Impedance Characteristics Analysis of Grid-Connected Voltage Source Converter based on Symmetrical Phase-Locked Loop

Impedance modeling is applied to analyze the stability of grid-connected voltage source converter (VSC) widely under weak inductive grid. The traditional phase-locked loop (PLL) will introduce frequency coupling characteristic due to the asymmetric structure, and the negative-and positive-sequence will influence each other which complicates the impedance analysis. This paper analyzes the characteristic of the symmetrical PLL, and the frequency coupling of the grid-connected VSC will be eliminated after employing the symmetrical PLL. In addition, the symmetrical PLL can ensure that the impedance characteristics of the grid-connected VSC remain unchanged with the grid voltage variations. Simulation results are conducted to verify the correctness of impedance model and the characteristics of the symmetrical PLL.