How to pair your UPS and generator

How to pair your UPS and generator

Data centres today invariably use UPS systems to protect their sensitive IT equipment from mains power aberrations and short-term cuts. However, events like the simultaneous failure of two National Grid power stations last August show that longer-term blackouts are also a threat.

Accordingly, protection against both short- and long-term power failures is essential – and the only way to provide this comprehensively is to operate a UPS and a generator as a complementary pair. The UPS shields the load from brief anomalies, while also giving the generator time to start up and come online if a blackout extends towards the UPS battery autonomy time. The generator, once running, can continue for as long as necessary, subject to fuel availability.

Diesel Generator UPS Schematic

During normal operation, power flows through the UPS – assuming it is an online type – to the load, while also keeping the UPS batteries charged. The generator, although powered down, 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 on to 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 10 seconds after mains failure detection. Similarly, the AMF signal usually keeps the generator running for at least two minutes after mains power is restored, to ensure that it is truly stabilised.

To perform successfully as a UPS complement, the generator must

  • Start reliably and quickly on demand
  • Provide the UPS with an AC waveform that is stable in both amplitude and frequency

Reliable generator starting and UPS synchronisation

Generators in standby applications use diesel engines similar to 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 generator engine speed of typically 1500 rpm. However, exact speed 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. Electronic governors are preferred, even though they are more expensive, because they offer better stability and faster response; this ensures that the load can be transferred safely between the UPS and generator output.

Choosing the right protected power system supplier

Correct generator sizing is important, with oversizing usually being advisable. Additionally, generators should be selected according to their continuous rather than standby rating, as this will equip them better for running at any time and for any duration. The generator’s ability to support a step load should also be reviewed, especially if the critical load does not have any soft start facility.

Achieving success with a UPS/generator pair depends not only on an understanding of each machine, but also of how they work together. Therefore, it makes sense to seek solutions from well-resourced, experienced suppliers like Kohler Uninterruptible Power, who can supply and install a complete, integrated, and tested implementation. They can also advise on essential environmental issues, such as complying with regulations governing fuel storage, and managing the generator’s heat and exhaust output.

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