What is Transformer K-Factor Rating

A K-Factor rated transformer is one which is used to deal with harmonic generating loads. Harmonics generate additional heat in the transformer and cause non-K-rated transformers to overheat possibly causing a fire, also reducing the life of the transformer. K-rated transformers are sized appropriately to handle this additional heat and tested to UL 1561 rigid standards for K-factor rated transformers. The way a K-rated transformer works is it uses a double sized neutral conductor and either change the geometry of their conductors or use multiple conductors for the coils. Quality transformers are manufactured with high grade silicon steel, copper windings, and more air ducts. It is a value used to determine how much harmonic current a transformer can handle without exceeding it’s maximum temperature rise level. K-factor values range from 1 to 50. K-factor of 1 is used for linear loads only, and a K-factor of 50 is used for the harshest harmonic environment possible. A K-factor of 13 is typical. When transformers use a K-factor value, they are said to be K-rated. The use of a K-rated transformer is anywhere non-linear loads are present. Prime uses would be in factory automation, computer rooms, and office buildings because of the high harmonic content in these areas. Typically a K-13 rated transformer is sufficient for most applications. Loads approaching 100% non-linear or more than 75% THD should incorporate a K-20 rated transformer. Over the past several years there has been dramatic growth in the use of equipment incorporating switching type power supplies. Examples are personal computers, video display terminals, fax machines, copiers, electronic high efficiency ballasts, UPS systems, variable speed drives and various medical electronic monitors. The nature of all these loads is non-linear; they only demand current during part of the cycle and/or change their impedance during the voltage cycle. This type of load creates harmonic currents, which in turn generate heat in the distribution equipment, neutral conductors and distribution transformers. Underwriters Laboratories (UL) has designated K-factor as a means of rating a transformers ability to handle loads which generate harmonic currents, UL recognizes K-factor values of 4, 9, 13, 20, 30, 40 and 50. The K-factor ratings are based on information contained in ANSI/IEEE C57.110-1986, Recommended Practice for Establishing Capability When Supplying Non-sinusoidal Load Currents. The K-factor number tells us how much a transformer must be de-rated to handle a definite non-linear load or, conversely, how much it must be oversized to handle the same load.

The following rules will generally result in an acceptable choice of K-factor value:
• Follow successful practice in sizing the transformer.
• Where the harmonic current producing equipment is less than 15 per cent, use a standard transformer.
• Where the electronic equipment represents up to 35 per cent of the load, use a K-4 rated transformer.
• Where the electronic equipment represents up to 75 per cent of the load, use a K-13 rated transformer.
• Where 100 per cent of the load is electronic equipment, use a K-20 rated transformer.
• Higher K-factor ratings are generally reserved for specific pieces of equipment where the harmonic spectrum of the load is known.

K-rating is a heat survival rating, not a treatment of associated power quality issues like voltage distortion, and efficiency isn’t typically discussed. Surviving the extra heat means using more core and coil material, and sometimes use of different construction techniques. Depending on the manufacturer’s design, harmonic losses may be reduced to varying degrees. Ironically, even though the designated use of the K-rated transformer is to feed nonlinear load, manufacturers publish their loss data under linear load conditions.

Understanding Transformer "K Factor Rating"

A K-Factor rating is an index of the transformers ability to supply harmonic content in its load current while operating within it temperature limits. For Dry Type Transformers a K-Factor calculation is made to determine the amount of the Harmonic Content present in a power system. K-Rated transformers are sized to handle 100% of the fundamental 60 Hz load, plus the non-linear load specified. The neutral of the K-Rated transformer is sized at 300% of the current rating of the phase connections. Industry literature and commentary refers to a limited number of K-factor ratings: K-1, K-4,K9, K-13, K-20, K-30, K-40. A transformer could be designed for other K-factor rating in between those values, as well as for higher values.

The commonly referenced ratings calculated according to ANSI/IEEE C57.11-1986 are as following
• K-Factor 1: A transformer with this rating has been designed to handle only the heating effects of eddy currents and other losses resulting from 60 Hz, sine-wave current loading on the transformer. Such a transformer may or may not be designed to handle the increased heating of harmonics in its load current. Applications are motors, incandescent lighting, resistance heating, motor generators (without solid state drives).
• K-Factor 4: A transformer with this rating has been designed to supply rated KVA, without overheating, to a load made-up of 100% of the normal 60 Hz, sine-wave, fundamental current plus: 16% of the fundamental as 3rd harmonic current; 10% of the fundamental as 5th; 7% of the fundamental as 7th; 5.5% of the fundamental as 9th; and smaller percentages trough the 25th harmonic. The "4" indicates its ability to accommodate four times the eddy current losses of a K-1 transformer. Uses are HID lighting, induction heaters, Welders, UPS with optional input filtering, PLC and solid state controls.
• K-Factor 9: A K-9 transformer can accommodate 163% of the harmonic loading of a K-4 transformer.
• K-Factor 13: A K-13 transformer can accommodate 200% of the harmonic loading of a K-4 rated transformer. These transformers are used for multiple receptacle circuits in health care facilities, UPS without optional input filtering, Production or assemble line equipment, Schools and classroom facilities.
• K-Factor 20, K-30, K-40: The higher number of each of these K-factor ratings indicates ability to handle successively larger amounts of harmonic load content without overheating. Some of these transformers are used in SCR variable speed drives, Circuits with exclusive data processing equipment, Critical care facilities and Hospital operating room.

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