NBS Engineering Services section V12 - Generator systems – has been extensively revised in Update 2009-3. We look at some of the issues to consider when specifying generator systems.
Fortunately, in the UK, the electrical supply throughout the 14 regional distributors is extremely reliable. The average Customer Minutes Lost (CML) during 2007/8* being less than 76 and, even when taking severe storms into account, only rising to around 83.
Severe storms in the UK are the most common cause of widespread loss of supplies but localized faults can occur at any time due, for example, to equipment failure, damage caused by excavation or vandalism. In some critical situations: hospitals, airports, data centres, etc, mains failure of only a few milliseconds duration is often sufficient to cause severe and often life threatening problems. Since generators often require several seconds to stabilize after being started, uninterruptable power supplies (UPS) are often used for short duration outages (and deal with voltage sags and dips). Generators, once up to speed and producing acceptable power quality, are reserved for longer power interruptions.
When specifying a generator, it is essential to consider the application for which it is intended to be used. BS ISO 8528-1 provides four definitions relating to application: continuous power, prime power, limited time running power and emergency standby power. The majority of generator systems used in the UK are installed for emergency standby purposes and are powered by water cooled diesel engines running at a constant 1500 rpm, driving a three phase alternator.
BS 7698-7 (also numbered BS ISO 8528-7) covers the requirements and parameters for the specification and design of a reciprocating internal combustion engine driven generating set. Some of the key issues to consider include:
- Sizing: To correctly size a generator set, the operating characteristics of the load(s) must be identified. Some load types impose substantial start up currents compared to their running currents (e.g. motors - unless fitted with ‘soft start controls’). Other load types, e.g. welding equipment, x-ray, magnetic resonance imaging or computerized axial tomography, draw high peaks during normal operation. Non sinusoidal load types, e.g. those drawn by IT equipment, uninterruptible power supplies or variable frequency drives, can cause distortion to both voltage and frequency. Specification of voltage and frequency recovery times, staged or full load acceptance may also have a considerable effect on generator sizing and cost.
- Location and service conditions: Whether a generator set is located inside a building or outside (perhaps within a plant room or dedicated enclosure), issues to consider include: mounting arrangements, ventilation, protection from the elements ( risk of flooding), security, fuel/ lubricant leaks, connection to distribution boards and automatic transfer switching equipment, and access for maintenance.
BS ISO 8528-1 requires that specifiers inform generator manufacturers of the prevailing site conditions; specifically, lower and upper levels of ambient air temperature, barometric pressure (or altitude above sea level), and humidity. In addition, specifiers are also required to include information confirming air quality (e.g. dusty or sandy), proximity to the coast (e.g. exposure to salt), whether the environment contains chemical pollution or radiation (including the nature and extent of any pollution), and if there is a requirement to continue to operate under conditions of external shock or vibration. - Fuel selection: For emergency standby applications in the UK, diesel is the most commonly selected fuel. Fuel storage is an issue to consider, particularly given that diesel deteriorates with time, typically lasting around two years. Therefore, storage tanks should be limited in size to the amount of fuel consumed during routine testing during that period of time. Advances in technology mean that modern diesel engines are smaller (for a given power output) and quieter, and produce lower emissions.
- Emissions: In the USA, the Environmental Protection Agency (EPA) publishes categories of allowable emissions (Tier 1-4), whilst in the EU, Directive 2004/26/ EC applies stages II-IV. Each tier or stage specifies lower amounts of four pollutants based on the number of grams per kilowatt-hour emitted from diesel exhaust. Whilst manufacturers continue to press for consistency of approach across world markets, differences exist between national boundaries. Emissions from stationary diesel generator sets, e.g. those used for prime or emergency standby power are not covered by EU legislation. However, some EU member countries apply their own restrictions, e.g. Germany and France, and generator manufacturers supply sets that meet these emission restrictions.
- Noise: Typical sound pressure levels of engines are 100 -110 dBA at 1m. Use of silencers will attenuate noise from the exhaust system, but to reduce noise from other sources (e.g. cooling fan, alternator, and mechanical engine noise) further measures are likely to be necessary. These include careful consideration of the location of the generator set, taking account of prevailing winds, proximity of buildings/ occupants, use of building materials, position of air inlets and outlets, use of anti-vibration mountings and sound attenuating enclosures.
Proprietary or generic specification
The source of funding for a project is likely to have a significant impact upon how a generator system is specified. Those projects funded from public finances (e.g. hospitals), where naming of manufacturers is not permitted, will require a generic specification to be prepared. On privately financed projects this approach is unnecessary, but a proprietary specification may limit competition and, therefore, a generic specification could be useful here too. The key issue is to ensure that any specification is robust and that the completed system when installed meets the original design requirements.
NBS Engineering Services section V12 offers the appropriate clauses together with extensive guidance to aid specifiers in making decisions relating to the selection of products and equipment. The software also facilitates proprietary or generic specification, either of the complete generator system or of the individual components – although care should be taken if considering specifying the latter, since non standard generator sets, although available, come at a substantial cost premium and, usually, have long lead in times relating to production.
* 2007/08 Electricity Distribution Quality of Service Report Ref: 166/08 published by Ofgem
