How are harmonic currents created. How can high frequency harmonics be mitigated in power transmission

In the simplest sense, harmonic loads are created by switching. When a switching event happens, a square wave is created and this will create notable harmonics up to 25th base frequency (Fourier Transform). Harmonics can be mitigated proactive by switching more slowly and reactively by using special transformers and passive circuits which cause the harmonics to see a higher impedance.