We have seen in previous posts how there is increase in non linear loads and in what way they effect the power systems. In a nut shell, because of increase in usage of modern electronic machinery like computers, printers, etc which are non linear in load affecting the fundamental wave shape of currents there by increasing the harmonic content in currents. These harmonics when superimposed on fundamental waveform creates a distorted wave resulting in Total Harmonic Distortion (THD). 
In power networks, transformers are the most vulnerable components for any faults. Understandably harmonics affect the nearest transformers.The result may be abnormal heating of transformers resulting in premature failure. The costs involved in replacing transformers and the cost of disruption in power is huge. To overcome this K-rated transformers are introduced.
If at all you come across K-rated transformer it means that the transformer is capable of withstanding harmonics to a extent mentioned on its rating plate. K-rated transformers are designed like any other standard transformer but with some precaution and variation in heat dissipation.

What is K-Factor?

It is the arithmetic sum of products obtained by multiplying square of Harmonic current (p.u.base) and the square of harmonics. In simple mathematical equation

K-factor = ∑ Ih2h2
Where h is the harmonic number and  Iis the current at that harmonic. Also ∑ Ih2 = 1

In other words K-factor is the ability of the transformer to take the harmonic load content without any overheating. You may observe a K factor of 1 means linear load. The higher the K factor the more is the harmonic load content. For example a K9 transformer can take harmonic load content up to K-factor 9 (refer above equation). If the harmonic load exceeds K9 then the transformer needs to be changed or derated.

How K-Factor Transformers Differ?

As mentioned above, K-factor transformers differ from standard transformers. They have basically additional thermal capacity to take on losses created by harmonic currents. Harmonic currents increase the stray losses of transformer thereby some local heating. Some design engineers design in such a way that eddy current losses are minimized. This is one way of keeping the heat generated by harmonic load under check. Even under the harmonic load content as per K-factor the winding temperature should not exceed the specified temperature rise. This will make the transformer costly as compared to a standard transformer.

Why not de-rate Standard Transformers?

There are some disadvantages in using de-rated standard transformers in place of K-rated transformers.

The over current protection, if used might trip because of high inrush currents of a high rating transformer. For example if you are using a 1250kVA transformer de-rated to 1000kVA in place of a K-rated 1000kVA transformer, the inrush current of 1250kVA transformer trips the breaker as the breaker might be set for 1000kVA currents. Alternatively if the protection is also set for 1250kVA then protection for 1000kVA transformer will never be in place.

The rating plate may show 1250kVA and everyone may follow de-rated loads initially. But over a period of time it would be difficult to manage de-rated loads.

The cost of de-rated transformer may be higher than the cost of K-rated transformer although depending on the K-rating.

Precautions While Selecting K-Rating

As mentioned above the cost of K-rated transformer depends on its K-rating. Hence while calculating the K-rating one should give importance to the extent of harmonic loads the transformer is subjected to. For example if the extent of harmonics is up to 13th and by over caution if it is specified as 25th harmonic then the Squared Harmonic current is multiplied to the square of the harmonic i.e. square of 25  which is 625. Please recall the equation mentioned above. This may come to a significant increase when summed up to the total K-factor. As the K-factor is increased the size of the transformer increases and thereby its cost also increases. Hence it is really important to select a realistic K-factor

One more precaution is that the neutral terminal and its conductor shall be rated double the size of the phase terminals and its conductors. The reason is obvious which is to take care of all unbalanced currents.

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