K-FACTOR TRANSFORMER

K-FACTOR TRANSFORMER

K factor has been a basis for measuring the capability of a transformer to carry nonlinear loads. It is the factor to determine whether the transformer can withstand transformer heating due to harmonic load currents.

ANSI/IEEE Std.C57-110 is the primary reference when it comes to the discussion of k-factor in a transformer.

Below are selected articles discussing the k-factor and the k-rated transformers. FAQs are also available for your reference to make yourself familiar with the nature of use of this k-factor for transformer.

K-Factor Definition

K-factor is a weighting of the harmonic load currents according to their effects on transformer heating, as derived from ANSI/IEEE C57.110. A K-factor of 1.0 indicates a linear load (no harmonics). The higher the
K-factor, the greater the harmonic heating effects. When a non-linear load is supplied from a transformer, it is sometimes necessary to derate the transformer capacity to avoid overheating and subsequent insulation failure. The reason for this is that the increased eddy currents caused by the harmonics increase transformer losses and thus generate additional heat. Also, the RMS load current could be much higher than the kVA rating of the load would indicate. Hence, a transformer rated for the expected load will have insufficient capacity. continue reading...

K-Factor Frequently Asked Questions

What is a K-factor? 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 Krated. How much K-rating do I need? 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. 

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K-Rated Transformer Tutorials

K-rated transformers are designed for the non-linear loads that standard transformers just can’t handle. Krated transformers are manufactured with heavier gauge copper and a double sized neutral conductor and have higher magnetic to resistive properties than a standard transformer which enables them to handle the heat generated by harmonic currents. De-rating of a standard transformer doesn’t assure optimal performance under nonlinear loads and aluminum wound transformers are prone to premature failure. The Ultra-K manufactured by Controlled Power Company is a shielded, copper wound, K-rated isolation transformer which incorporates precise design techniques to handle the heat generated by high harmonic currents. 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 for additional transient protection. continue reading...

Common Power Quality Factors Affecting Transformers

K-factor is a specific measure of the heating effect of harmonics in general and on transformers in particular. It differs from the THD calculation in that it emphasizes the frequency as well as the amplitude of the harmonic order. This is because heating effects increase as the square of the frequency. A K-4 reading would mean that the stray loss heating effects are four times normal. A standard transformer is, in effect, a K-1 transformer. As with THD, it is misleading to make a K-factor reading at the load or receptacle because there will be a certain amount of upstream cancellation; transformer K-factor is what counts. Once the K-factor is determined, choose the next higher trade size. K-factor rated transformers are available in standard trade sizes of K-4, K-13, K-20, K-30, etc. K-13 is a common rating for a transformer supplying office loads. The higher ratings tend to be packaged into PDUs (Power Distribution Units) which are specially designed to supply computer and other PQsensitive installations.continue reading...