Overview of PLL methods for distributed generation units

Distributed energy resources are increasingly being connected to the utility grid by means of an inverter. The basic information necessary for these inverter-connected distribution units are the frequency and phase angle of the utility grid. The phase angle can be estimated using phase-locked loops (PLLs). Voltage unbalance, harmonics and other kinds of undesirable pertubations are common conditions in the electric utility which are detrimental for the operation of the PLL. In this paper, three commonly known PLL methods are discussed: the single-phase synchronous reference frame (SRF) PLL, the three-phase SRF PLL and the double SRF PLL. The effect of voltage unbalance on those PLL methods will be discussed in this paper.