In short. After an outage or a transfer, switching every load back at once can overload the generator and cause a second blackout. Restoration should bring loads on in stages, largest or most critical first where appropriate, with delays so voltage and frequency settle between steps.
Why restart needs design
Loads draw more at start than in running: motors may pull five to seven times full-load current, transformers draw inrush, UPS rectifiers and chargers draw recharge current, and refrigeration and cooling loads restart together. The sum can far exceed the generator’s step capacity.
What sets the stage size
| Constraint | Meaning |
|---|---|
| Generator step capability | Maximum block load that keeps voltage and frequency within limits |
| Voltage dip limit | Contactors and sensitive loads must not drop out |
| Frequency recovery | Engine speed and governor recovery time |
| Transformer inrush | Several times rated current for a few cycles |
| Process dependencies | Some loads must start before others |
See load-step screening and motor-starting capacity.
Building the sequence
- Group loads into steps no larger than the generator can accept.
- Order steps by priority and dependency: controls and cooling before process loads, for example.
- Insert delays that let voltage and frequency recover, typically several seconds per step, to be confirmed on site.
- Use permissive signals where possible: the next step starts when the generator reports stable output.
- Limit the largest motor starts using soft starters or drives.
Special cases
- UPS recharge: use rectifier walk-in and charge current limiting.
- Chillers and compressors: restart delays to protect the equipment itself.
- Transformers: stagger energisation.
- Retransfer to utility: avoid a second restart storm.
Automatic versus manual
Automatic sequencing is faster and repeatable, and suits unattended sites. Manual restoration lets operators judge conditions but depends on staffing and training. Some sites automate the first tiers and leave the rest to operators. See automatic restoration control.
Worked illustration
A generator of 800 kVA must restore 600 kW of load including a 150 kW pump with direct-on-line starting. If all loads return together, the pump’s starting current of about six times its full-load current coincides with transformer inrush and UPS recharge, and the generator voltage collapses. A sequence is created: step 1, control and lighting at 0 s; step 2, cooling fans at 10 s; step 3, the 150 kW pump on a soft starter at 20 s; step 4, UPS rectifier with a 30-second walk-in at 30 s; step 5, deferrable loads at 60 s. Each step is within the generator’s capability.
Verification
- Test with the real loads and record voltage and frequency at each step.
- Adjust step size and delay from measurements.
- Check the behaviour if the generator is already at high load.
Restart is a design problem
When power returns, chillers, compressors, pumps and UPS rectifiers all want to start at once. Their combined inrush can exceed the source and cause another outage. Sequencing spreads the demand.
| Step | Load group | Typical delay |
|---|---|---|
| 1 | Controls, lighting, IT, monitoring | Immediate |
| 2 | UPS input (with walk-in) | 5 to 15 s |
| 3 | Cooling circulation pumps and fans | 10 to 30 s |
| 4 | Chillers one by one | 30 to 60 s each |
| 5 | Non-critical loads | Last, when source loading is stable |
Constraints on restart
- Compressors and chillers need a minimum off time and pressure equalisation before restart.
- Motors with high inertia need the supply to be stable.
- Generators accept limited step sizes, so sequence in blocks.
Check each step
- Is the voltage and frequency within limits before the next step?
- Is generator loading below the allowed block?
- Has the previous load reached steady state?
When mains returns
After transfer back, the load transient repeats. Re-sequence rather than closing all loads at once.
See automatic restoration and motor starting.
Common mistakes
- Simultaneous restart after an outage.
- Fixed delays not matched to the actual recovery time.
- Not considering the sequence after a retransfer.
What to send us
Send the load list, motor sizes and start methods. Send your application details. See also load shedding.
Questions engineers ask
Why did the generator trip after power came back?
The combined restart load of motors, transformers and chargers likely exceeded its step capability. Sequencing fixes this.
How long should delays between steps be?
Long enough for voltage and frequency to recover. Determine this by testing, not by a fixed rule.
Does a soft starter help?
Yes. It reduces starting current and the voltage dip, so larger motors can be added in fewer steps.
Should restoration be automatic?
For unattended and critical sites, usually yes, with an operator override.
This guide gives general principles. Sequences must be tested on the real system.