Comparison of Phase-Locked Loops Used for Frequency Measurements in a Low-Inertia Power Grid with Wind Generation

Abstract

The need for more precise frequency measurements in electric power systems is increasing because of the growing penetration of renewable energy and the subsequent reduction in inertia, which gives rise to more intense frequency fluctuations. Phase-locked loops (PLLs) are now more accurate and present faster dynamic responses, helping to implement primary frequency controllers for renewable generators connected to the grid through power electronics. However, there are differences among PLL implementations that affect their behavior when estimating frequency, depending on factors as location, voltage level, or penetration of renewable energy, among other things. How this affects the ability of PLL-based frequency controllers to adequately estimate the instantaneous mismatch between generation and demand, especially in low-inertia power systems, is still unclear, and constitutes an open research topic. This paper contributes to this research effort by presenting a comparative study of different PLL configurations in order to gain insight into their performance in different scenarios in a low-inertia power system with a high share of wind energy. Constant and variable wind speed scenarios are considered, and PLL behavior is assessed in terms of the absolute mean value and the root-mean-square value of the frequency error, and the absolute value of the frequency derivative error. The results show that the PLL behavior depends on the voltage level and the location, and it was found that the further away the PLL is from the source that controls the frequency, the more noise is produced, which impacts the quality of the frequency measurement.