Non-linear stability boundary assessment of offshore wind power plants under large grid disturbances

Recent research has identified that the dynamics of the phase-locked loop (PLL) converter control contributes to grid-synchronisation instability when a wind turbine (WT) system is perturbed with large disturbances (i.e. severe grid faults). The metric for such stability assessment is the region of attraction (ROA), which signifies a subset of the system's state-space in which all the trajectories converge to a stable equilibrium point. This paper reviews the performance of transient stability assessment methods such as the time-domain methods, i.e. time-domain simulations, and phase portrait analysis; and the analytical methods, i.e. equal-area criteria, and Lyapunov's direct method, for estimating the ROA of a post-disturbance WT system. Additionally, some highlights of advanced methods for transient stability assessment, such as the sum-of-squares optimisation technique and machine learning methods, are also presented. The time-domain and analytical methods are applied on an equivalent swing equation model of a WT, where it is seen that the time-domain methods are less complex and provides accurate ROA, accompanied by a high computation burden. On the other hand, the analytical methods are complex and provides either slightly optimistic or highly conservative estimates of the ROA; however, such methods are very fast due to closed-form solutions.