Double synchronous reference frame PLL for power converters control

This paper deals with one of the most important issues in the control of grid-connected converters, which is the detection of the positive sequence fundamental component of the utility voltage. The study carried out in this paper conducts to a fast, precise, and robust positive sequence voltage detector offering a good behavior, even if unbalanced and distorted conditions are present in the grid. The proposed detector utilizes a new "Double Synchronous Reference Frame PLL" (DSRF-PLL), which completely eliminates the existing errors in conventional synchronous reference frame PLL systems (SRF-PLL) when operating under unbalanced utility voltages. In the study performed in this paper, the positive and negative sequence components of the unbalanced voltage vector are properly characterized. When this unbalanced vector is expressed on the DSRF, the analysis of the signals on the DSRF axes permits to design a decoupling network which isolates the positive and negative sequence components. This decoupling network gives rise to a new PLL structure which detects the positive sequence voltage component quickly and accurately. In this paper, conclusions of the analytical study are verified by simulation and experiment.