Picking Power: Application considerations for selecting an engine generator
The reliability of an onsite power system depends on the readiness of the engine generator and what it’s connected to. Understanding some basic application considerations will help users specify the right generator.
This article considers only reciprocating engine generators, those using diesel, natural gas or liquefied propane for a piston-type prime mover turning a connected alternator. In general, these generators are 10 kWe and larger.
Required Duty and Electrical Output
The first consideration to correctly specify a generator is to understand how it will be used. There are three duty types. Standby duty is when a generator is to be used in the event and for the duration of a power outage. Prime duty is when the generator supplies power in place of utility power. Continuous duty requires the generator to provide power up to its rated maximum for an unlimited amount of time. Each rating has a different effect on performance and equipment selection.
The next step is to configure the required output frequency (hertz), voltage and amperage.
In regards to frequency, some applications in Canada and the United States are 50 Hz; however, most are 60 Hz. Engine generators are constant speed machines operating at 1,800 rpm (sometimes 3,600 rpm) for 60 Hz and 1,500 rpm for 50 Hz.
Different voltage outputs can be made by alternator selection and connection. Typical low voltages are 120-600V single and three-phase and medium voltages 2,400-13,800V three-phase. There are other frequency and voltage combinations that can be generated but most of these would be for special applications and are not as common.
The total amperage output and load starting characteristics required of the generator is a huge consideration. This determines in large part the physical size of the generator and the cost. Determining the required engine horsepower and correct alternator combination is the process of sizing.
Sizing takes into account the number of loads and load steps required to be run and for how long. It is always best to have the most comprehensive load list prepared and analyzed to correctly size the generator. This load list should include all of the various electrical appurtenances (loads) organized in the order they would be started (load steps). Knowing start sequences for the loads is important. For instance, a generator for a pump house with multiple pump motors (total load) would have to be sized differently to start all the motors at once as opposed to individually (same total load, different load steps). Engine generator manufacturers have sizing software that can correctly size a unit based on this list. Certain loads require special consideration, such as UPSs, variable frequency drives and large motors. The better the load list, the more accurate the sizing.
• Fuel Selection.
The decision about what fuel to use comes down to the application and what is available at the job site.
Natural gas delivered by utility offers an unlimited supply without truck delivery to the site. There are some regional variations in BTU content so be aware of the heat value of the gas. Natural gas can be subject to supply interruption and the possibility of interruption can disqualify the use of natural gas in certain applications.
Liquefied propane (LP) gas is often used in a locale where utility source gas is not available, or prohibited from use. LP must be stored onsite so truck delivery is required, but it transports and stores well. There can be issues vaporizing enough gas off the top of the storage tank to fuel the engine in cold weather. This requires the LP be sent to the engine as a liquid to be vaporized just before entering the combustion stream.
Diesel, the most popular choice for standby duty, is a good reliable fuel for engine generators. It does require on-site storage, how much to store depends on how many hours of operation at load are required before refuel is available. Diesel does not store indefinitely: two years is its approximate storage life before it starts to settle or separate. Diesel can be susceptible to gelling in very cold temperatures. Winter grades, or fuel heaters, are available for cold climates; in tropical areas, a microbicide may be needed.
• Environmental Considerations.
Job site temperatures and elevation must be considered for generator selection. There should be provision to address any harsh conditions, such as temperature extremes, dust or dirt, humidity, sea air or corrosive environments. Consider if the job site is in a high-wind area or subject to heavy snow loads. Seismic certifications are required in certain areas.
Generators can either be located indoors in a specially designed generator room or outdoors in their own enclosure. The enclosure design should provide protection from the elements and unauthorized entry. Consider noise from the unit and proximity to distribution switchboards and transfer switches. Ensure engine exhaust will disperse away from other building openings and vents.
Generator room applications require adequate space for service access and clearances required by electrical code. There must be adequate airflow for combustion and cooling. Vibration isolators should be installed between the generator base and floor.
Outdoor enclosures are typically made of sheet metal, steel or aluminum. They can be skintight or walk in with space to allow personnel entry. The structure of the enclosure can be designed to attenuate sound. Many provisions to address harsh environments are by enclosure design and options.
No other concern about engine generators is more stringently regulated than emissions. Both Canadian and U.S. environmental protection agencies set standards for allowable emissions from diesel and gaseous generators. Standards are imposed based on engine horsepower and application. There can be significant legal and monetary penalties for violating standards. Keep in mind there can also be regional or local standards that exceed federal standards. A professional should be consulted to make sure all regulations are known, the equipment being supplied is legal for the application and permitting processes are followed.
• Standards of Safety and Performance.
The International Organization for Standardization (ISO) has relevant standards for generators in regards to ratings and performance, as well as standards for the manufacturing process. Underwriters Laboratories (UL) and Underwriters Laboratories of Canada (CUL) set standards and makes a listing available for demonstrated product safety and integrity. The National Fire Protection Association (NFPA) sets fire prevention standards, establishes safety and performance standards for transfer switches and addresses the performance of emergency standby generators in critical applications. Some regions are subject to the provisions of the International Building Code (IBC) that require the generator to be designed and tested to operate after an earthquake and to withstand high wind loads. Additional sources of standards include the Canadian Electrical Code and US National Electrical Code, the Canadian Standards Association and CE mark. These are common for generator professionals to understand and your chosen manufacturer should be knowledgeable about how they apply.
Find a Professional
Good distributors and manufacturers welcome the opportunity to help you. Be prepared to discuss the duty type. Share your load list and particulars of your job site. Let them know fuel preference and how many hours you intend for the generator to run. Determine if the generator is intended for indoor or outdoor installation. Find out if there are noise restrictions and absolutely understand emissions regulations and permitting processes prior to purchasing any equipment. This should give you a good start on selecting the right generator.
Rob Hilkemeier is the Western regional sales manager for Baldor Generators. For more information, visit www.baldor.com.