Choosing the right generator

This post looks at why your UPS may need a generator, and how to set up a matched generator-UPS pair.

Online UPS ensure power quality while the mains supply is available, and provide emergency backup from their energy storage resource when it isn’t. One inevitable problem with energy storage, typically provided by lead-acid VRLA batteries, is that its capacity is always finite. While extra battery banks can increase capacity, the risk of an extended blackout exceeding this cannot be eliminated.

Accordingly, most data centres running business-critical applications, where even a graceful shutdown is not an option, install generators to complement their UPS. Unlike batteries, generators can support the critical load through any length of blackout, provided they are adequately fuelled and maintained. However, there is a range of electrical and mechanical factors to consider when pairing a generator and UPS within a secure power protection system.

Within generators used for standby applications, a diesel engine uses fuel to create mechanical energy, which an alternator converts into a single-phase (230V) or three-phase (400V) voltage. The engine, which is similar to those found in large goods vehicles, must be well-maintained, with an adequate fuel supply and a healthy battery for starting. Oil and coolant levels must also be maintained.

The value of the alternator’s output voltage depends on how it is wound, while its precise amplitude and stability is controlled by an automatic voltage regulator (AVR). Voltage frequency – usually 50 Hz in the UK – is set by engine speed, which in turn is controlled by a governor that regulates fuel flow. Governors can be mechanical or electronic; mechanical types are lower cost, but are less responsive and provide less stable engine speed and frequency regulation. In contrast, electronic regulators are highly responsive with very stable engine speed regulation, so are almost always used within UPS backup generators.

If a blackout becomes extended, the generator must be signalled, allowing it to automatically start, stabilise and take over the load within the remaining UPS battery autonomy. The generator must also be instructed to stop when utility mains power is restored. These start and stop signals are supplied by the generator’s Automatic Mains Failure (AMF) detection panel.

However, to avoid the AMF panel starting the generator on every mains disturbance, it is usually set to operate only after the mains supply has failed for a significant time, usually 2 to 10 seconds. It is equally important not to shut down the generator immediately after the mains is restored; the reconnection may be part of the utility company’s fault location procedure or the result of an automatic breaker operation. The fault may still exist, causing another disconnection almost immediately.

Accordingly, most AMF controlled generators start within 10 seconds of a utility power failure, and continue running for at least two minutes after mains power is restored.

The generator should be prepared to ensure that it responds as quickly as possible to any start signal. The engine should be kept warm with a mains-powered engine water heater (sometimes called a jacket heater), while a trickle charger should keep the battery fully charged.

Compatibility between the generator and UPS is also important. The generator’s frequency range may exceed the UPS’s capabilities and, in the worst case, UPS synchronisation may not be possible. This could be either because the generator frequency is outside limits or it is varying too quickly for the UPS to follow without subjecting the load to risk.

The safest, most reliable and ultimately most cost-effective way of specifying and achieving a well-matched generator/UPS pair is to discuss your requirements with a supplier that has the experience and the products to assemble the right package. UPSL, for example, supply both UPSs and generators, and is well placed to bring them together into integrated solutions with assured interoperability. UPSL can also advise on related issues such as generator sizing and neutral conductor switching.


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