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What is a K-Rated transformer?ukkrata1 22 April1998

General_____________________________________________________________________

A standard transformer is not designed for high harmoniccurrents produced by non-linear loads and will overheat and

fail prematurely when connected to these loads. For thisreason a special transformer has been designed. Thistransformer is a K-rated transformer. K-rated transformersare: able to handle the heat generated by harmonic

currents; not effected by harmonics; very efficient whunder their K-factor value; manufactured with heavie

copper and a double sized neutral conductor; and hahigher magnetic to resistive properties than a standatransformer. All these properties are needed to reduheating and distortion effects of non-linear loads

Non Linear Loads____________________________________________________________

Over the past few decades there has been dramatic growthin the use of equipment that incorporate switching type powersupplies; i.e personal computers, fax mach-ines, copiers,electronic high efficiency ballasts, variable speed drives, andvarious medical equipment. These loads are non-linear innature; meaning they only demand current during part of thecycle. This type of load creates harmonic currents, which in

turn generate heat in the distribution equipment, neutralconductors, and transformers. The temperature rise limit ofthe insulation in a transformer is usually 150 C or 200 C

When this limit is exceeded, the life expectancy of thtransformer will be cut in half for every 10 C to a poithe insulation breaks down destroying the transformsome cases this creates a fire. Because they are despecifically for non-linear loads, K-rated transformeroverheat when subjected to harmonic loads. They alwithin the temperature rise limit of the insulation, and

be used wherever harmonic generating loads are pr

K Factor____________________________________________________________________

There are different amounts of harmonic currents produced.The term for the total amount of harmonic current present iscalled "Total Harmonic Distortion (THD)". Since this valuehas a wide range, there needs to be an appropriate way tosize the K-rated transformer to the load. This is where K-factor comes in. K-rated transformers have an associatedK-factor rating. K-factor ratings range between 1 and 50.The higher the K-factor, the more heat from harmonic

currents the transformer is able to handle. A standardtransformer that is designed for linear loads is said to have aK-factor of 1, whereas a transformer with a K-factor of 50 isdesigned for the harshest harmonic current environmentpossible. Transformers rated with K-factors of 40 and 50 areextremely rare, very expensive and generally are not used.

Making the correct selection of K-factor is extremelyimportant because it affects cost and safety. Calculatiof harmonic content produces a precise value of K-fabut power loads change constantly rendering thecalculated value questionable. New constructioninstallations have no data to assist in selecting theappropriate K-factor rating. In these cases, empiricalallows us to use past practices to obtain the correct K

factor rating. Table 1 shows what K-factor rating to uwhen the electronic equipment represents a certainpercentage of non-linear current. This table is basedpast practices

Non-Linear Load K-rating

Incidental electronic equipment representing

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As previously stated, a standard transformer can not handlethe heat generated by non-linear loads. What somemanufacturers will do is specify an oversized standard K-1rated transformer as a K-13 or K-20 rated transformer athalf its capability. This transformer may be capable of 100

KVA as a K-1 rated transformer, but only capable of 50KVA as a K-13 rated transformer. This transformer is largerand more costly than a 50 KVA K-13 rated transformer. De-rating a transformer doesnt assure that it will performcorrectly when subjected to non-linear loads. The magneticand resistive properties of a transformer are enhancedwhen it is designed specifically for non-linear loads. Astandard transformer doesnt take these considerations intoaccount and may fail even when subjected to light non-linear loads.

Other manufacturers will use aluminum windings instecopper because it is less expensive. However, to achithe same resistive property, it takes 1.6 times as mucaluminum as it does copper. Since the terminals to thtransformer are copper, there is a need to bond alumi

copper. Achieving a strong aluminum to copper bondbeen proven to be extremely difficult. Even with a tighaluminum expands and contracts easier than copperheat variations which will loosen the connection. Eventhough an aluminum wound transformer is initially lesexpensive, it is larger and more prone to failure than acopper wound transformer and will cost more in the lo

Summary___________________________________________________________________

K-rated transformers are designed for the non-linear

loads that standard transformers just cant handle. K-rated transformers are manufactured with heavier gaugecopper and a double sized neutral conductor and havehigher magnetic to resistive properties than a standardtransformer which enables them to handle the heatgenerated by harmonic currents. De-rating of a standard

transformer doesnt assure optimal performance under n

linear loads and aluminum wound transformers are pronpremature failure. The Ultra-Kmanufactured by ControllPower Company is a shielded, copper wound, K-rated istransformer which incorporates precise design techniquehandle the heat generated by high harmonic currents. TKhas K-ratings of K-4, K-7, K-13, and K-20, and the optiTVSS and high frequency filters for additional transientprotection.