Using a UPS with a Generator

When a UPS and a generator are used together, the generator supply must be compatible with the UPS. If the generator frequency fluctuates outside the UPS tolerance, the UPS switches back to battery; when the battery autonomy (for example 5 minutes) runs out, the loads shut down even though the generator is running.

For this reason, it is not sufficient for the UPS and the generator to be suitable individually. A UPS covers the outage in zero seconds, but its battery autonomy is limited. A generator supplies power for hours, but typically takes 10–15 seconds to come online. If the two are going to work together, the generator must be sized for the UPS and the settings must be matched to each other.

How does the system work?

1. The mains fails. The UPS switches to battery without delay.
2. The ATS panel detects the outage and starts the generator.
3. The generator settles on voltage and frequency. The ATS transfers the load to the generator.
4. The UPS rectifier gradually moves onto the generator supply. Battery charging begins.
5. When the mains returns, the ATS transfers the load back to the mains. The UPS carries on without interruption.

How much battery autonomy is needed?

In theory, a few seconds are enough. In practice, they are not. The generator’s first start attempt may fail; the control panel makes a second and third attempt. If an alarm comes up, someone has to intervene. That is why projects usually specify battery autonomy of a few minutes up to 10–15 minutes. In critical facilities, the target time is written into the project specification.

What kVA generator does a UPS need?

For a generator, a UPS is not an ordinary load. The calculation includes:

• The load the UPS supplies.
• UPS losses: rectifier and inverter efficiency.
• Battery charging power: After an outage, while the discharged batteries are recharging, the UPS input rises noticeably. This is the item most often forgotten.
• Harmonics: The rectifier draws non-linear current. The alternator’s impedance is higher than the grid’s. The same harmonic current creates more voltage distortion on a generator.

Widely accepted approximate ratios by rectifier type:

UPS rectifier type	Input current harmonics (THDi, typical)	Generator / UPS rating (approximate)
IGBT (active) rectifier	< 5%	1.25–1.5 ×
12-pulse thyristor + filter	~10%	~1.5 ×
6-pulse thyristor, no filter	~25–35%	2 × or more

Example: for a 200 kVA UPS with an IGBT rectifier, a generator of approximately 250–300 kVA is considered at the preliminary selection stage. For an older 6-pulse UPS of the same rating, this figure can rise to 400 kVA and above. The ratios are for preliminary selection; the final decision is made with the UPS and generator manufacturers’ data. Loads other than the UPS (air conditioning, lighting, pumps) are added separately; see what kVA generator do I need and the power calculator.

Common problems on site

The UPS drops to battery on the generator. When load is applied to the generator, frequency dips briefly. If the UPS input tolerance is set narrow, it treats this as a “bad mains” and goes back to battery. Solution: widen the UPS input frequency tolerance and bypass synchronization window to suit the generator, and use an electronic speed governor on the generator. This is the most common cause of the scenario at the start of this article.

The whole load comes on at once. If the UPS rectifier takes the load in one go, a sharp voltage and frequency dip occurs on the generator. Solution: have the rectifier take the load gradually over a few seconds (walk-in / soft start). If there are several UPSs, stagger their transfers a few seconds apart.

Voltage wanders at low load. The input filters of some UPSs produce capacitive (leading) reactive power at low load. Alternators have limited capability with capacitive loads; the AVR may become unstable. Solution: switch the filters in together with the load, and check the alternator’s capability curve.

The voltage waveform is distorted. With harmonic-heavy loads, alternator selection matters. Permanent magnet excitation (PMG) or an auxiliary excitation system helps the AVR run stably with these loads.

Test: simulate the outage

A UPS–generator system is only proven by a real outage. Running the generator off-load, or doing the UPS battery test separately, is not enough. The recommended approach: at a suitable time, open the mains breaker and follow the chain from start to finish. Did the UPS switch to battery? How many seconds did the ATS take to transfer? Did the UPS accept the generator supply, or did it stay on battery? What happened to the generator load when battery charging started? Was the retransfer smooth when the mains returned? If the answer to any of these questions is “we don’t know”, the system has not been tested yet. During the test, voltage, frequency and UPS alarm logs should be recorded.

Where does this come up?

• Data centres: UPS for the servers, usually redundant generators for long-duration supply.
• Hospitals: UPS for medical equipment and information systems, generator for the whole facility.
• Telecom, banking, industrial control and security systems.

Checklist

• Are the UPS rectifier type and input THDi value known?
• Has battery charging power been added to the generator calculation?
• Is the battery autonomy comfortably longer than the generator’s time to come online?
• Have the UPS input frequency tolerance and bypass window been set?
• Is the rectifier walk-in feature enabled?
• Has the switch-on of multiple UPSs been staggered?
• Has leading power factor at low load been checked?
• Has the system been tested with a real outage by opening the mains breaker?

Next step

A UPS and a generator can be matched with confidence when the data from both sides is assessed together. Share your UPS model and load information with the Terex Diesel team, and we will determine the right generator rating together: contact us.