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Application Notes  Ultra-K-UK #1

 Tell a Buddy  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 overheatand fail prematurely when connected to these loads. Forthis reason 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 whenused under their K-factor value; manufactured with heaviergauge copper and a double sized neutral conductor; andhave higher magnetic to resistive properties than astandard transformer. All these properties are needed toreduce the heating and distortion effects of non-linearloads

Non Linear Loads ______________________________________________________________ 

Over the past few decades there has been dramaticgrowth in the use of equipment that incorporate switchingtype power supplies; i.e personal computers, fax mach-ines, copiers, electronic high efficiency ballasts, variablespeed drives, and various medical equipment. These loadsare non-linear in nature; meaning they only demandcurrent during part of the cycle. This type of load createsharmonic currents, which in turn generate heat in thedistribution equipment, neutral conductors, andtransformers. The temperature rise limit of the insulation ina transformer is usually 150° C or 200° C

When this limit is exceeded, the life expectancy of thetransformer will be cut in half for every 10° C to a pointwhere the insulation breaks down destroying thetransformer it. In some cases this creates a fire. Becausethey are designed specifically for non-linear loads, K-ratedtransformers will not overheat when subjected to harmonicloads. They also stay within the temperature rise limit ofthe insulation, and should be used wherever harmonicgenerating loads are present.

K Factor ______________________________________________________________________ 

There are different amounts of harmonic currentsproduced. The term for the total amount of harmoniccurrent present is called "Total Harmonic Distortion(THD)". Since this value has a wide range, there needs tobe an appropriate way to size the K-rated transformer tothe load. This is where K-factor comes in. K-ratedtransformers have an associatedK-factor rating. K-factor ratings range between 1 and 50.The higher the K-factor, the more heat from harmoniccurrents the transformer is able to handle. A standardtransformer that is designed for linear loads is said tohave a K-factor of 1, whereas a transformer with a K-factor of 50 is designed for the harshest harmonic currentenvironment possible. Transformers rated with K-factorsof 40 and 50 are extremely rare, very expensive andgenerally are not used.

Making the correct selection of K-factor is extremelyimportant because it affects cost and safety.Calculations of harmonic content produces a precisevalue of K-factor, but power loads change constantlyrendering the calculated value questionable. Newconstruction installations have no data to assist inselecting the appropriate K-factor rating. In these cases,empirical data allows us to use past practices to obtainthe correct K-factor rating. Table 1 shows what K-factorrating to use when the electronic equipment representsa certain percentage of non-linear current. This table isbased on past practices

Non-Linear Load K-rating  Sometimes a de-rated K-1transformer is used to

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Incidental electronic equipment representing <5%Harmonic producing equipment representing <35%Harmonic producing equipment representing <50%Harmonic producing equipment representing <75%Harmonic producing equipment representing <100% 

K-1 K-4K-7K-13K-20 

Table 1

obtaing a greater K-rating.For example to achieve a K-13 rating, a K-1 ratedtransformer would have tobe oversized 200%.

Other Technologies _____________________________________________________________ 

As previously stated, a standard transformer can nothandle the heat generated by non-linear loads. Whatsome manufacturers will do is specify an oversizedstandard K-1 rated transformer as a K-13 or K-20 ratedtransformer at half it’s capability. This transformer maybe capable of 100 KVA as a K-1 rated transformer, butonly capable of 50 KVA as a K-13 rated transformer. Thistransformer is larger and more costly than a 50 KVA K-13rated transformer. De-rating a transformer doesn’t assurethat it will perform correctly when subjected to non-linear

loads. The magnetic and resistive properties of atransformer are enhanced when it is designed specificallyfor non-linear loads. A standard transformer doesn’t takethese considerations into account and may fail evenwhen subjected to light non-linear loads.

Other manufacturers will use aluminum windings insteadof copper because it is less expensive. However, toachieve the same resistive property, it takes 1.6 times asmuch aluminum as it does copper. Since the terminals tothe transformer are copper, there is a need to bondaluminum to copper. Achieving a strong aluminum tocopper bond has been proven to be extremely difficult.Even with a tight bond, aluminum expands and contractseasier than copper due to heat variations which will loosenthe connection. Even though an aluminum wound

transformer is initially less expensive, it is larger and moreprone to failure than a copper wound transformer and willcost more in the long run.

Summary ______________________________________________________________________ 

K-rated transformers are designed for the non-linearloads that standard transformers just can’t handle. K-rated transformers are manufactured with heaviergauge copper and a double sized neutral conductor

and have higher magnetic to resistive properties than astandard transformer which enables them to handlethe heat generated by harmonic currents. De-rating ofa standard

transformer doesn’t assure optimal performance under non-linear loads and aluminum wound transformers are prone topremature failure. The Ultra-K  manufactured by ControlledPower Company is a shielded, copper wound, K-rated

isolation transformer which incorporates precise designtechniques to handle the heat generated by high harmoniccurrents. The Ultra-K  has K-ratings of K-4, K-7, K-13, and K-20, and the options of TVSS and high frequency filters foradditional transient protection.

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