Harmonics in transformers
could you explain what it is that causes harmonics in transformers?

A transformer is only approximately linear, and that only within limits.
The net current in a transformer (the magnetizing current) creates an &quot;H&quot; field proportional to the instantaneous current. The &quot;H&quot; field induces a &quot;B&quot; field, but the relationship between H and B is non-linear. Changes in the &quot;B&quot; field, create induced voltage in the windings, and this voltage is proportional to the rate of change of &quot;B&quot;.
The relationship between &quot;H&quot; and &quot;B&quot; becomes especially non-linear when the core is &quot;saturated&quot;. That is, when &quot;H&quot; is very high, a change in &quot;H&quot; causes very little change in &quot;B&quot; (compared to the change in &quot;B&quot; when &quot;H&quot; is small).
All this entails that if the voltage is a perfect sine wave, the current will not be a perfect sine wave, but a distorted sine wave. Every distorted sine wave is equal to the sum of a sine wave of the fundamental frequency and sine waves of harmonic frequencies.