## Forum Replies Created

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- Steven MillParticipant
@guest said:

**very interesting article…bt tell me abt fuzzy logic and lfc**Thanks. I’ll write an article about LFC, you could read it at the beginning of next week.

Steven MillParticipantHello, you’re welcome!

Jeff Williams, sure next week I’ll post an article about AGC first.

Steven MillParticipantThank you for your feedbacks!

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

@Spir Georges GHALI said:

Dear;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 * I*_{L}

As I

_{L}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.

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