An Enhanced Type 3 Phase-Locked Loop with Improved Frequency Estimation Capability Under Grid Distortion

This paper proposes an enhanced Type 3 phase-locked loop (ET3-PLL) to improve the frequency estimation performance of the conventional Type 3 synchronous reference frame PLL (SRF-PLL) under harmonic distortion. Frequency response analysis of the conventional Type 3 SRF-PLL reveals its limited ability to reject harmonic disturbances in frequency estimation. To address this limitation, the ET3-PLL is introduced. Its small-signal model is established, and comparative frequency-domain analysis shows that the ET3-PLL provides superior frequency estimation performance under harmonic distortion. Moreover, large-signal analysis of the phase error dynamics indicates that the ET3-PLL achieves exactly the same phase-tracking performance as the conventional Type 3 SRF-PLL. Experimental results under various grid conditions validate the effectiveness of the proposed method.