A Novel Phase-Locked Loop Structure to Enhance Converter Stability in Weak Grids

The phase-locked loop (PLL) is widely used to synchronize the converters with the grid. However, the asymmetric dynamic characteristics of the PLL may cause frequency coupling effects and even cause sub/supersynchronous oscillation in weak grids. This article analyzes the frequency coupling effects of the conventional dual second-order generalized integrator PLL. A novel PLL structure is proposed to eliminate the frequency-coupling effects, simultaneously controlling the d -axis and q -axis voltage components. The interharmonic components can be suppressed, and the perturbation propagation paths through the PLL can be prevented. In this way, the synchronization stability of converters under weak grids is improved. Also, the design procedure of the structure and parameters of the PLL are provided. Finally, the simulations and experimental results verify the effectiveness of the proposed solution.