Harmonics are disturbances in the public power supply. Also frequency inverters or inverters can cause additional waves at the 50 Hz mains frequency. A lot of mains disturbances can destroy connected devices.
Interconnected transformers will be heated additionally due to an increase of losses in the transformer.
Remarkable is that the losses occur independent of the load, meaning the drawn power. So called iron losses of the transformer core come up even in no-load.
The example shows a transformer with a clean sine wave input and alternatively one that is stressed by an input with harmonics.
The type size is 115 kVA, the weight is 450 kg.
Assumed operation: 12 hours per day and 260 days per year.
Electricity costs: 0,20 €/kWh.
Without harmonics the transformer has losses of 1250 W, with harmonics 1650 W. The difference of 400 W equals around 250,-€ additional costs in the electricity bill per year.
Corrective actions would be to prefix chokes or sinusoidal filters or to use transformer cores that are resistant to harmonics.