A Systematic Approach to Design Amplitude Estimators for NTD-PLL: Performance Improvement Under Abnormal Grid Disturbances

Nonfrequency-dependent transport delay-phase-locked loop (NTD-PLL) is one of the simple grid synchronization approaches that aims to improve the phase and frequency estimation under frequency deviation. However, its amplitude estimation still exhibits double-frequency oscillations that are larger in magnitude as compared to the conventional transport delay (TD)-PLL counterpart. To overcome this challenge, two systematic approaches to design amplitude estimators (AEs) for NTD-PLL are presented in this article. One approach includes eliminating double-frequency oscillation error present in \boldsymbol{dq}-frame component {\boldsymbol{\upsilon }}_{\boldsymbol{d}}, whereas another uses Pythagorean trigonometric identity in \boldsymbol{\alpha \beta }-frame for accurate amplitude estimation. Due to the general form of AE proposed in \boldsymbol{\alpha \beta }-frame, it can be easily applied in the design of other TD-based PLLs. The design aspects of these methods are discussed in detail, and the efficacy of the proposed structures is finally evaluated through numerical and experimental results. Some methods to enhance the performance of the AEs are explored and analyzed. This article provides useful insight to the designers regarding the advantages and disadvantages associated with the proposed AEs for their specific applications.