Two-Stages Sliding Fourier Transform for High Performance Phase Angle and Frequency Tracking

Sliding discrete Fourier transform (SDFT) gives excellent performance at nominal frequency operation. However, at off-nominal frequency, it generates errors in both magnitude and phase angle due to spectral leakage. Moreover, its harmonic rejection ability is greatly impaired in this case. This paper introduces a method to apply sliding Fourier transform under off-nominal frequency at fixed sampling rate while fixing the defects associated with it off-nominal frequency operation. This method involves the use of two stages sliding Fourier transform (SFT). A fixed window-width SFT in the first stage is used to drive the variable window-width SFT in the second stage. The proposed technique (SFT–SFT) is tested in real time on dSPACE MicroLabBox using pre-generated voltage vectors to simulate the most inconvenient grid conditions. The testing scenarios proved its superior performance compared to the decoupled stationary reference frame PLL method.