In this article, I have discussed the difference between a conventional transformer and a K-factor rated transformer.
In modern day plants, Variable Speed Drives (VSD) are widely used. The project I am working with now have more that a hundred units VSD ranging from fractional kilowatt up to several hundred kilowatts. Just imagine how much harmonics will these VSD be contributing to the power system once they operate.
The transformers were sized to provide spare capacities thus a conventional transformer may be suitable. It is, however, more desirable and economical on the long term to install K-factor rated transformers with future expansion of the plant considered.
IEEE Std C57.110 - IEEE Recommended Practice for Establishing Transformer Capability When Supplying Nonsinusoidal Load Currents provides sample calculation on how to determine the K-factor rating of transformers based on expected harmonic currents.