In the AC power grid, due to the operation of many nonlinear electrical equipment, the voltage and current waveforms are actually not complete sine waveforms, but non-sine waves with different degrees of distortion. Harmonic refers to the amount of electricity contained in the current whose frequency is an integer multiple of the fundamental wave, and generally refers to the Fourier series decomposition of the periodic non-sinusoidal charge, in addition to the electricity generated by the fundamental frequency, the rest of the electricity generated by the current greater than the fundamental frequency is called harmonics. The harmonic order is the ratio of the harmonic frequency to the fundamental frequency (n=fn/f1). The harmonic waveform diagram is shown in Figure 1 below, and the harmonic decomposition diagram is shown in Figure 2 below.

The fundamental frequency is the grid frequency (power frequency 50Hz), the harmonic frequency (h) is the integer ratio of the harmonic frequency to the fundamental frequency, and the intermittent wave frequency (ih) is the ratio of the intermittent wave frequency to the fundamental frequency. For example, when the fundamental wave is 50Hz, the second harmonic is 100Hz, and the third harmonic is 150Hz, as shown in Figure 3 below. According to the "Power Quality Public Grid Harmonics GB/T 14549--93", the total voltage distortion rate is qualified within 5%.

Figure 3

2.The main equipment that generates harmonics

Non-linear loads: switched-mode power supplies (SMPS), electronic fluorescent lamp ballasts, speed-regulating drives, uninterruptible power supplies (UPS), magnetic core equipment (transformers, motors, etc.) and household appliances (e.g. televisions)
Rectifier equipment: Generally speaking, multiphase commutator equipment is the largest number of harmonic sources in the power system, and its main circuit usually does not have neutral conductors, rectifiers (AC-DC), inverters (AC-DC-AC), and frequency converters (AC-DC-AC, AC-AC)

3. The harm of harmonics
1．Harmonics may cause voltage resonance in the power system, produce overvoltage, and may break down the insulation of line equipment. When a large number of 3rd harmonics flow through the neutral wire, the line may overheat or even cause a fire.

2．Harmonics will cause the relay protection and automatic devices of the system to malfunction, and at the same time lead to the measurement of electrical measuring instruments is inaccurate.

3．The impedance of the capacitor to the higher harmonics is very small, so when the voltage containing the higher harmonics is applied to both ends of the capacitor, the capacitor is easily overloaded and damaged.

4．Harmonic current will lead to an increase in the power loss of power lines.