Modelling Power-Frequency Interactions between Voltage Source Converters with PLLs and Power Networks with Reduced Inertia

Traditionally, renewable energy sources have been connected to the grid by using current-controlled voltage source converters (CC-VSCs) based on phase-locked loops (PLLs). This approach simplifies the design and implementation of control algorithms and it has been well studied in the literature. Nevertheless, the massive integration of CC-VSCs in electrical systems can lead to undesired dynamic interactions. In particular, the connection of CC-VSCs to weak (high-impedance) grids have been thoroughly analysed. However, the connection to low-inertia power networks has received less attention. Therefore, the main issue addressed in this paper is: how do CC-VSCs and power grids with reduced inertia interact? Simple power-frequency models are developed to analyse this issue, especially under transient grid conditions where the existing modelling tools fail to provide a clear answer. Theoretical and experimental results are used to validate the main contributions of this work.