Harmonics in the power system come from electrical equipment, that is, from power generation equipment and electrical equipment. Since the magnetic field generated by the rotor of the generator cannot be a perfect sine wave, the voltage waveform sent by the generator cannot be a sine wave without distortion. At present, there are two kinds of generators used in China: Hidden pole machine and salient pole machine. Hidden pole machine is mostly used for steam turbine generator, and salient pole machine is mostly used for water turbine generator.

For the harmonic component, the hidden pole machine is better than the salient pole machine, but with the progress of science and technology and the input of electronic excitation devices such as thyristor and IGBT, the harmonic component of the generator has increased. When the terminal voltage of the generator is more than 10% higher than the rated voltage, the third harmonic of the voltage will increase significantly due to the magnetic saturation of the motor. Similarly, when the power side voltage of the transformer exceeds the rated voltage by more than 10%, the third harmonic of the secondary side voltage will also increase significantly. Since the grid voltage offset is below ± 7%, the harmonic components generated by power generation and transformation equipment are relatively small, much lower than the national assessment standard. Therefore, power generation and transformation equipment are not the main contradiction affecting the quality of grid voltage waveform.

Therefore, the main contradiction affecting the voltage waveform quality of the power grid is the nonlinear electrical equipment, that is, the nonlinear electrical equipment is the main harmonic source. The nonlinear electrical equipment mainly includes the following four categories:

• electric arc heating equipment: such as electric arc furnace, electric welding machine, etc.

• DC electric equipment with AC rectification: such as electric locomotive, electrolysis, electroplating, etc.

• AC rectifier and inverter electrical equipment: such as variable frequency speed regulation, variable frequency air conditioning, etc.

• switching power supply equipment: such as medium frequency furnace, color TV, computer, electronic rectifier, etc.