Impact of Inner Control Loops on Small-Signal Stability and Model-Order Reduction of Grid-Forming Converters

The analysis and efficient dynamic simulation of power systems with a large number of distributed generators using grid-forming converter control requires reduced order models. The natural separation of time scales allows for the simplification of the inner control loops, i.e., voltage and current controller. This work provides a structured overview of model reduction approaches of the inner loops and introduces new reduced models based on transfer function approximation. The models are thoroughly tested using frequency response analysis, eigenvalues sensitivity analysis, time-domain simulation and stability regions for various scenarios, varying controller and electrical parameters. Furthermore, the influence of simplifications of the network lines is investigated. It becomes apparent that there is no single reduced model that is suitable for all scenarios. Models ought to be selected depending on the specific scenario. Guidelines for the model selection as well as the choice of controller parameters are provided.