The ideal voltage provided by the public grid should be a single and fixed frequency and a specified voltage amplitude. The appearance of harmonic current and harmonic voltage is a kind of pollution to the public power grid. The environment in which it uses electrical equipment deteriorates, and the surrounding power electronic equipment is widely used. People have studied and recognized the harm of harmonic before, but at that time, the severe punishment of harmonic pollution did not attract enough attention. In the past thirty or forty years, various power electronic devices have been rapidly developed. With the increasingly serious harmonic pollution of the power grid, various faults and accidents caused by harmonics have also occurred continuously, and the seriousness of the harm of harmonics has aroused people‘s high attention. The harm of harmonics to public power grid and other systems is as follows.

    (1) Harmonics cause additional harmonic losses to the components in the public power grid, which reduces the efficiency of power generation, transmission and electrical equipment. When a large number of third harmonic flows through the neutral line, the line will overheat and even cause fire.

    (2) Harmonics affect the normal operation of various electrical equipment. In addition to additional losses, the influence of harmonics on the motor will also produce mechanical vibration, noise and overvoltage, which will cause serious overheating of the transformer. Harmonic wave causes overheating, aging of insulation, shortening of service life and even damage of capacitor, cable and other equipment.

    (3) Harmonic will cause local parallel resonance and series resonance in the public power grid, thus amplifying the harmonic, which will greatly harm the above (1) and (2), and even cause serious accidents.

    (4) Harmonics will lead to the misoperation of relay protection and automatic devices, and will make the measurement of electrical measuring instruments inaccurate.

    (5) Harmonics will cause interference to adjacent communication systems, or noise to reduce communication quality; The heavy ones lead to the loss of accommodation, which makes the communication system unable to work normally.