| Summary: | The frequency-locked loop based on a second-order generalized integrator has been widely used in grid synchronization systems but featuring unfavorable harmonics filtering performance. To solve this issue, a frequency-locked loop based on multiple second-order generalized integrators was proposed, which is to realize harmonics attenuation and frequency synchronization concurrently. Nevertheless, it is inconvenient for implementation for the complex structure and burdensome calculation. The target of this work is to solve these issues by proposing two novel frequency-locked loop structures based on repetitive controllers. The proposed solutions show many favorable features simultaneously, $i$ ) they are resilient to all the commonly confronted grid interferences, such as harmonics, dc components, voltage sages, frequency variations, etc.; ii) they are convenient to implement thanks to the simpler structure and significantly alleviated computation load; iii) they have satisfying dynamic performance. Eventually, the effectiveness of the proposed frequency-locked loops is comprehensively verified by experiments under different working conditions.
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