Grid-voltage Synchronization Algorithms Based on Phase-locked Loop and Frequency-locked Loop for Power Converters

Abstract

The purpose of this thesis is to study and find the appropriate grid-voltage synchronization method for grid-connected converters under different kinds of faults like phase unbalancing, harmonics, offset and voltage sags. The main purpose of grid-synchronization algorithms is to estimate the positive- and negative-sequence components of the utility voltage under unbalanced and distorted condition. The existing most advanced phase-locked loop (PLL) and frequency-locked loop (FLL) methods are well known method for grid-synchronization. The fundamental variable estimated by the PLL is the grid-phase angle, whereas the grid frequency is the one for the FLL. The most extended technique used for grid synchronization in three-phase three wire system is a synchronous reference frame PLL (SRF-PLL). The SRF-PLL works accurately during balanced condition, but cannot estimate voltage components during unbalanced condition. The Decoupled Double Synchronous Reference Frame PLL (DDSRF-PLL) is might be a substantiation solution for the estimation of the sequence components of the utility voltage under unbalanced condition. Another method based on the FLL, a double second-order generalized integrator FLL (DSOGI-FLL) has also the ability to detect the positive- and negative-sequence components of the utility voltage under unbalanced condition. DDSRF-PLL and DSOGI-FLL algorithms are tested under different kinds of faults and compared with each other. The results show that their performance under harmonic-distorted condition is not really acceptable. A new algorithm based on SRF-PLL, decoupled multiple synchronous reference frame PLL (DMSRF-PLL) might be a better solution for accurate detection of the positive- and negative-sequence voltage components under unbalanced and harmonic distortion condition.