Stability Control of Grid-Connected Converter Considering Phase-Locked Loop Frequency Coupling Effect

Abstract

Given the problems that the phase-locked loop frequency coupling effect (PLL-FCE) in a weak grid reduces the quality of the output current waveform and brings challenges to maintaining a steady running of the grid-connected converter (GCC), this paper analyzes the coupling relationship between the FCE of the PLL, grid impedance and the output impedance of GCCs under a weak grid. It elucidates the role of the above coupling relationships in system stability and then proposes a stability optimization control method. Firstly, this paper delves into the frequency coupling phenomenon and its coupling mechanism in GCCs operating within weak grid conditions. Through analysis using small signal disturbance, it elucidates the significance of the PLL-FCE, particularly in medium- and low-frequency ranges, by establishing the coupling admittance model. Secondly, it presents the output impedance model for a three-phase LCL-type GCC, incorporating the characteristics of PLL frequency coupling. This model elucidates the interplay between the GCC’s output impedance, the PLL-FCE and the grid impedance. It also unveils the impact of the PLL-FCE on system stability in weak grid scenarios. Building upon these insights, this paper proposes an enhanced PLL based on the Second-Order Generalized Integrator (SOGI). It provides a detailed parameter design process for implementing these improved PLL structures. Finally, the study conducts simulation and experiment verification under weak grid conditions. The findings indicate that the PLL-FCE indeed undermines the stability of GCCs in the weak grid, with this effect becoming more pronounced as the grid impedance increases. However, the implementation of the SOGI-PLL successfully mitigates the adverse impact of the PLL-FCE on the stability of the converter–weak grid interactive system, thereby enhancing the adaptability of GCCs to weak grid environments.

Keywords

• frequency coupling effect
• grid-connected converter
• stability
• grid impedance
• SOGI-PLL