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How to choose a UPS standby generator

Image: Adobe Stock / Connect world

Data centres with critical loads need protection from longer-duration blackouts along with short-term power aberrations and cuts, providing protection to this level calls for a generator, as well as a UPS.

To date, the UK has had a reasonably robust national grid for power distribution, backed up by access to power from continental Europe. However, that doesn’t mean power cuts in the UK are impossible, nor are they unknown. 

For example, in November 2016, London’s West End was blacked out for several hours, while in March 2018, the Harbour Exchange data centre in London experienced a long-duration power cut. This impacted connected cloud service providers and communications companies with services remaining unavailable or degraded throughout the day. 

These events could become more common, however. It has been reported that Great Britain faces its greatest risk of blackouts for six years this winter, as old coal plants and nuclear reactors shut down and energy demand rises as the economy emerges from Covid-19 restrictions.

Incidents like these, coupled with concerns over the loss of aging nuclear and coal-fired power station capacity, motivate data centre operators to protect themselves against longer-term blackouts, as well as mains power aberrations and failures lasting milliseconds or seconds. Such protection must come from a UPS-generator pair, rather than just a UPS.

The generator and UPS as a pair

The correct set up of a generator/UPS pair starts with understanding the two components’ complementary roles. During normal operation, the generator is powered down, although maintained and ready to start immediately on demand; it is described as a standby generator. Meanwhile, the utility mains supply flows through the UPS to the critical load. The UPS handles any short-term power aberrations or dropouts. However, there is always the possibility of a power blackout that lasts longer than the battery autonomy, irrespective of how much energy storage capacity is provided.

Accordingly, the generator is connected to an Automatic Mains Failure (AMF) detection panel, which initiates a generator start-up if a power cut becomes critically extended. Once the generator output has stabilised, it is switched onto the essential parts of the facility’s load, including the UPS. While the AMF should start the generator early enough to allow for this stabilisation, it should not signal a start during every minor supply disturbance. To avoid this, the AMF signal is typically delayed for two to ten seconds after mains failure detection.

It’s equally important to avoid switching the load back to the mains prematurely. If mains power re-appears, it may be part of the utility company’s fault location procedure or because of an automatic breaker operation. The fault may still exist, causing the supply to disconnect again almost immediately. To prevent these false starts, most AMF-controlled generators will continue to deliver power for at least two minutes after the mains supply is restored.

AMF support is essential – but generators need two further features to fulfil their UPS standby function. The first concerns their ability to start reliably and quickly on demand, and the second is about supplying the UPS with an AC waveform that’s stable in both amplitude and frequency.

Generators in standby applications use diesel engines like those found in large lorries. To ensure reliable start-up, they should be well maintained, with an adequate fuel supply and a healthy battery for starting. Sufficient coolant and oil are also essential. The generators can be kept warm by mains-powered engine water heaters, (or jacket heaters); a mains-powered battery charger is used to trickle-charge the starter battery.

The diesel engine drives an alternator that converts its mechanical power into electricity. In the UK, this is usually single-phase 230 V or three-phase 400 V. The voltage amplitude is set by how the alternator is wound, while its stability is controlled by an Automatic Voltage Regulator (AVR). The alternator’s output voltage frequency – usually 50 Hz in the UK – is determined by the engine speed. Typically, generators have an engine speed of 1500 rpm that equates to 50 Hz electrical output frequency, although this depends on the alternator design. In any case, frequency stability, which is essential for UPS synchronisation, is assured by a mechanical or electronic engine governor that regulates the engine’s fuel feed.

Mechanical governors use springs and spinning weights to regulate fuel flow. They are lower-cost than electronic types, but they are less responsive and provide less stable engine speed – and therefore voltage frequency – regulation. Electronic governors regulate by counting the teeth on the alternator flywheel as it rotates. This approach is highly responsive and provides very stable engine speed regulation; accordingly, it is nearly always used by UPS standby generators.

Without this stability, the generator’s frequency range or slew rate may be wider than the UPS can accept. In the worst case, the UPS will set an alarm signal to warn that it will not transfer the load from the UPS to the raw generator output, in the event of a fault.

Such situations can be avoided by using electronic governors, ensuring that the generator is designed specifically for UPS support and thoroughly tested during commissioning.

Sizing and environmental considerations

Correct generator sizing is important, with oversizing usually being advisable. In most facilities, the generator must support air conditioning and emergency power, as well as the UPS load. As an approximate guideline, 1.5 x nominal UPS capacity should be allowed for transformerless UPS’; this rises to 2 x nominal capacity for transformer-based UPS’; and 3 x nominal capacity for air conditioning. Technology of engines and the load has evolved and it is more important than ever to discuss the specific requirements with the supplier when sizing the installation.

Another sizing recommendation is to select generators according to their continuous rather than standby rating, as this will equip them better for running at any time and for any duration. Additionally, the generator’s ability to support a step load should be reviewed, especially if the critical load does not have any soft start facility.

The generator, once selected, must be installed into its environment correctly. Most standby generators use a base tank which is double-bunded to ensure capture of any spilt fuel. Acoustic noise is another form of pollution, especially if the generator runs at night, but acoustic housings, with various noise attenuation ratings and costs, are available to mitigate this issue.

When running, generators create considerable heat and exhaust fume volumes. Usual practice is to install them outdoors in weatherproof and acoustic enclosures, to ensure sufficient airflow for cooling – it also simplifies the venting of exhaust fumes.

The generator must be installed on a flat, level surface such as a purpose-built concrete slab. It should also be positioned as close as possible to the AMF panel and/or the essential services board, to minimise the required power and signal cable lengths.


Most data centres with critical loads regard generators as essential complements to UPS’ within their power protection systems; they cannot afford to shut down during a power failure, even if the UPS allows them to do so gracefully. This article has shown how generators can fulfil their role successfully, if they are well-prepared, carefully matched to the UPS, and installed into their environment with sufficient care.

Picture of Alex Emms
Alex Emms
Technical Director at Kohler Uninterruptible Power

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