The K-rated transformers are isolation transformers specifically designed to withstand the phenomena due to the fact that non-linear loads generate high levels of harmonic distortion. Typical non-linear load are computers, variable speed drives, inverters, welders and so on. Harmonic generation results in increased current absorption and therefore higher losses.
In order to ensure that the thermal class temperature limit is respected, the transformer design must keep into consideration the presence of said harmonics. The K factor is an indication of how much the harmonic distortion affects the system: the higher is K, the higher is the harmonic content that the transformer must withstand without any damage.
It is worth reminding that a K-rated transformers does not cancel the harmonics in the system (except for third and triplen harmonics), but is able to tolerate their effects. In order to reduce or eliminate the harmonic content, it is necessary to arrange suitable filtering equipment.