Sub-Synchronous Oscillations and Transient Stability of Islanded Microgrid

Sustained sub-synchronous oscillations are observed in an islanded microgrid consisting of grid-forming converters under a large transient disturbance. Such oscillations are studied in terms of various types of hidden attractors, namely, hidden periodic orbit, hidden quasi-periodic orbit, and hidden chaotic attractor, which are shown to coexist with the stable equilibrium point, share different basins of attraction, and be transformed mutually for different parameter values. Moreover, the basins of attraction have fractal boundaries, making the nearby trajectories unpredictable. The transient stability of the islanded microgrid can be improved by eliminating the hidden attractors through adjusting control parameters and triggering a homoclinic bifurcation. Furthermore, transient disturbances can be mitigated by protecting critical converters and accelerating the restarting process. Besides, the emergence of sub-synchronous oscillations can be eliminated through tripping off one or more specific converters. Full-circuit cycle-by-cycle simulations are performed for islanded microgrids comprising i) 5 grid-forming converters, ii) 5 grid-forming converters and a synchronous generator, and iii) 30 grid-forming converters to verify the analytical findings.