Model, Analysis, and Design of a q-Axis Self-Synchronizing Current Control for Three-Phase Grid-Connected Converters

This article introduces a q-axis self-synchronizing current control strategy for three-phase grid-connected converters with LCL filters, encompassing its modeling, analysis, and design. Grid-forming converters synchronize with the grid by adjusting terminal voltage magnitude and frequency based on deviations in active and reactive power references. In contrast, grid-following converters use a phase-locked loop for synchronization, where a controller regulates output current by commanding the converter voltage. Building on these concepts, the proposed strategy achieves self-synchronization by monitoring deviations from current references and dynamically adjusting the amplitude and frequency of the synthesized voltage. This method enables a self-synchronizing current control that achieves grid synchronization independently of grid voltage measurements. Real-time hardware-in-the-loop testing validates the effectiveness of the q-axis self-synchronizing control. It also demonstrates the accuracy of the linearized model in representing system dynamics. The control strategy maintains stable operation under both ideal and nonideal grid conditions. Furthermore, this article provides a root-locus analysis of the linearized state-space model, a detailed design methodology, and a comprehensive stability analysis, including the impact of the operating point on system dynamics. Practical aspects such as a start-up procedure and an LCL reactive power compensation method are also addressed to support real-world implementation.