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Power topic #7005 | Page 2

Category one: No standby power generator required. The business is able to tolerate brief utility outages without threat to life and safety or significant loss of production or customer service. Threat of an extended outage is deemed an acceptable risk. However, even the smallest businesses today have battery-powered UPS systems to maintain computer power during short nuisance outages. The UPS system also provides time for proper shutdown of computers to avoid loss of data in the event of longer outages.

Category two: Need minimal backup power for life- safety, security and computer systems (emergency lighting, alarm systems, elevators, building egress, and UPS system for electronic data). This level is typical of code-driven standby power systems.

Category three: Need substantial standby power to maintain all production or business operations during outages of short duration. Facility types likely to be in this category include airports, hospitals, apartment complexes, office buildings, semiconductor manufac- turers, and most municipal and government buildings.

Category four: Need near-total standby power to maintain all factory or business operations for extended periods of time. Examples include regional medical centers, Internet service providers, telecommunications, broadcasting, and key government facilities.

P elimina y powe system sizing

Once an individual company’s risks have been as- sessed and the degree of standby power protection required has been determined, an initial sizing estimate

Cirent Semiconductor in Orlando, FL, has a 6 MW standby power system to prevent costly raw material losses during a utility outage.

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of the standby power system can begin. The first step in sizing a standby power system is to establish project parameters.

  • Minimum genset load/capacity: In general, a standby power system should be sized to provide a 20 percent reserve margin of power for better stability and to accommodate load growth over time. However, running a generator set at less than 30 percent of rated load can lead to engine damage, and reduced reliability.

  • Maximum allowable frequency and voltage dips: As you reduce the maximum allowable frequency and voltage dips, the size of the specified genset increases.

  • Altitude and temperature: Based on the geographic location, the size of the genset must increase for a given level of performance as altitude and ambient temperature rise.

  • Fuel: Diesel generator sets are the most popular for standby power applications, but units running on natural gas or LP are also available, and favored for extended-hour applications.

  • Other factors: Generator voltage, number of phases and generator frequency should all be specified before selecting a specific power system.

Identifying loads

Typically, a standby generator powers a variety of loads having different operating characteristics. There are linear and nonlinear loads, loads that are extremely sensitive to voltage disturbances, loads like motors that have very high starting current requirements, loads that cycle on and off, and loads that exhibit high peak load demand when operated. In some applications, multiple loads may be required to be picked up by the generator simultaneously, while most applications will allow step sequence loading that can have a big impact on reducing the required generator capacity. The genera- tor must accommodate the highest anticipated starting, peak and running load expected.

Most modern electrical loads are nonlinear in nature. They are electronic loads that consume power in bites, creating load current harmonics. These harmonics cause additional heating in the alternator, and generator output voltage distortion. Untreated, these harmonics may cause alternator and load overheating. Typically, oversized generators are recommended in these instances.

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