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

Abstract

This 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.