In short. The UPS protects the instant, the ATS protects the transfer, and the STS protects against source failure to dual-corded loads. They must be coordinated on timing, windows and settings. Most real failures happen at the interfaces, such as a UPS rejecting generator voltage.
Roles
| Device | Role | Speed |
|---|---|---|
| UPS | No-break supply from battery | Instant |
| STS | Fast electronic transfer between two sources | Fraction of a cycle to a few milliseconds |
| ATS | Mechanical transfer between utility and generator | Cycles to seconds |
Coordination points
- Source acceptance: the UPS input window must accept the generator’s voltage and frequency range. Measure at the UPS input during commissioning.
- Transfer timing: the UPS battery must bridge the full sequence from utility failure to generator on load, with margin for a failed start.
- Rectifier walk-in: staged ramp of rectifier load so the generator is not hit with a sudden step.
- Recharge limit: limit battery recharge current when on generator.
- Bypass control: prevent paralleling out-of-sync sources through bypass.
- STS source logic: prefer source, retransfer, and sync window with the sources it connects.
Generator mode in UPS settings
Many UPS have a generator mode that widens input tolerances, limits input current and extends walk-in. Check that it is enabled and set to match the generator’s behaviour. See integrated system studies.
Bypass hazards
Maintenance bypass operations are a high-risk area for human error. Use clear labelling, interlocks and a written sequence. See interlocking.
Integrated tests
- Utility failure with the UPS on battery.
- Generator start, acceptance and UPS transfer to generator.
- Battery recharge on generator.
- Utility return and retransfer.
- Failure of first start or STS source loss.
Record the time stamps. See transfer architecture.
Illustrative example
A data hall UPS rejects the generator input intermittently. The test shows the generator’s frequency overshoots briefly after loading. The UPS input window is widened with the manufacturer’s agreement, the rectifier walk-in is extended to 20 seconds, and the governor is tuned so the overshoot disappears.
Three devices, one load
| Device | Role |
|---|---|
| UPS | Holds load through the interruption, battery-backed |
| Static transfer switch (STS) | Switches the load between two sources in milliseconds |
| ATS | Moves a bus between utility and generator in seconds |
The UPS protects the instant; the generator protects the duration.
Sequence on mains failure (illustrative)
- Mains fails; UPS goes to battery in zero time.
- Generator starts and ATS transfers in 10 to 15 seconds.
- UPS input detects the generator, applies walk-in so its rectifier ramps load gradually over 5 to 30 seconds.
- UPS recharges the battery, increasing generator load.
Interaction hazards
- UPS input current distortion on generator supply.
- Generator voltage and frequency outside the UPS tolerance window.
- Generator unable to take the UPS step plus recharge.
- STS transfer between sources that are out of phase.
Control coordination
Define the permissives: generator in window, walk-in enabled, charging current limited while on generator. The STS should transfer only to a source in sync, or accept a controlled break.
Test
Test the whole chain with real load: mains cut, generator start, transfer, UPS walk-in, return. See UPS and nonlinear loads and hybrid system studies.
The master engineering question
At each handover, does the next device accept what the previous one delivers, and what carries the load if it does not?
What to send us
Send UPS, ATS and generator data. Send your application details.
Questions engineers ask
Is an STS the same as an ATS?
No. An STS is electronic and much faster, for dual-source critical loads.
Why does the UPS keep rejecting the generator?
Voltage or frequency may be outside its window, or distorted.
Should the UPS recharge on generator?
Yes, at a controlled rate so the generator is not overloaded.
Who is responsible for coordination?
The project engineer, with input from each manufacturer.
This guide gives general principles. Follow manufacturer instructions.