Re: Stability Factor: K-Factor

Spir Georges GHALI


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.

– 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 ?

– 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 ).

– 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.

– It’s mentioned that “ K-rated Transformers could be used at places where the loads are either inductive or nonlinear ”, and as known, some inductive loads generate harmonics but not all inductive loads.

– 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 ?

– The K-Factor value is the percentage of Harmonics current’s percentage to fundamental current, but this percentage’s value can be only known after industries’ running, so, what the Electrical Engineers Designers should do at designing step ? For this point, the IEC determines a curve that helps the Electrical Engineers Designers to define the transformer’s powers that should be used depending on the percentage of “ Non Linear Loads ”.