Ngai-Man Ho

Grid Synchronisation Based on Frequency-Locked Loop Schemes

This chapter presents synchronisation algorithms aimed to provide an estimation of the angular frequency and the fundamental component of a reference signal (grid voltage). Schemes for both single-phase and three-phase cases are presented. The proposed schemes do not require transformation of variables into the synchronous frame coordinates as conventional phase-locked loop (PLL) schemes. Therefore, they are not based on the phase angle detection. Instead, the angular frequency is detected and used for synchronisation purposes. The design of the proposed schemes is based on a complete model description in terms of stationary frame coordinates of a reference signal subject to a considerable amount of harmonic distortion. The proposed schemes include explicit harmonic compensation mechanisms to alleviate the effect of such harmonic distortion. Therefore, they are intended to perform properly under considerable distortion caused by low order harmonics, and are robust against angular frequency variations in the reference signal. Moreover, in the three-phase case, the model description involves both positive and negative sequences in stationary coordinates of the fundamental and harmonic components. Therefore, this scheme is able to perform well under severe unbalanced conditions, providing clean estimates for both the positive and negative sequences of the fundamental component of an unbalanced and distorted three-phase reference signal.