In the three-phase three-wire system of three-phase symmetry (power supply symmetry, load symmetry) system, it does not contain even harmonics and harmonics of integral multiple of three. For asymmetric systems, even harmonics and even DC components are possible.
First, the quality of the power supply is not high to produce harmonics
Because the three-phase winding is difficult to achieve absolute symmetry in the production of the generator, the core is difficult to achieve absolute uniformity and some other reasons, the power supply will produce some harmonics, but in general, very few.
Second, the transmission and distribution system produces harmonics
Due to the saturation of the transformer core, the nonlinearity of the magnetization curve, and the consideration of economy when designing the transformer, the working magnetic density is selected on the near saturation section of the magnetization curve, so that the magnetization current is a peaked waveform, and thus contains odd harmonics.