Small-signal Stability Analysis of an Offshore Wind Farm with Two Grid-Forming HVDC Terminals

This paper presents an analysis of small-signal stability and parametric sensitivity in the isolated ac network of a large-scale offshore wind farm with two HVDC terminals. The wind farm is assumed to be based on Type-4 wind turbines with grid-following (GFL) control organized in two clusters, each associated with a HVDC converter terminal. The two parts of the wind farm are interconnected with an ac cable, and the HVDC converters are controlled as Virtual Synchronous Machines (VSMs) to provide grid-forming (GFM) functionalities and ensuring shared frequency and voltage control in the islanded ac grid. Two different approaches for the small-signal analysis have been considered, based either on explicit analytical modeling or on numerical analysis in Matlab/Simulink, and the advantages or disadvantages of both approaches are discussed. Sensitivity analysis is conducted to examine the influence of key internal parameters of the VSM implementations, especially considering the virtual impedance required for parallel operation. While the analysis confirms that sufficient small-signal stability margins can be ensured in a wide range of parameters and operating conditions, it is also shown how too high or too low values of the virtual impedance can cause stability issues depending on the VSM implementation.