Automatic reclosing does the same job on any overhead line, but it is applied differently depending on the voltage level. On medium voltage the recloser does it, with several attempts and coordination with fuses. On transmission the line relay does it, with a single attempt, sometimes single-pole, and with a synchronism or dead-line check at closing.
The same function, two applications
The reclosing function (79 in IEEE C37.2) controls the automatic reclosing of a breaker and its lockout when reclosing fails. The reason is the same on any overhead line: most faults are transient and disappear once the voltage is removed. This guide compares how it is applied on a medium-voltage circuit and on a transmission line. General operation is covered in the guide on automatic reclosing 79.
Comparison
| Aspect | Medium-voltage feeder | Transmission line |
|---|---|---|
| Where the function lives | In the recloser, a breaker with its integrated control | In the line relay or a separate reclosing IED |
| Attempts | Several, per the utility’s philosophy | One attempt is common |
| Fuse saving | Yes: the first trip is fast so the recloser clears before the branch fuse blows; later trips are timed | Not applicable |
| Reclosing type | Three-pole | Three-pole or single-pole |
| Check at closing | Normally no synchronism check | One terminal closes on dead line and live bus; the other with synchronism check 25 |
| Coordination | With fuses, other reclosers and sectionalizers | With the remote terminal, breaker opening time and nearby generation |
| Underground cables | Not reclosed | Not reclosed in the underground section |
What changes in practice
Dead time. It must exceed the arc deionization time, which grows with voltage. In single-pole it is longer, because the healthy phases sustain a secondary arc.
Reclaim time. It must be longer than the breaker mechanism recharge time and than the time of the slowest protection that may trip again.
What initiates and what blocks. At both levels, instantaneous or teleprotection trips usually initiate reclosing. Time-delayed zones, breaker failure, transformer differential and transfer trips usually block it.
Nearby generation. A fast reclose onto a permanent fault near a generator is very severe for its shaft. On a line connecting a solar or wind plant, anti-islanding must disconnect the plant before the dead time expires.
What is tested
On a feeder, the full recloser sequence is tested: number of trips, fast and slow curves, dead times and lockout. On a line, synchronism, dead line, single-pole reclosing and pole discrepancy are also tested. Both use secondary injection, and on lines they are completed with end-to-end tests.
Frequent mistakes when moving between levels
- Copying the multi-attempt sequence of a feeder onto a transmission line.
- Letting a time-delayed zone initiate reclosing on a line.
- Not distinguishing the underground section on a mixed line.
- Setting pole discrepancy below the single-pole dead time.
Do you have doubts about the reclosing settings of your feeder or line? At Tiza Group we review and configure them remotely. Learn about remote relay configuration or contact us.
Related guides
- Automatic reclosing 79 (RREC)
- Single-pole and three-pole reclosing
- The recloser does not reclose
- Recloser
- Distribution feeder protection
- Fuse, recloser and sectionalizer coordination
- Synchronism check 25
References
- IEEE C37.2-2008, function 79, ac reclosing relay.
- IEEE C37.104, Guide for Automatic Reclosing on AC Distribution and Transmission Lines.
Frequently asked questions
Why are several reclosing attempts allowed on medium voltage but usually only one on transmission?
On distribution, transient faults are frequent and the recloser is coordinated with fuses, so several attempts increase the chance of restoring service. On transmission, reclosing onto a permanent fault is a severe operation for the system and for nearby generators, so a single attempt is the usual practice.
What is single-pole reclosing and why is it typical of transmission?
In single-pole reclosing only the faulted phase opens and recloses. It requires breakers with independent pole operation and a longer dead time because of the secondary arc. It keeps power transfer through the other two phases, which matters on transmission lines.
Should an underground section be reclosed?
Generally not. Cable faults are usually permanent and reclosing makes the damage worse. On a line with overhead and underground sections, logic that tells where the fault occurred is needed.