Re: Stability Factor: K-Factor

Steven Mill

Thank you for your feedbacks!


Spir Georges GHALI, sorry for the late reply and thank you for your interesting remarks.

@Spir Georges GHALI said:

It’s very good and important to mention the effects of Harmonics on the Transformers, but I have some remarks that are :

– I think the percentages of Harmonics currents mentioned in this topic mean the percentage of the “ THD-I ” ( Total Harmonic Distortion of Current ). If not, please clarify.

Yes these percentages are of THD. Sorry If that confused you.

– It’s known that the effect of K-Factor is to over-size the Transformer’s power, but what is the formula between the K-Factor and the Transformer’s power ?

Normally in order to calculate the K-Factor you keep into account two things the KVA & load Harmonic Current. So accordingly the K-Factor would be:


K – Factor = KVA * IL

As IL is the load harmonic content so you can easily calculate the K-Factor value to be used.

– You mention that “ the inductive loads like Motors are known as harmonic generating loads ”, but most of motors especially “ Squirrel Cage Motors ”, and after the transient running case are always “ Linear Loads ”, but for these loads the Harmonic currents can be generated or not depending on the kind of running equipment used to run these motors. ( for exp. Contactors with Thermal relays doesn’t generate any harmonic, VFD equipment generates harmonics ).

I was specifically referring to induction motors which are widely used in industry as compared to squirrel cage motors which are most suited for small tasks. Very rare have I seen squirrel cage motors used in industry. Induction motors generate harmonics and come in the domain of non-linear loads. Also in industries the Switch mode power supplies & DC rectifiers also add to the non-linearity of the system.

– It’s mentioned that “ the working principal of K-rated Transformers involves the use of a double sized neutral conductor ”, but in general, depending on the Harmonics Currents value or percentage, we over-size “ double or even more ” the cables’ sections of Phases & Neutral “, and also depending on the value or percentage of the “ Third multiple Harmonics ” and the “ Unbalance current’s value ” we decide the final section of neutral conductor.

Yes you are right about conductor size. It depends on the harmonic currents value. I also wrote in the article that Some K-rated transformers use more than one conductor. Normally you don’t need a transformer with K-factor rating more than K-4 or K-13 in industries which are coherent to the design specs presented in the article.

– As mentioned, the K-Factor Value’s range is from “ 1 ” to “ 50 ”, and also, the rules mentioned the K-Factor Transformers that should be used depending on the Harmonic Current, ( exp. for Harmonics Currents more than 75% the “ K-20 Transformers ” should be used ) but, at which levels or where the values “ K-30  or  K-35 Transformers ” or more should be used ?

Transformer size increase as you increases the value of K-Factor & so does the heat tolerance due to harmonic currents. Normally transformers of K-Factor 30, 40 & 50 are used where harmonic currents are somewhere b/w 125-150%. These are worst harmonic conditions. As mentioned before the size of transformer increases with the increase in K-Factor so the optimal K-Factor should be chosen which would be a tradeoff between transformer size & its heat tolerance. Normally the K-50 transformers are very expensive & much larger in size so manufacturers recommend a transformer of K-30 with modified designs as a replacement.