An LVRT strategy with quantitative design of virtual impedance for VSG

Grid-forming inverters controlled as Virtual Synchronous Generators (VSGs) can provide voltage and frequency support for the power grid, which enhances the stability of the grid. However, VSGs face three challenges under grid voltage sags, i.e., maintaining power angle stability, avoiding overcurrent, and providing reactive power support, while previous virtual impedance-based LVRT strategies for VSGs cannot meet those three requirements simultaneously. In this paper, an LVRT strategy with frozen power loops and quantitatively designed virtual impedance is proposed. By freezing the power loops under grid voltage sags, the power angle stability is maintained. Meanwhile, the virtual impedance is quantitatively designed taking both the tolerable current of the inverter and the reactive power requirement of the grid code into account, which limits the overcurrent and fulfills the requirements of the grid code simultaneously. The experimental results verify that the proposed LVRT strategy not only meets the above three requirements but also makes full use of the maximum output capacity of the inverter during grid voltage sags.