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Harmonic distortion, also known as harmonic interference, refers to the odd wave (3, 5, 7...) in the sine wave. , the change has nothing to do with the frequency, but has absolutely to do with the change of voltage and current. Harmonics are of no use in electronic circuits. They only cause overheating of electronic equipment or damage to electronic parts.

Generation of harmonics:

In the past 30 years, power electronic devices have been widely used and become a large harmonic source. In all kinds of power electronic devices, rectifier device accounted for a large proportion. At present, most of the commonly used rectifier circuits are thyristor phase-controlled or diode rectifier circuits, among which three-phase bridge and single-phase bridge rectifier circuits are the most common. The harmonic pollution and power factor lag caused by rectifier circuits with resistive load have been well known. The diode rectifier circuit with capacitive filter on the dc side is also a serious source of harmonic pollution. The phase of the fundamental component of the input current of this circuit is approximately the same as that of the supply voltage, so the fundamental power factor is approximately 1. However, the harmonic component of the input current is very large, which causes serious pollution to the power grid and makes the total power factor very low. In addition, the phase-controlled ac power adjustment circuit and the power electronic devices such as the cycle converter also generate a large amount of harmonic current on the input side.

Ideally, the voltage provided by a utility network should be a single, fixed frequency and a defined voltage amplitude. In the past 30 or 40 years, the rapid development of various power electronic devices has made the harmonic pollution of public power grid more and more serious, and various faults and accidents caused by harmonics have also occurred continuously. Therefore, the severity of harmonic hazard has aroused people's high attention. The hazards of harmonics to public power grid and other systems are as follows:

⑴  Harmonic causes additional harmonic loss of components in the public power grid, which reduces the efficiency of power generation, transmission and electric equipment. When a large number of third harmonics flow through the neutral line, the line will overheat and even fire will occur.

⑵  Harmonics affect the normal operation of various electrical equipment. In addition to the additional losses caused by the harmonics, mechanical vibration, noise and overvoltage will also be generated, causing severe local overheating of the transformer. Harmonics cause equipment such as capacitors and cables to overheat, deteriorate insulation, shorten service life and even damage.

⑶  Harmonic will cause the local parallel resonance and series resonance in the public power grid, so that the harmonic amplification, which makes the harm of the above success and the harm greatly increased, and even cause serious accidents.

⑷  Harmonics can lead to the wrong action of relay protection and automatic device, and make the measurement of electric measuring instrument inaccurate.

⑸  Harmonics will interfere with the adjacent communication system, which will cause noise and reduce the communication quality. Heavy result in the loss of accommodation, so that the communication system can not work properly.