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K-Factor Transformers

Three-Phase, Type KT, 60 Hz, for Non-Linear Loads Product Description

o Type KT Suitable for indoor or outdoor applications (with weathershield).Ventilated enclosures (DT-3).220C Insulation system, 150C Rise standard (self extinguishing).Type DT-3 is available in ratings of 15 1000 kVA and up to 4160 volts.

Application DescriptionCutler-Hammer KT Transformers include several major design improvements that addressthe problems caused by non-linear loads and harmonics. They are designed to withstandthe effects of harmonic currents without exceeding the temperature rating of the insulationsystem. The KT design compensates for the stresses on a transformers winding insulation

which prevents insulation breakdown and premature failure. The net result is longertransformer life.

Design Features o Core

A high grade, nonaging, grain-oriented silicon steel with high magnetic permeabilityprovides reduced coreinduction levels, preventing saturation as a result of thehigher frequency harmonics and resultant peak voltages. In a core approachingsaturation, the current in the coil will increase as voltage drops because the corecannot absorb the additional magnetic flux. This core also provides reduced eddycurrents or induced currents in the steel caused by the high ratios of peak-to-RMScurrents and voltages found in harmonic loads.

o Coils

Windings are continuous wound aluminum or optional copper construction sizedand configured to reduce overheating caused by harmonic currents. These coilsreduce skin and proximity effect losses which occur when current carryingconductors next to each other and coiled around steel generate magnetic fields.These magnetic fields push the currents in the conductors away from each othercausing increased losses and additional heating.

o Neutral BusThe neutral bus is sized and configured to accommodate at least 200% of the ratedcurrent. This compensates for the increased neutral currents found in non-linearloads thus reducing heat.

o The K FactorA common industry term for the amount of harmonics produced by a given load isthe K Factor. The larger the K Factor, the more harmonics are present. Linear

loads, for example have a K Factor of 1. Transformers may carry a K Factor ratingto define the transformers ability to withstand the additional heating generated byharmonic currents.

o Calculating the K FactorAll nonlinear waveforms can be broken down mathematically into a fundamentalfrequency and its harmonics IEEE C57.110 establishes a direct relationshipbetween these harmonics and transformer heating. Underwriters Laboratories hasestablished a similar relationship, the K Factor, which is derived by summing thesquare of the percentage current at a given harmonic level multiplied by the squareof the harmonic order.

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For example: a load that is 90% of the fundamental, 30% of the third harmonic,and 20% of the fifth harmonic would yield (.9)2 (1)2 + (.3)2 (3)2 + (.2)2 (5)2 or a KFactor of 2.62. This load would require an Eaton Cutler-Hammer KT-4 Transformer

with a K Factor rating of 4. Transformers that carry a K Factor rating define thetransformers ability to withstand a given harmonic load while operating within thetransformers insulation class. An analysis of harmonic loads and a calculation ofthe K Factor must be made to properly apply transformers in any building or facility.Note that the calculated K Factor is not constant since non-linear loads changethroughout the day as equipment and lighting is turned off and on. These harmonicloads also change over the life of the building or facility as equipment is added orremoved.

o Harmonic CurrentsHarmonic currents are found in nonlinear loads. These currents are generated byvarious types of equipment including switching mode power supplies that abruptlyswitch current on and off during each line cycle. Switching mode power supplies ordiodecapacitor power supplies convert AC line volt-age to low voltage DC. Thisprocess accomplished by charging capacitors during each line cycle with narrowpulses of current that are time-coincident with line voltage peaks. Examples of thisequipment include electronic ballasts for fluorescent lighting, personal computers,printers, fax machines, electronic and medical test equipment, uninterruptiblepower supplies, and solid-state motor drives.

Note: Nonlinear is synonymous with the term non-sinusoidal. Features, Benefits and Functions

o 600 volt class standard.o Three-phase, 48 208Y/120 volt standard.o 150C rise standard, 80C and 115C available.o Neutrals sized for 200% of rated current.o Electrostatic shield.

Note: Electrostatic shields do not reduce harmonic levels. However, because of the nature

of switching mode loads, these shields do reduce transient noise in the system which mayaffect sensitive computer loads.

o Aluminum windings (copper optional).o Class 220C insulation.o Reduced core flux density.

Note: Reduced core flux prevents the core from saturation and overheating due to voltagedistortions caused by harmonic currents.

o Indoor enclosures (weathershields optional, for outdoor applications).o Coils designed to minimize stray losses.o K4, K13 standard.o K9, K20, K30, K40, K50 optional.

Standards and Certificationso Industry Standards

All Cutler-Hammer dry type distribution and control transformers are built andtested in accordance with applicable NEMA, ANSI, and IEEE Standards. All 600volt class transformers are UL listed unless otherwise noted.

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o Seismic QualifiedAll Cutler-Hammer dry type distribution transformers are seismically qualified, andexceed requirements of the Uniform Building Code (UBC) and California Code Title24.

Product Specifications o Frequency

Cutler-Hammer standard dry type distribution transformers are designed for 60Hertz operation. Transformers required for other frequencies are available andmust be specifically designed.

o Overload CapabilityShort-term overload is designed into transformers as required by ANSI. Dry typedistribution transformers will deliver 200% nameplate load for one-half hour; 150%load for one-hour; and 125% load for four-hours without being damaged providedthat a constant 50% load precedes and follows the overload. See ANSI C57.96-01.250 for additional limitations. Continuous overload capacity is not deliberatelydesigned into a transformer because the design objective is to be within theallowed winding temperature rise with nameplate loading.

o Insulation System and Temperature RiseThe design life of transformers having different insulation systems is the same

the lower temperature systems are designed for the same life as the highertemperature systems.o Enclosures

Cutler-Hammer ventilated transformers, Type KT, utilize a NEMA 2 rated (drip-proof) enclosure as standard, and are rated NEMA 3R with the addition of

weathershields.o Winding Terminations

Primary and secondary windings are terminated in the wiring compartment.Ventilated transformers have leads brought out to aluminum pads that are pre-drilled to accept Cu/Al lugs. Lugs are not supplied with these transformers. Cutler-Hammer recommends external cables be rated 75C for ventilated designs.

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