Power quality analysis of medium frequency induction melting furnaces using sinusoidal coding

In this paper, the medium frequency induction melting furnace (IMF) system, has been represented by field-data-based models, developed specifically for power quality studies. The IMF is an electrical furnace in which heat is applied by induction heating of the metal and its operation injects various time-varying harmonic and interharmonic currents to the power system, which should be analyzed in terms of power quality. In this research work, the IMF operation has been represented by a variable series RL circuit to model the fundamental components of electrical quantities, and a shunt-connected current source, to model the generated harmonics and interharmonics over a typical melting cycle. Based on the similarity between the spectrums of speech signals and that of the IMF current, sinusoidal coding method is applied to represent the major harmonics and interharmonics of the supply line currents obtained from field measurements of the IMFs. It has been shown that the sinusoidal coding method can be used to satisfactorily represent the operation of the IMF system as seen from the supply terminals, with a minimum data storage requirement, and can be used to construct efficient IMF databases for power quality studies. The power quality problems of the IMF system, in particular the problematic time-varying interharmonic voltage distortion have been investigated in detail, using the developed model.