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On the phase sequence characteristics of interharmonics

Phase sequence is an important attribute of voltages and currents in three phase power systems. This paper investigates the sequence characteristics of interharmonic components. Three common cases involving interharmonics are analyzed. These are variable frequency drives (VFD), motors with fluctuating mechanical loads and general fluctuating impedance loads. The results reveal that interharmonics can have either positive or negative sequence and are rarely zero sequence. Its sequence cannot be determined by examining the interharmonic frequency alone. The general rule is that the sequence of interharmonics is the same as that of the supply system harmonic components being modulated. If the resulting interharmonic frequency produced by the modulation process is negative, the sequence of the interharmonic component will change from positive to negative or vice versa. The findings of this paper will help to improve our understandings on the characteristics of interharmonics and can be useful for the development of interharmonic simulation programs.