Three-Phase PLLs by Using Frequency Adaptive Multiple Delayed Signal Cancellation Prefilters under Adverse Grid Conditions

Phase-locked loops (PLLs) are the most widely adopted techniques for synchronization of distributed generation units to the utility grid. However, when the grid voltage is polluted with severe unbalances and harmonics, the accuracy and performance of the conventional PLL will be deteriorated. This paper proposes a novel filtering technique, called the multiple delayed signal cancellation (MDSC), for improving the dynamic performance of the conventional synchronous reference frame (SRF)-PLL under adverse three-phase grid conditions. Theoretical developments of dq-frame and \alpha \beta-frame MDSC operators are investigated. The analysis shows that these MDSC operators can provide more flexibility to configure the lowest undesired harmonics. Since the MDSC operators possess multiple delayed signals in parallel, the delay time introduced by the operators can be reduced by selecting a suitable number of transfer delays. Therefore, the MDSC operators can be configured to have less delay time in comparison with the cascaded delayed signal cancellation operators with the same harmonic elimination capability. These two MDSC operators are utilized as a prefiltering stage to the conventional SRF-PLL to improve the dynamic responses. Since MDSC operators are dependent on grid frequency excursions, a new frequency adaptation scheme for the MDSC operators is also proposed for providing fast grid frequency estimations in designing the MDSC prefilter based PLLs. Experimental verifications demonstrate the effectiveness of the proposed PLLs and provide good dynamic responses when compared with the recent advanced PLLs.