Stability analysis of PLL influenced by control loops in grid-connected converters under fault disturbance

When the power grid suffers from grid faults that cause phase disturbances, the grid-connected converter becomes destabilized by the interaction between the phase-locked loop (PLL) and the control loop. In this paper, the stability of the PLL affected by the control loop under transient grid faults is studied. First, the equivalent model of the PLL under the influence of the control loop is established. Then, different response processes of PLLs under the ground fault with various control parameters are qualitatively analyzed. Furthermore, a small-signal model is proposed to assess the stability of the PLL under different control loop parameters. The system poles can be calculated to show the physical origin of the instability. Finally, simulations of a three-phase 21-level modular multilevel converter (MMC) built in PSCAD and a down-scale experiment is performed to verify the parameter influence of the control loop on the PLL.