In this work a phase-locked loop (PLL) is presented, which is able to provide an estimation of the angular frequency, and both the positive and negative sequences of the fundamental component of a three-phase signal. These sequences are provided in fixed reference frame coordinates, and thus the proposed algorithm is referred as fixed reference frame PLL (FRF-PLL). In fact, the FRF-PLL does not require transformation of variables into the synchronous frame coordinates as in most PLL schemes. The design of the FRF-PLL is based on a complete description of the source voltage involving both positive and negative sequences in stationary coordinates and considering the angular frequency as an uncertain parameter. Therefore, the FRF-PLL is intended to perform properly under severe unbalanced conditions, and to be robust against angular frequency variations in the three-phase source voltage signal. Although not considered in the design, it is shown that the scheme is also robust against harmonic distortion present in the source voltage signal.
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