Design and analysis of PLLs for line-connected converters in three phase systems

This paper is concerned with phase locked loop (PLL) structures for use in the control of line connected power electronic converters in three phase systems. Such converters have to operate under conditions in which the power system phase voltages are often unbalanced. The unbalanced and distorted line voltage vector as observed in the stationary reference frame generally has instantaneous rates of change components in the radial as well as in the angular directions. The essential task of a PLL in this context is usually to track an appropriately filtered behavior of the angular rate of change. This paper presents and analyzes methods to achieve this filtering and tracking. The positive sequence vector component in the input phase voltages is extracted using geometrical properties of the unbalanced vector trajectory.