A software phase locked loop for unbalanced and distorted utility conditions

A software phase locked loop (PLL) for custom power devices is described in this paper. The PLL uses the previous and current samples of the 3-phase utility voltages and separates them into the sequence components. The phase angle and frequency of the estimated positive sequence component are tracked by the first two stages of the PLL. The third stage estimates the parameter Δθ which is required by the first stage. Simulations of the performance of the PLL have been presented, as well as the performance of a dynamic voltage restorer incorporating the proposed PLL. The proposed PLL performs well under unbalanced and distorted utility conditions. It tracks the utility phase angle and frequency smoothly and also extracts the sequence components of the utility voltages. A DVR incorporating the PLL provides voltage support to sensitive loads and also acts as a harmonic filter.