Damping Torque Analysis of the PLL Low-Frequency Components for Grid Following VSG

Abstract

Phase-locked loop (PLL) is the key tool for the grid-connected inverter to track the frequency and phase of the grid voltage when synchronizing with the power grid. Therefore, the stability of PLL is particularly important. This paper proposes a damping torque analysis method to study the stability of the low-frequency component of PLL for the grid following a virtual synchronous generator (VSG). Firstly, the small signal model is established for the grid following VSG with a dual-loop structure. Secondly, the damping torque analysis method is provided especially for the stability analysis of PLL low-frequency components. And the corresponding stability criterion is constructed. Finally, significant factors that affect the stability of PLL low-frequency components are studied by the test system and the corresponding mechanism is revealed, including the influence of line reactance, PLL control parameters, and virtual damping. Results show that the stability of PLL is greatly dominated by the low-frequency component, whose stability can be improved by adopting a smaller integral coefficient of PLL, smaller virtual inertia, and larger virtual damping D.