Assessing the Impact of Offshore Wind Farm Grid Configuration on Harmonic Stability

This paper presents a methodology for assessing the harmonic stability of an offshore wind farm considering the wind farm configuration and the onshore AC grid impedance. Using the impedance models of the wind farm configurations, respective loop gains are derived. Thus, the impedance-based stability criterion can be applied to assess the harmonic stability. An iterative process is carried out to investigate potential interactions within the overall system consisting of several components such as the wind turbine converter and transformer as well as the offshore transmission line and onshore AC grid equivalent. The impedance models are validated by comparison with the frequency response obtained from simulations of respective time domain models. The results show that the derived methodology indicates low margins and unstable systems at particular frequencies. Corresponding time domain models agree with the prediction showing a marginally stable system oscillating at the predicted frequencies. It is demonstrated that the harmonic stability depends on the wind farm configuration, as it is shown that oscillations in the grid current are influenced by the placement of the turbines within the wind farm.