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This paper presents an explanation of theimportance of testing generator sets at ratedpower factor. Typically, a generator set hasa rating expressed in kW and power factoror kW and kVA. In order to check for properoperation of all emergency system supportsystems, it is important to test the generatorat rated power factor or kVA. A generatoroperating at rated kW at 0.8 power factor

lagging load requires more kW from the enginethan when running at rated kW on a resistiveload bank. This is due to a change in alternatorefficiency and will result in increased operatingtemperatures and fuel consumption.

Cummins Power Generation recommends every gen-erator set be tested at rated power factor either at the

factory or on-site during acceptance testing, but notnecessarily both. Cummins tests production generatorsets at rated load and power factor at the factory priorto shipment, hence, specifications need not require 0.8PF testing of Cummins generator sets during on-siteinstallation acceptance.

Our distributors are seeing an increasing number ofspecifications requiring on-site acceptance testing ofthe generator set at full rated load and power factor(rated kW and 0.8 power factor). These specifications

are likely driven by generator set manufacturers whohave not made the capital expenditure for reactive loadbanks and, therefore, cannot offer rated power factortesting as a factory service. NFPA 110, Standard forEmergency and Standby Power systems, allows theoption of full load rated power factor testing either atthe manufacturers factory or during installation accep-tance testing on site. If factory tested at rated powerfactor, then NFPA 110 permits on-site acceptancetesting at unity power factor.

Cummins has made the investment in inductive loadbanks for production testing, because we do not

want to use the customers installation for our testing.Therefore, to avoid the higher costs and the problemsassociated with inductive load banks on-site, weencourage consulting engineers to require factorytesting at rated load and power factor in their specifica-tions. If rated load and power factor testing cannot becertified by the manufacturer, then the specificationsmust require on-site testing at rated power factor (usingload banks to supplement facility loads as required).

Factory testing constitutes a necessary and adequateconfirmation of the generator set capability to carry

> White paper

By Jim Iverson, Senior Applications Engineer

Rated power factor tests andinstallation acceptance of emergencyand standby power systems

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Power topic #6004| Technical information from Cummins Power Generation

FIGURE 1

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www.cumminspower.com

2006 | Cummins Power Generation

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rated power factor load. Once demonstrated by factorytesting, subsequent on-site testing at unity powerfactor is sufficient. Rated load and power factor testingis conducted to verify the following:

1. The engine must be capable of delivering thenecessary power to the generator at rated power

factor. Generator efficiency is a function of bothload kW and load power factor and that efficiencycan be a few percent lower at 0.8 power factorthan at unity power factor as illustrated in Figure 1.This requires the engine to develop a few percentmore kW at 0.8 power factor than at unity powerfactor.

2. A higher level of generator excitation (generatormain field current) is required to maintain ratedoutput voltage at 0.8 power factor than at unitypower factor as illustrated in Figure 2. Thegenerator produces a higher output current to

support the kVA required by the load at 0.8 power

Power topic #6004| Page 2

FIGURE 2

factor. The generator and all excitation systemcomponents (main generator, exciter and voltageregulator) must be tested to ascertain they arecapable of operating at the higher current levels forextended operation at 0.8 power factor.

Cummins Power Generation endorses full load and

rated power factor testing on every emergency andstandby generator set as the only way to positivelydetermine that each generator set is capable ofsupporting lagging power factor load when installed.However, installation acceptance testing should beperformed according to the alternative outlined in NFPA110. According to NFPA 110, paragraph 7.13.6 7.13.6.1requires a full load test as follows:

A load shall be applied for a two-hour, full load test.The building load shall be permitted to serve as partor all of the load, supplemented by a load bank ofsufficient size to provide a load equal to 100 percent

of the nameplate kW rating of the EPS (EmergencyPower Supply), less applicable derating factors for siteconditions.

A unity power factor shall be permitted for on-sitetesting, provided that rated load tests at rated powerfactor have been performed by the manufacturer of theEPS prior to shipment.

The purpose of the field acceptance testing requiredby NFPA 110 is to confirm that the auxiliary supportsystems perform satisfactorily when the generator setoperates at full load. These systems include fuel supplyand delivery, intake air, exhaust air, ventilation and

cooling systems. Usually there is not enough buildingload to allow the generator set to operate at full ratedload and the building load must be supplemented bya load bank. This can be a fairly expensive propositionconsidering that most applications will require portableload banks to supplement on-site load.

If the generator set has been tested by the manufac-turer at rated load and power factor prior to shipment,NFPA allows the field acceptance test to be run atgenerator set rated kW at unity power factor. NFPArecognizes that the site support systems are typicallydesigned conservatively and that on site problems

would be uncovered with rated kW load testing.Although it is true that the auxiliary support systems willbe stressed slightly more when the generator set runsat lagging power factor, engineers need only specifyfield acceptance testing at unity power factor.

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Summary

To verify full load generator operating characteristics,testing at full rated kW and power factor is required.Project specifications should include the specificreference to NFPA for full load testing. Generatormanufacturers that conduct factory production testingat full load and rated power factor should be allowed torun the field acceptance tests at full rated load at unitypower factor.

For additional technical support, please contact yourlocal Cummins Power Generation distributor. To locateyour distributor, visit www.cumminspower.com.

Switchgear & Controls, and Technical Marketing &Sales. Jim provides technical direction to CommercialMarketing & Sales, contributes to domestic industrycodes and standards development, offers sales andservice training, offers technical input for publishedliterature and software, publishes technical papers onindustry topics, and provides application engineeringsupport to customers.

About the author

Jim Iverson is a senior applicationsengineer for Cummins Power Generation,with an M.S. in Engineering Science,and a B.S. in Electrical Engineering.Since 1976, he has managed TransferSwitch Design, Systems Engineering,

www.cumminspower.com

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PT-6004 (12/06) formally 900-0281

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