Study And Apply On Inter-harmonics Emission Characteristics Of Medium-frequency Power Supply

With the rapid development of industry today, steel is one of the important raw materials of national industrial production. Medium-frequency induction furnace and other metal smelting equipments are widely used to meet the social demand for steel. With the extensive use of Medium-frequency induction furnace and users’ demand further enhanced of the power quality in power system, the problem of harmonic produced by Medium-frequency induction has been attracted more and more attention. The characteristics of integer harmonic produced by medium-frequency power supply have been deeply study, but a lot of inter-harmonics are detected in the enter-wire of Medium-frequency induction furnace. It becomes one of the most important factors to affect the grid power quality and the safe and stable operation of power equipment.The works of rectifier and inverter are studied in this paper, the mathematical models of harmonic transmission characteristics in both of them are built, and the mathematical models of inter-harmonic are derived by combining harmonic transmission characteristics in both of them in medium-frequency power supply. Then, inter-harmonic is divided into positive sequence component and negative sequence component by studing the theory of inter-harmonic sequence component decomposition. The mathematical model is also applied to multi-wave controlled medium-frequency-power supplys’ inter-harmonic emission characteristics analysis. The sources and transmission process of inter-harmonic are explained by inter-harmonic emission characteristics, the theoretical reasons are provided for inter-harmonic prevention and governance.Medium-frequency power supply with twenty-four pulses rectifier is analyzed by simulation, the simulation results show that the mathematical model and the theroy of inter-harmonic sequence component decomposition are proved to be reliable and accurate. Then, the content of harmonic current and the effective value trends of inter-harmonic are analyzed by the current measuring data which is tested from the enter-wire of twenty-four pulses rectifier and twelve pulses rectifier. The measurement results show that inter-harmonics emission characteristics of medium-frequency power supply and the theory of inter-harmonic sequence component decomposition can meet the demand of engineering application as well. Finnaly, the problem of 11th filter branch is deeply analyzed and researched in this project case. The fusing reason of 11th filter branch is the inter-harmonic which launched by medium-frequency power supply. The inter-harmonic is overlapping with the parallel resonance point of power supply system and filter branch formed. It causes that inter-harmonic which launched by medium-frequency power supply is magnified, and the level of inter-harmonic far beyond the rated current of 11th filter branch. In this peper, the single-tuned filter branch is reformed in the case, and the problem of 11th filter branch is solved.