A Comparison of Grid-Forming and Grid-Following Control of VSCs

Sammanfattning: Variable renewable energy sources are today increasingly integrated in the power system as a step towards the renewable society. The large-scale introduction of converter-based energy sources brings challenges in terms of reduced damping to the power system due to the reduced number of synchronous generators. This can be manifested as high rate-of-change-of-frequency and decreased grid stability. To forestall this reduced performance, it is suggested that the grid-following control of today’s converters are restructured to a grid-forming control, enabling the converter to behave closer to a synchronous machine.   This thesis compares grid-following and grid-forming control and seeks to further describe this grid-forming behavior by applying a grid-forming control method on an energy storage enhanced STATCOM-system. A continuous time model and a linearized model based on state space representations are constructed in order to investigate the grid-forming behavior but also how the converter stability is affected by a restructure from grid-following to grid-forming control.   The results indicate that the investigated grid-forming control method displays a behavior similar to synchronous machines and incorporates the ability to provide frequency response services and so called “synthetic inertia” to the grid.  The results also show that the stability of the converter (the ability to provide a bounded output when the system is perturbed) is ensured when the control method is restructured from grid-following to grid-forming and that the investigated grid-forming method is stable also in weak grid situations.