The Association of Manufacturers and suppliers of Power Systems and ancillary equipment

10 points to consider when specifying a generating set

1. What is the application?

What kind of generating set is required? What type do you need?

There are many, and clarity at the outset helps you look in the right places, and saves you time. Is it for mobile use, or permanently installed? Is it for standby use as a back up to the mains, or will it be the prime source of electricity? Is it to be used occasionally or constantly in use? Residential or commercial, and so on, these are all considerations that assist you with identifying the right supplier and finding the right solution.


2. What power is required?

Knowing the application is a starting point, but equally to specify any generating set can only be done by establishing how much power that it (or they) are required to provide. There are many ways of calculating power requirements, from simply adding up the connected load, to calculating its diversity of use over time, to considerations of the maximum demand.

You should consider power in terms of both real (KWe) and apparent (KVA) power, the difference is known as the power factor* and generating sets are specified by convention at 0.8pf – so a 500KVA Generating Set may only provide 400KWe of power. In considering the power, you should also consider the way in which the power is required.

Generating sets can only provide a portion of their rating in a single go (known as a load step) – if you require a lot of power all at one step, this may affect the size of the generating set or you may find it more economic to consider staging the power demand. Some equipment – e.g. motors and pumps – also have differing loads when they are first started (a typical motor may present a load six times of its running current and two and a half times its running power); this must also be considered.


3. What performance characteristics are required?

Electrical equipment is sensitive to changes in supply, both in terms of voltage and frequency. When specifying a generating set you need to know how sensitive your equipment is and what disturbance level it can withstand. Generating sets are supplied at a number of performance classes, which define their stability for voltage and frequency under steady load conditions and also under conditions of changing load.


4. What rating is required?

Engines have different power ratings depending on how they are to be used. An engine will have a different rating if it is to be used constantly rather than if it is to be used only occasionally in emergency situations. The basic ratings are known as COP (Continuous Operating Power), PRP (Prime Rated Power), LTP (Limited Time Power) and ESP (Emergency Stand-by Power). The difference between these four ratings for an engine can be as high as 30% of the engine rating. It is therefore important that you select the right rating for your application. 


5. What fuel is the generating set to use?

Generating sets can be powered by a wide variety of fuels including Diesel, BioDiesel, Petrol, Natural Gas and LPG. The most common fuel for standby power is diesel, owing principally to storage stability, cost and availability. The  type of fuel determines the type of engine. N.B. Particular care must be taken with BioDiesel or BioDiesel-blends as it can degrade in storage and cause the generating set to fail. Special fuel cleaning or ‘polishing’ may be considered.


6. What are the ambient conditions?

The ambient temperature, and altitude and even humidity in coastal locations, will affect the performance of a generating set and its constituent components. 

High temperatures and altitudes can limit power whilst low temperatures can result in difficulties starting. The full range of ambient conditions over which the generating set is expected to operate must be taken into consideration.


7. What autonomy is the generating set required to have?

The autonomy of a generating set is how long it is expected to be able to run, with or without human intervention. It is normally expressed in hours against load (e.g. the generating set must run for 72 hours at Full Load). This affects the size of fuel storage (and therefore the physical size of the generating set as a whole) and other aspects such as cooling and lubricating oil systems.


8. What standards or regulations is the generating set required to comply with?

A key consideration for any generating set is what standards it is required to comply with. For generating sets being supplied into the EU, many of these will be product-based and encompassed under the requirements of CE Marking, such as the Machinery Directive, Low Voltage Directive, and more. In addition it is important to know of others which are specific to an application or locale-based, such as emissions, noise or fuel storage regulations, especially where generating sets are to parallel (synchronize) with the main utility.


9. What are the requirements of the physical environment that the generating set is to go in?

Generating sets have many inherent hazards. It is therefore important that where they are to be sited or installed is considered. Generating sets may increase the fire risks, due to presence of heat and fuels – how will that be managed? Building Regulations give much guidance on installation of generating sets in buildings, and state what must be achieved; indeed planning permission may be required. They cause environmental risks due to the stored fluids, fuel, coolant etc. How will this be managed? 

In the UK, the Pollution Prevention Guidelines give much guidance on this subject. Generating sets produce hot exhaust gases – where will these be discharged to prevent risk to people or the environment? They are very noisy – what noise can be accepted on site or remote from the site? Much legislation applies to this both in the UK and across Europe. They require access for maintenance and fuel deliveries etc. – how will this be managed?


10. How is the generating set to electrically interface with the site?

The generating set has to be connected to the physical equipment it is to support. What are the basic requirements of this – what voltage, what frequency? Will the interface be automatic, manually operated, and will there be paralleling with other sources of generation? Another generating set or the mains? Regardless it is also essential that the electrical protection of the generating set, the equipment it is connected to and people using it are considered and appropriately provided for.

It is essential that all electrical work is carried out by suitably qualified personnel. It will involve determining what protective devices are required, together with arrangements for protective earthing, neutral earth referencing. When supplies from different sources are present (e.g. a generating set and the mains supply); they can never be allowed to operate in parallel to a piece of equipment without appropriate switching equipment permitting them to do so.

Our member companies join AMPS for many reasons but commitment to the Trade Association is a good indication that they are also committed to the industry and their reputation. If you need advice, information or suppliers then approaching AMPS is probably your best first step.


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