Stability Region Analysis for HVDC-Connected Offshore Wind Farm Participating in Grid Frequency Support

When the high-voltage direct current-connected offshore wind farm (HVDC-connected OWF) participates in grid frequency support, excessively high frequency controller gains may exacerbate the electromagnetic oscillations within system. Therefore, it is essential to study the stability region for frequency controller gains of HVDC-connected OWF, thus ensuring the grid secure operation during frequency events. Currently, numerical methods are used to analyze the operational stability region of HVDC-connected OWF. However, these methods involve exhaustive calculations to obtain the stability region, resulting in a significant computational burden. To address the issue, this work proposes an analytical stability region identification method for HVDC-connected OWF when it participates in the grid frequency support. Specifically, a comprehensive small signal model, tailored for analyzing frequency support-induced oscillation, is first developed by considering both the frequency regulation characteristics and the power converter dynamics. Further, using the Padé approximation technique, the reduced order equivalent model is derived while preserving its accuracy. With this equivalent model, the mechanism underlying frequency support-induced oscillation is qualitatively elucidated, and the stability region for frequency support is also analytically derived. Finally, the analysis results are validated through electromagnetic transient simulations.