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What is the K-Factor for Transformers

  • What is the K-Factor for Transformers

     

    K-factor transformers are designed to be able to be used for loads with harmonic distortion without the necessity of de- rating. If a K-factor transformer is to be used in an application it is necessary to know the load characteristics and the harmonic content over the whole load cycle and then to calculate the K-factor and specify a transformer with the required K-factor value. For most general applications a K-factor rating of 15 or less is adequate.

     

    Because they must be designed to reduce the level of eddy current generation in the windings, or to allow better dissipation of losses, K-factor transformers are:

    • more expensive (about two times)
    • heavier (about 15-20 % more)
    • larger

    when compared to standard power transformer designs of the same kVA rating.

     

    They may have a shield between the two windings to limit harmonic induction: the basic conductor section size making up the transposed windings (particularly the low voltage) are made smaller to limit eddy currents (skin effect) while the overall conductors may be made larger to reduce ohmic heating by the power frequency current. Neutral conductors may be made larger to limit the heating effects of triplen harmonics.

     

    The core is often made of better quality magnetic steel with lower hysteresis loss and perhaps thinner laminations to reduce core eddy current losses. The overall core size may be made larger to reduce operating flux density and hence eddy current and hysteresis losses. However the core losses are only a minor part in this aspect if the supply is **** of harmonics and the only harmonics are in the load current.

     

    Cooling is also enhanced in K-factor transformer design.

     

    Overall the fundamental property of K-factor transformers is that they have lower losses than standard transformers of the same rating for the same level of harmonic distortion.

     

    In general, only the transformer winding loss is used in the K-factor calculation. The core loss is not important in this determination as at full load the load loss is the much higher level. However some designs do have lower core loss as discussed above.

     

    Dry-type transformers are more susceptible to harmonic effects because of their lower heat dissipation coefficients. This lower heat dissipation results from the lack of oil. Oil provides a much more efficient convection loss method (whether natural or forced) compared to the mainly conduction loss mechanism which dominates the thermal dissipation process in dry-type units.

     

    K-factor transformers will also have a lower impedance than the equivalent rating standard transformer design.

     

    Reference: ee.unsw.edu.au

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