What are K-Factor Rated Transformers?

The life of a transformer is defined as the life of the paper insulation. Paper insulation can deteriorate due to several factors, mainly including temperature, moisture content, and oxygen content. Of all these factors, the temperature has a major impact because modern oil preservation systems minimize the effect of moisture or oxygen on insulation degradation.

The transformer temperature, usually considered as hot spot temperature (HST), is mainly controlled by the ambient temperature and the load, where the ambient temperature is the main factor. On the one hand, certain types of loads are directly influenced by the ambient temperature, e.g. cold loads in summer: the higher the ambient temperature, the higher the load. 

On the other hand, the load capacity of a transformer is usually influenced by the ambient temperature. The transformer must be operated under the specified HST limits and the ambient temperature is an uncontrollable factor among the HST influencing factors (ambient temperature and load); therefore, if the ambient temperature is high, the load capacity of the transformer is always low and vice versa.

Dry Type Transformer          

What are K-factor rated transformers?

K-factor rated transformers are capable of withstanding the heating effects caused by harmonic load currents. They are specially designed for non-linear harmonics generating loads. It is defined in the ANSI/IEEE C57.110 standard.

Electronic circuits, switched-mode power supplies, and semiconductor switches draw a large amount of non-linear current causing distorted current. This distorted current results in harmonics which have an adverse effect on transformers. It is necessary to properly rate the transformers so that they can withstand the temperature rise caused by the harmonics generated by non-linear loads.

Definition of K-Factor rating

It is the value that determines how effectively a transformer can handle harmonic currents while maintaining the temperature rise well within the limits.

H-Class Dry Type Transformer     

Formula of k-factor

Where, h is the order of harmonics and Ih is the fraction of total rms load current at h-order harmonics.

It ranges from 1 to 50. Transformers that have a K-factor of 1 can handle linear loads only and those rated 50 can withstand harsh harmonics. For attained better K-factor ratings, transformers should be made capable of withstanding extra heats caused due to harmonics.

K-factor rating transformer

Harmonics

Design consideration

A K-factor transformer is designed to withstand core and conduction losses caused due to harmonics. Their construction varies slightly from the standard transformers.

The neutral conductor of these transformers is 200% oversized than the standard transformers. This can help in mitigating the effects of third-order harmonics that sum up in the neutral conductor.

The heat produced due to the eddy current can be controlled by providing electrostatic shielding between primary and secondary windings.

Transformer leads are made bigger.

A ferromagnetic core with lesser inductance is used.

Multiple smaller conductors are used in the secondary winding of the transformer to reduce the skin effect.

Harmonics are generated by all non-linear loads. If transformers are not capable of withstanding the heating effects of harmonics, they may fail. In order to overcome heating effects, transformers must be oversized, so that they will run at a fraction of their rated capacity or K-rated transformers have to be used. K-factor transformers are specially designed to withstand the harmonics and operate at full load without de-rating.

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