Symmetrical voltage sag support strategy based on photovoltaic virtual synchronous generator

During symmetrical voltage sags caused by grid faults, the inadequate active support capability of the conventional photovoltaic virtual synchronous generator (PV-VSG) may lead to system collapse or even grid disconnection. This paper examines the dynamic interaction between the photovoltaic array and virtual synchronous generator (VSG), proposing a variable step-size voltage tracking algorithm specifically designed for photovoltaic arrays. By predicting the maximum power output of the photovoltaic array and setting suitable load-shedding ratios, the suboptimal power point is calculated based on the power capacity of the grid-connected system. The inertia-induced oscillations of the system's active and reactive power curves during voltage sags are considered. An adaptive power angle control function is incorporated to address instability caused by increases in the power angle. A voltage feedforward compensation algorithm, based on the voltage sag ratio, is then employed to enhance the VSG's voltage support capability during voltage sags, enabling flexible responses to varying degrees of symmetrical voltage dips. Additionally, a dynamic impedance function is introduced to design virtual impedance, reducing system underdamping, minimizing oscillations, and buffering overcurrent surges during voltage dips or recoveries. Comparative stability analysis of the photovoltaic grid-connected system demonstrates a significant improvement in the active support capability of the PV-VSG during grid voltage sags.