Harmonics in power system
Figure 1. A three-phase rectifier and resultant waveforms. Source: Invertek.


Power converters use diodes to rectify the AC supply and feed the internal DC link.
The rectification causes non-sinusoidal current waveforms. See Figure 1.
Any repeating waveform can be constructed from a series of sines of integer multiples of the fundemental frequency of the waveform. These are the harmonics.




Figure 2. This fabulous illustration of the Fourier Transform by Lucas V. Barbosa on Wikipedia's Fourier transform page shows the transformation of a periodic waveform from the time domain to the frequency domain. The frequency plot shows the relative strength of the harmonics with clarity that could not be obtained from staring at the time plot.


It should be apparent that the more square the time domain waveform is then the more harmonics you will have and these should be visible in the frequency domain.



  How does power converters inject harmonics in power system?


It is because of the non-sinusoidal current waveforms caused by the rectifier diodes.