Waveform Quality Evaluation Method of Variable-Frequency Current Based on Curve Fitting

Shengquan Zhao and Yaozong Liu ()
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Shengquan Zhao: College of Intelligence Science and Technology, National University of Defense Technology, 410073 Changsha, China
Yaozong Liu: College of Intelligence Science and Technology, National University of Defense Technology, 410073 Changsha, China

Energies, 2022, vol. 15, issue 20, 1-16

Abstract: Since total harmonic distortion (THD) is mainly used as the evaluation index for the waveform quality of periodic signals, it cannot be applied to variable-frequency signals. However, there is scarce research on the evaluation methods and indicators of variable-frequency signals in the literature. In this paper, an evaluation method of the waveform distortion (WD) of variable-frequency signals based on curve fitting is proposed. First, the variable-frequency current expression of the coefficients to be optimized is obtained through theoretical derivation. Second, the coefficients are optimized by curve fitting in the time domain through the nonlinear least-squares method. Then, the waveform distortion of the variable-frequency current ( I WDVF ) is calculated. In order to validate the proposed evaluation method, the simulation model of a synchronous motor driven by a cascaded H-bridge five-level inverter is built. The simulation results show that, for the same constant-frequency current, the current THD ( I THD ) obtained by the FFT method is the same as the current WD ( I WD ) obtained by the curve fitting method, which verifies the equivalence of the two methods. The influence of different sampling frequencies on the I WD and I THD is compared. The higher the sampling frequency, the more sampling points, and the more accurate the results. For the linear variable-frequency current, the I WDVF obtained by the curve fitting method is close to the average value of the I THD obtained by FFT, which indicates that the time-domain curve fitting method is reasonable to solve the I WDVF . For nonlinear variable-frequency current waveforms, the curve fitting method can also reasonably calculate the I WDVF . The curve fitting method can solve the evaluation problem of the variable-frequency current, and provide an evaluation means for the optimal design of a modulation strategy aiming at the optimal waveform quality of the variable- frequency current.

Keywords: variable-frequency current; waveform distortion evaluation; curve fitting; THD (search for similar items in EconPapers)
JEL-codes: Q Q0 Q4 Q40 Q41 Q42 Q43 Q47 Q48 Q49 (search for similar items in EconPapers)
Date: 2022
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