relaying.

Frequently asked questions