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With the continuous development of the national economy and the constant improvement of the power system, the safe operation of the power system and the reliability of the power supply become increasingly important.

Traditionally, the 3~35kV (including 66kV) power grid belongs to the medium and low voltage transformer distribution network; the **** value of the internal overvoltage in such power grid is not high, so the main factor that threatens the safety level of the grid insulation is not the internal overvoltage, but the atmospheric overvoltage (i.e., lightning overvoltage), therefore, the overvoltage protection measures that have been adopted for long are only to prevent the atmospheric overvoltage from harming the equipment. The main technical measure only relates to the installation of various arresters, whose discharge voltage is more than 4 times of the phase voltage. Designed to avoid the internal overvoltage, it is only effective for protecting lightning damage, but has no protection against the internal overvoltage.

However, the operation experience indicates that when such grid develops to a certain scale, the internal overvoltage, especially the grounding overvoltage generated during the single-phase intermittent grounding of the grid and the ferromagnetic resonance overvoltage generated under special conditions, has greatly threatened the safe operation of power grid equipment. The single-phase intermittent grounding overvoltage will be the greatest threat.

In recent years, the operating experience in enterprises relating to metallurgy, coal, petroleum, chemical, power supply, etc. indicates that to absorb the overvoltage with the optimization cabinet to **** extent, matched with the line selection and harmonic elimination modules, has the desired results, and is welcomed and recognized by users in eliminating the system intermittent overvoltage and ferromagnetic resonance overvoltage, judging the single-phase earthing fault line and other aspects.