VSC control for AC/DC distribution grids

Abstract

Current renewable energy sources (RES) connection to the distribution grids is generally performed using voltage source converters (VSCs). These devices are able to convert and control AC/DC power efficiently. They enable fast and precise control of voltage/frequency even in the event of disturbances which appear in the grids. This work focuses on the two main VSCs control structures: Grid-following (GFL) and Grid-forming (GFM). The control approaches mentioned above are vital for the grid connection of the RES, meaning they areabletomaintainstableoperation. Thefirststageofworkcoversimplementationofboth controls on separate standalone models. For the GFL part we used conventional PLL control along with inner current loop. GFM control was based on P–f and Q–V droops. This way weacquire the necessary subsystems which lately will be implemented to the distribution grid system. The second stage consists of gathering the obtained models together with European Low Voltage distribution grid. Since in this system we use a transformer only for the grid equivalent to step down the voltage, the rest of the grid operates at the converter’s nominal voltage. As is typical for low-voltage distribution networks, the grid follows a radial structure, where power f lowis unidirectional- from source to loads. Asthe ACpart of the grid was operating as expected, we add the DC part by slightly adjusting theconfigurationoftheGrid-followingVSC.ItenablestotracktheDCvoltageandconsequently regulate the amount of power we want to flow through the DC part of the system.