### Generators Sizing Calculations – Part Three

 Subject of Previous Article Article Glossary of Generators – Part One Glossary of Generators – Part Two

Today, we will start explaining the Generator Sizing Calculations.

 Generator Sizing Calculations

 Note: These articles will explain the design of stationary generators. They will not cover the design of stationary–designed commercial generators used as mobile generators, which are generally considered to be an unintended application.

 First: Reasons for having on-site generators

 There are Mandatory reasons for having generators in many installations as follows: A- To meet building code requirements: Loss of the normal power supply would introduce life safety or health hazards B- To meet economic requirements: To mitigate of the risk of loss of services, data, or other valuable assets due to loss of electric power To reduce load rates offered by the electric utility C- To meet the loads’ specific requirements: A wide range of specific requirements will result in the need for on–site electric generators. Some common needs are: Lighting: Egress lighting for evacuation, illuminated exit signs, security lighting, warning lights, operating room lighting, elevator car lighting, generator room lighting, etc. Control Power: Control power for boilers, air compressors, and other equipment with critical functions. Transportation: Elevators for fire department use. Mechanical Systems: Smoke control and pressurization fans, waste water treatment, etc. Heating: Critical process heat, Refrigeration: Blood banks, food storage, etc. Production: Critical process power for laboratories, pharmaceutical production processes, etc. Space Conditioning: Cooling for computer equipment rooms, cooling and heating for vulnerable people, ventilation of hazardous atmospheres, ventilation of pollutants or biological contamination, etc. Fire Protection: Fire pumps, jockey pumps, alarm and annunciation, Data Processing: UPS systems and cooling to prevent data loss, memory loss, program corruption. Life Support: Hospitals, nursing homes, and other health care facilities. Communications Systems: 911 service, police and fire stations, hi–rise building public address systems, etc. Signal Systems: Railroad, ship, and air traffic control.

 Second: Applicable performance standards for generator sets

 Applicable performance standards for generator sets include: International Electrotechnical Committee: Standard for Rotating Electrical Machines, Part 1 (Rating and Performance), IEC 34–1. International Standards Organization: Standard for Reciprocating Internal Combustion Engine Driven Alternating Current Generator Sets, Parts 1 through 9, ISO 8528. National Electrical Manufacturer’s Association: Standard for Motors and Generators, NEMA MG1–1. Canadian Standards Association: CSA 22, Canadian Electrical Code. CSA 282, Emergency Electrical Power Supply for Buildings. Underwriters Laboratories: UL 2200 Stationary Engine Generator Assemblies. In North America, many safety (and environmental) issues related to generator set applications are addressed by the following standards of the National Fire Protection Association (NFPA): Flammable and Combustible Liquids Code–NFPA 30 Standard for the Installation and use of Stationary Combustion Engines and Gas Turbines– NFPA 37 National Fuel Gas Code–NFPA 54 Storage and Handling of Liquified Petroleum Gas—NFPA 58 National Electrical Code–NFPA 70 Health Care Facilities Code–NFPA 99 Life Safety Code–NFPA 101 Emergency and Standby Power Systems–NFPA 110

 Third: Selection Factors Used For Generators Sizing Calculations

 Here we will describe preliminary factors for selecting a generator for certain project, which will be as follows: Generator Power Ratings, Application type, Location Considerations, Fuel Selection Considerations, Site Considerations, Environmental Considerations, System Voltage and Phase, Acceptable percent of voltage & frequency dip, Acceptable duration of the voltage & frequency dip, Percent and type of loads to be connected, Load step sequencing, Future needs.

