What are Total Harmonic Distortion and its effects on electrical system?

To put it simply, harmonics are unwanted, predictable, and repeatable noise riding on an electrical waveform.

In power applications, harmonics can damage equipment, overheat Transformers, and cause erratic operation.

This is especially true if the harmonic is a multiple of the base frequency (100 Hertz, 150 Hertz, etc. for a 50Hz reference).

Total Harmonic Distortion (THD) is a term to describe the total waveform distortion by all the harmonics present on the waveform.

You should have as little as possible. Many power utilities require a THD limit at the point where a facilities connects to the utility.