A time-frequency mixed method for on-line monitoring of harmonics and interharmonics

To meet the on-line monitoring requirements of the interharmonic national standards, a hybrid method of time domain and frequency domain anlalysis is proposed to analyze and evaluate interharmonics. First, the parallel structure of the enhanced PLL (phase-locked loop) is used to detect the fundamental and harmonic components in the time domain. The structure detects the fundamental frequency signal of high amplitude with the main PLL, and the measured base frequency is output to sub-parallel PLL structure to lock the corresponding harmonic components. This method can track the time-varying fundamental and harmonic components rapidly and accurately, so a separate analysis of interharmonic components can be more effectively achieved with the suppression of the fundamental and harmonic leakage effects. Then the interharmonic components with fundamental frequency components and harmonics components removed are analyzed separately in the frequency domain by the harmonic group method, which can deal with wide distribution characteristics of interharmonic frequencies. Finally, an on-line monitoring platform for harmonics and interharmonics based on the proposed method is realized in LabVIEW. The results of simulation and experiments show the effectiveness of the method.