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The presence of dc offsets results in fundamental frequency oscillations in the frequency estimation and it is a challenging problem in single-phase grids. In this article, a four consecutive samples (4CSs) based method is proposed to estimate grid frequency with strong dc offset rejection capability. In details, a 4CS model is established based on the relationship among the fundamental grid voltage, dc offsets, and their consecutive sample signals. Different from the traditional 3CS model, the proposed 4CS model can reject the negative effects of dc offsets completely. Based on the obtained model, a frequency estimation is proposed and stability analysis is provided according to the adaptive control theory. Unlike the traditional 3CSs relying on prefilters, the proposed method is filter-free and it still has strong suppression capability against dc offsets. The theoretical sound and experimental results indicate that the proposed method achieves accurate frequency estimation with good dc offset rejection capability. [ABSTRACT FROM AUTHOR]

The cascaded delayed signal cancellation (CDSC) based PLL in dq rotate frame ( dq CDSC-PLL) can totally eliminate the disturbances of phase error. However, its open-loop bandwidth is significantly affected after incorporating CDSC into the PLL structure, so the transient response performance of three dq CDSC-PLLs are deteriorated. This paper proposes an improved dq CDSC-PLL by cascading a digital filter based phase-lead compensator into CDSC-based PLL to improve the dynamic response without sacrificing the stability. The filter shows flat gain in the passband, which can compensate the phase delay induced by CDSC and simultaneously maintain the attenuation of the notch point. The improved dq CDSC-PLLs significantly increases the open-loop bandwidth and enhances the dynamic performance. Besides, in order to further minimize the undesirable steady-state error, the frequency adaptive control was adopted without using any additional circuits. In order to verify the effectiveness of the proposed method, the simulation based on Matlab/Simulink is carried out for various conditions. Furthermore, experimental results for typical scenarios are provided, which highlight the advantage of the improved dq CDSC-PLL in terms of the dynamic performance. [ABSTRACT FROM AUTHOR]