Power sources such as transformers need to be operated at reduced capacity when connected to harmonics-causing loads. For instance, a 100 kVA transformer may have to be operated at 60 kVA.
The K rating is a factor that is developed to indicate the amount of harmonics that the load can generate. The K rating is extremely useful when designing electric systems and sizing components.
The formula for K rating is
Where Ih is the total harmonic current of particular harmonic. Ih is expressed as p.u. basis
The k rating of loads helps decide on the specifications of the power source such as transformers and also the size and capacity of the conductors. The amount of harmonics drawn by a load determines its k rating. Thus loads such as resistance heaters, transformers and motors with less harmonics have a k rating of 1. Welders and induction heaters have a k rating of 4 as they generate a substantial amount of harmonics.
Loads which have higher k rating are connected to transformers having a corresponding k rating. Thus a transformer of 100kVA with a k rating of 4 will deliver 100kVA as long as the k rating of the load is 4. At higher k ratings, the transformer may have to be derated.
Transformers with higher k ratings are made of thicker conductors and are designed to withstand higher eddy currents. This leads to the transformers having bigger size. These transformers also occupy higher space. The neutral conductors in these transformers are thicker to permit the circulation of the triplen harmonics through them.