Harmonic wave

Strictly speaking, harmonic refers to the electric quantity contained in the current whose frequency is an integral multiple of the fundamental wave. Generally, it refers to the electric quantity generated by decomposing the periodic non sinusoidal electric quantity with Fourier series, and the other electric quantity generated by the current whose frequency is greater than the fundamental wave frequency. In a broad sense, since the effective component of the AC power grid is a single frequency of the power frequency, any component that is different from the power frequency can be called harmonic. At this point, the meaning of the term "harmonic" has become somewhat inconsistent with its original meaning. It is precisely because of the broad concept of harmonics that there are expressions such as "fractional harmonics", "inter harmonics", "sub harmonics", and so on.

The main reasons for harmonic generation are: due to the sinusoidal voltage being applied to a nonlinear load, the fundamental current undergoes distortion and generates harmonics. The main nonlinear loads include UPS, switching power supply, rectifier, frequency converter, inverter, etc.

The frequency of a harmonic must also be an integer multiple of the frequency of the fundamental wave. A wave with a frequency of three times the fundamental wave is called a third harmonic, a wave with a frequency of five times the fundamental wave is called a fifth harmonic, and so on. No matter how many harmonics, they are all sine waves.