 1- Generator Power Ratings

There are four different types of ratings that must be considered when specifying a generator set for an application:
1- Industry Standard ratings (ISO-8528-1:2005)
This is an industry standard that defines the performance parameters required in various onsite power applications.
ISO-8528-1:2005 defines basic generator set rating categories based on four operational categories:
• Emergency standby,
• Prime power,
• Limited time prime,
• Continuous power.
2- Manufacturers’ ratings
Most manufacturers follow the ratings detailed in ISO-8528-1. However, some manufacturers have slight exceptions to these ratings in order to best fit their customer requirements, performance capabilities, or maintenance schedules.
3- EPA emission ratings (Governmental regulations on engine Emissions)
The U.S. Environmental Protection Agency has environmental ratings and regulations for generator sets that vary by drive-engine horsepower and application.
The EPA began to enforce limits on off-road engines use in generator sets in 2006 and began phasing in regulations by Tier levels and engine type. Some definitions one needs to be aware of are the differences between:
• Stationary emergency,
• Stationary nonemergency,
• Mobile generator set engines.
4- Custom industry ratings
Various organizations and industry segments have created custom ratings to fit particular operational needs. For example, One mission-critical organization called the
Uptime Institute has set design standards for data centers, with the intention that data centers are designed with similar practices to ensure data safety.
 Uptime Institute Tier Level 1 TIER 1 TIER 2 TIER 3 TIER 4 acceptable MTU Onsite Energy rating ESP rated generator set taking into consideration site conditions ESP rated generator set taking into consideration site conditions and 85% average load factor Method for approval No approval necessary Contact your MTU Onsite Energy representative for a letter to comply with Uptime’s requirements
Note:
The proper understanding of the standards will ensure the best rating selection for the purpose the generator set serves, which is the basic foundation to customer satisfaction.

 2- Application Type

The application of On–site power generation systems can be determined by system type and generator set rating.
 Generator Set Rating Standby Prime Continuous System Type Emergency Prime Power Base Load Legally–Required Standby Peak Shaving Co–Generation Optional Standby Rate Curtailment
Table-2: Generation System Types

 1- Standby System Types A- Emergency Systems Emergency systems are generally installed as required for public safety and mandated by law. They are typically intended to provide power and lighting for short periods of time for three purposes: to permit safe evacuation of buildings, for life support and critical equipment for vulnerable people, or for critical communications systems and facilities used for public safety. Code requirements typically specify the minimum load equipment to be served. B- Legally–Required Standby Legally–required standby systems are generally installed as mandated by legal requirements for public safety. These systems are typically intended to provide power and lighting for short periods of time where necessary to prevent hazards or to facilitate fire–fighting operations. Code requirements typically specify the minimum load equipment to be served. C- Optional Standby Optional Standby systems are generally installed where safety is not at stake, but loss of power could cause an economic loss of business or revenue, interrupt a critical process, or cause an inconvenience or discomfort. These systems are typically installed in data centers, farms, commercial and industrial buildings, and residences. The owner of the system is permitted to select the loads connected to the system. In addition to providing a standby source of power in case of loss of a normal power supply, on–site generation systems are also used for the following purposes.

 2- Prime System Types A- Prime Power Prime power installations use on–site generation in lieu of a utility electricity supply, typically where utility power is not available. A simple prime power system uses at least two generator sets and a transfer switch to transfer supply to the loads between them. One or the other of the generator sets runs continuously with a variable load, and the second generator set serves as backup in case of a failure, and to allow downtime for required maintenance. A changeover clock within the transfer switch alternates the lead generator set on a predetermined interval. B- Peak Shaving Peak shaving installations use on–site generation to reduce or flatten peak electricity use for the purpose of saving money on energy demand charges. Peak shaving systems require a controller that starts and runs the on–site generator at the appropriate times to flatten the user’s peak demands. Generation installed for standby purposes may also be used for peak shaving. C- Rate Curtailment Rate curtailment installations use on–site generation in accordance with electric energy rate agreements with the serving electric utility. In exchange for favorable energy rates the user agrees to run the generators and assume a specified amount of load (kW) at times determined by the utility, typically not to exceed a specified number of hours per year. Generation installed for standby purposes may also be used for rate curtailment.

 3- Continuous System Types A- Continuous Base Load Continuous base load installations use on–site generation to supply a constant power (kW) typically through interconnection equipment into a utility grid. These installations are usually owned by electric utilities or under their control. B- Co–Generation Often, continuous base load generation is used in Co–Gen application. Simply put, Co–Gen is utilizing both the direct electricity generation and waste exhaust heat to substitute for utility supplied energy. The waste heat is captured and either used directly or converted to electricity.

In the next article, we will continue explaining other Selection factors used for Generators Sizing Calculations. So, please keep following.

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