Standby Power Generators Guide

Generators provide a source of mechanically derived electrical power and are used an alternative power source to a mains power supply. Generator sizing is dependent upon the use of the generating set within a power protection plan. This may be to provide power to critical uninterruptible power supply protected loads, essential loads and non-essential loads. There are two fundamental ways for a generator to be rated:

  • Prime Power Rating (PPR): where the generator is sized for continuous running as a replacement mains power supply and the most commonly used rating for use with an uninterruptible power supply.
  • Standby Power Rating (SPR): where the generator is sized for short duration runtimes as an alternative to the mains power supply. SPR rated generating sets can be 10% larger than PPR rated generators as they are sized for overload and sudden demand changes.

See the EcoPowerSupplies standby power generator collection for individual product details.

For UPS installations, Prime Power Rating is the preferred choice and the generator must be sized to power the UPS itself including an overload percentage. In addition the UPS must be able so synchronise to the generator supply in terms of its rectifier and static switch inputs, for load acceptance to occur. For some uninterruptible power supplies, generator output frequency can be an issue when synchronising and it is may be necessary to adjust the UPS input frequency window to cope with a fluctuating generator output.

High efficiency uninterruptible power supplies can be matched more closely to a similarly rated generator and the recommended sizing can be 1:125 as a minimum benchmark. This means the generator is 125% the size of the UPS rating. Factors that can increase this sizing ratio include the rectifier input required to charge the battery set, UPS rectifier harmonics and the essential and non-essential loads to be protected by the generator itself. A high ambient temperature, can also be important, leading to the need for to de-rating. For large applications including major datacenters, generators can be installed in parallel configurations to increase the total amount of output power available, and/or to provide additional resilience through redundancy.

Generator Construction

Generators are classified as to the type of fuel they burn. Diesel is the most common fuel-type followed by liquid petroleum gas (LPG). ECO-friendly fuels can include bio-diesel and bio-fuels such as vegetable oil. Large generating sets can be powered from natural gas and this is common in large combined heat and power (CHP) sets within a hospital installation for example. Here the CHP plant allows the hospital to generate its own electrical supply whilst capturing exhaust heat to route through its boiler system; to provide an additional boost to its heating infrastructure. The basic components of a generator include:

  • Engine: diesel generators typically operate at speeds from 750-3000 RPM with most 50Hz generators un at 1500 RPM, and 60Hz at 1800 RPM. A generator governor controls the engineer speed and most sets include an oil lubricating system to ensure moving surfaces are oiled before the crankshaft is turned.
  • Governor: controls the fuel flow and therefore the engine speed. Electronic governors are typically used for turbo-charged generators because of their ability to react rapidly to changes in load.
  • Fuel tank: the most common size is a day tank which is sized to carry fuel for an 8 hour day. Bulk tanks can provide further fuel storage and both can be refilled as required. For Health & Safety reasons fuel tanks should be bunded to prevent leakage into the environment.
  • Alternator: converts the generated mechanical energy into an electrical supply, regulated by a built-in automatic voltage regulator (AVR) to an output to within a set output window (voltage and frequency).
  • Exhaust: an exhaust pipe system will take fumes and gases generated during fuel burn away from the generating set. Exhaust runs should be short and taken to building exit points away from local workforces and populations.
  • Air Handling Systems: generators are normally air-cooled and require a low-temperature, clean and dry source of air for cooling to be effective and non-corrosive.
  • Starter Battery: lead-acid automotive batteries are typically used as they are more suited to the load characterstics (short runtime and high discharge rate). Failed batteries are a common cause of generator start-up failures.
  • Automatic Mains Failure (AMF) Panel: monitors the state of the mains power supply and instructs the generator to start-up or shutdown after a preset time period.
  • Electric Heater Jacket: helps to prevent freezing temperatures from affecting generator performance (optimum thermal efficiency and excessive exhaust ‘soot’) and are typically mains powered.
  • Accoustic Hood: helps to attenuate (reduce) audible noise levels (typically by 10%) within the surrounding site
  • Weatherproof Enclosure: can protect a generator situated outside a building (ground floor, rooftop or remote), with the added advantage of some sound attenuation.

EcoPowerSupplies can provide a complete generator installation and consultancy service including earthing, neutral connections, 24-7 remote monitoring, maintenance and emergency response services.

Please call us on 0800 612 7388, complete an enquiry form form or email us for more information on the EcoPowerSupplies range of standby power generators.