A Second Look on Nonfrequency-Dependent Transport Delay-Based PLL: Performance Enhancement Under Frequency Deviations

Several single-phase transport delay phase-locked loops (TD-PLLs) have been reported, among which a nonfrequency-dependent TD-PLL (NTD-PLL) has been proposed to supposedly deal with the challenge of phase offset and double-frequency oscillatory errors in the estimation quantities under the frequency deviations. However, research works reported the presence of double-frequency oscillatory errors in the estimations, which remains a crucial challenge in NTD-PLL. The main aim of this letter is to provide an analysis of the NTD-PLL and propose a modified NTD-PLL (mNTD-PLL). It is demonstrated through the numerical and experimental results that, with the appropriate implementation of the transformation matrix, the mNTD-PLL eliminates the double-frequency oscillatory errors from the estimation quantities. Furthermore, a linear model of the NTD-PLLs with higher accuracy is also derived in the linear time-periodic framework.