The harmonic problem of power system has attracted people's attention as early as the 1920s and 1930s. At that time, in Germany, the voltage and current waveforms were distorted due to the use of static mercury arc converters. J.C.Read's paper on converter harmonics published in 1945 is a classic paper on early harmonic research. In 1950s and 1960s, due to the development of HVDC transmission technology, a large number of papers were published on harmonic problems caused by converters in power systems. Since 1970s, due to the rapid development of power electronic technology, various power electronic devices have been widely used in power system, industry, transportation and home, and the harm caused by harmonics has become increasingly serious. All countries in the world attach great importance to harmonic problems. Many international academic conferences on harmonics have been held, and many countries and international academic organizations have formulated standards and regulations to limit harmonics in power systems and electrical equipment. Harm of harmonics is very serious-harmonics reduce the production, transmission and utilization efficiency of electric energy, make electrical equipment overheat, produce vibration and noise, make insulation aging, shorten service life, and even break down or burn out. Harmonic will cause local parallel resonance or series resonance of power system, amplify harmonic content, and lead to the burning of capacitors and other equipment. Harmonics can also cause misoperation of relay protection and automatic devices, leading to confusion of electric energy measurement. Outside the power system, harmonics will cause serious interference to communication equipment and electronic equipment.