Enabling islanding and re-synchronization of grid-forming offshore wind turbine generators

This paper explores the capability of grid-forming (GFM) offshore wind turbine generators (OF WTGs) to operate in an islanded mode and to re-synchronize with the grid. The transient stability of the WTG during the onset of the islanding and during resynchronization is investigated for several islanding durations. Two novel re-synchronization methods are proposed for the GFM OF WTGs while maintaining their voltage source behaviour. The first of the proposed methods enables a smooth power rampup during the re-synchronization period following the end of islanding. It ensures transient power-angle stability of the GFM converter with the grid by fixing the islanded GFM converter’s phase angle. The second proposed method relies on reconnection followed by a phase-match after a cycle. This method can elongate the overall islanding duration, increasing the power disruption duration; however, it still provides a simple yet reliable way to enable islanding clearance. A comparison between the two methods is also made to investigate their applicability, benefits, and drawbacks. For future power grids such as grid-forming offshore wind power plants (GFM OF WPPs) connected to HVDC grids, these methods provide a potential solution to withstand momentary and longer islanding without imposing severe mechanical and electrical stress on the WTG’s mechanical and power-electronic equipment.