Automatic service restoration is the logic that, after a total loss of voltage coming from the grid, opens the substation or plant breakers on undervoltage and, when the voltage returns and is stable, closes them again in stages and by priority. It does not act on circuits that tripped for a fault of their own. Unlike reclosing 79, it does not respond to a protection trip but to the loss of the source.
What automatic service restoration is
When the network feeding a substation or a plant goes down, the voltage drops to zero on all its buses. What happens when the voltage returns depends on the state the installation was left in. If all the breakers stay closed, the whole load is energized in one go. If they are left open without any logic, someone has to close them one by one, sometimes from a distant control room.
Automatic restoration is the logic that handles that moment: it prepares the installation during the loss of voltage and returns it to service in a defined order when the source comes back. In IEEE C37.2 terms it relies on undervoltage 27, on breaker close control and, when there are several sources, on device 83, automatic selective control or transfer.
How the logic works
- 01Loss of voltageUndervoltage on the bus, sustained longer than normal dips
- 02Undervoltage openingThe load breakers enabled for restoration open
- 03Waiting for stable voltageThe source voltage returns and stays within range
- 04Staggered closingCircuits close by priority, with a pause between stages
- 05End of sequenceCircuits with a fault or in manual control stay open and are indicated
Detecting the loss of voltage. The undervoltage must last longer than the dips cleared by network protection and longer than the dead time of upstream reclosing. If the logic acts during a dip, it causes an interruption that was not needed.
Undervoltage opening. The enabled load breakers open after the set time. In plants, large motor starters are usually opened too, since they will need their own restart sequence.
Waiting for stable voltage. When the voltage returns, the logic does not close immediately: it waits for the voltage to stay within range long enough to rule out a momentary return, such as an upstream reclose that trips again.
Staggered closing. Circuits close in groups, by priority. Essential loads or station service first; the rest afterwards. The pause between stages lets the inrush current of each group die away and the voltage recover.
Essential blocking
- Protection trip: a breaker that opened on overcurrent, differential or another protection is not restored. If the fault is still there, closing would feed it again.
- Manual opening: what the operator opened on purpose does not close by itself.
- Local control or breaker not available: racked out, springs discharged, low pressure.
- Insufficient source: if the voltage returns from a backup source of smaller capacity, such as an emergency generator, the logic limits which stages it restores.
- Voltage circuit failure: an open VT fuse looks like a loss of voltage and must not start the sequence.
How it differs from reclosing and load shedding
| Logic | What starts it | What it acts on |
|---|---|---|
| Automatic reclosing 79 | Trip of its own protection | The same breaker, to test whether the fault was transient |
| Automatic restoration | Loss of source voltage | Breakers opened by undervoltage, with no fault of their own |
| Underfrequency load shedding | Generation deficit on the grid | Loads disconnected to save the system |
All three can coexist. A load shed on underfrequency, for example, should not be restored by this logic, but when the system operator authorizes it. That coordination is part of the design.
Cold load pickup
After a long outage, loads lose their diversity: thermostats, compressors and HVAC equipment all start at once and the current can exceed normal demand for minutes. That cold load pickup can operate the feeder overcurrent protection. Besides staggered closing, many feeder relays have a cold load function that temporarily raises their pickup. Trips on energization are reviewed in overcurrent trips on starting or energization.
Where it is used
- Unmanned distribution substations, fed by one or two lines, where manual restoration would take too long.
- Industrial plants, where restoration is coordinated with motor restart and the process sequence, and where a voltage dip must not be mistaken for an outage.
- Station service of substations and power plants, combined with automatic transfer of sources.
- Utility networks that apply it in distribution substations with their own criteria; where the utility has a restoration procedure, the logic must follow it.
What goes wrong in practice
- Restoration during a dip: undervoltage time delay shorter than the grid’s reclosing.
- Closing onto a fault: the logic does not tell the breaker that tripped on protection from the one that opened on undervoltage.
- Everything closes at once: stages without enough pause, and the incoming supply trips on inrush current.
- Nothing is restored because the source voltage never enters the set range or because a selector was left in manual.
- Loads shed on underfrequency are restored without the system operator’s authorization.
How it is tested
In general terms, secondary injection simulates the loss of voltage, dips and the return of voltage, and each time delay, each stage and each block is checked: a circuit with a fault, opened by hand, in local or with the VT open. Where possible, the final test is done with a real loss of source during a window agreed with the utility.
We can help with your restoration logic
At Tiza Group we do the secondary engineering of the restoration logic in bay or station controllers or PLCs, and the remote relay configuration of its time delays and blocking, for substations and industry.
Related guides
- Undervoltage protection 27
- Automatic reclosing 79
- Undervoltage load shedding (UVLS)
- Medium-voltage automatic transfer: main-tie-main logic
- FLISR: fault location, isolation and service restoration
- Automatic switching sequences in substations
- Substation interlocking: types, levels and logic
References
- IEEE C37.2, Standard for Electrical Power System Device Function Numbers, Acronyms, and Contact Designations (devices 27, 79, 83).
- IEEE C37.104-2022, Guide for Automatic Reclosing on AC Distribution and Transmission Lines.
- IEEE Std 242 (Buff Book): recommended practice for protection and coordination of industrial and commercial power systems.
- IEC 61850-7-4, Compatible logical node classes and data object classes (PTUV, CSWI, XCBR).
Frequently asked questions
Why open the breakers if the voltage is going to come back?
Because if they stay closed, when the voltage returns the whole load is energized at once: transformers with their inrush current, motors starting and loads that have lost their diversity. That cold load pickup can trip the incoming supply or pull the voltage down. Opening and closing in stages spreads the duty.
How does it differ from automatic reclosing?
Reclosing 79 closes again the same breaker that its own protection opened, to test whether the fault was transient. Restoration acts on breakers that opened for lack of voltage, with no fault of their own, and closes them when the source returns. Both can coexist in the same substation, with cross-blocking.
What happens to a feeder that had a fault when the voltage was lost?
If its protection tripped, the logic excludes it from restoration and leaves it open for the operator to inspect. That is why restoration must tell a breaker opened by undervoltage from one opened by protection, by manual command or by other logic.