A PLL‐equivalent model for low‐frequency stability analysis of voltage source converter connected to weak grid

Abstract

AbstractThis study focuses on stability of weak grid connected voltage source converter (WG‐VSC) in Low‐frequency mode (LFM) (around 1–10 Hz), which is dominated by interactions among phase‐locked loop (PLL), outer loop control and weak grid condition. In order to clearly reveal LFM mechanism of WG‐VSC, a simple but effective PLL‐equivalent model has been proposed. First, a generic small signal model consisted of PLL and ‘outer loop & WG’ two parts is derived. Then, the transfer function of complicated ‘outer loop & WG’ part is divided into two parallel channels, representing the impact of active side control and reactive side control on LFM of WG‐VSC respectively. Finally, based on the multi‐mode decomposition theory, a simple but effective second order PLL‐equivalent (E‐PLL) model is obtained for LFM stability analysis of WG‐VSC. The effectiveness of E‐PLL model has been verified by simulation results in PSCAD/EMTDC and experimental results based on a hardware platform.