Optimisation of transmission expansion decisions under uncertainty for a North Sea grid

Abstract Offshore wind energy is expected to play a central role in the decarbonisation of future power systems, particularly in the North Sea region, where deployment targets are highly ambitious. To support the large-scale integration of offshore wind while maximizing social welfare under long-term uncertainties, this paper proposes a novel stochastic transmission expansion planning framework. The approach relies on a reformulation amenable to Benders decomposition, yielding a scalable and parallelizable algorithm capable of finding exact solutions despite the presence of mixed-integer recourse decisions. The performance of the method is first demonstrated using an illustrative toy example. Subsequently, it is applied to a larger and more realistic representation of the North Sea offshore grid, enabling tractable solution of instances that are computationally challenging for conventional methods. The results reveal clear interconnection patterns, indicating that a highly interconnected offshore grid with multiple cross-border links can provide substantial benefits. Although the resulting topology varies with uncertain demand, the analysis shows that demand uncertainty has relatively limited influence on optimal first-stage investment decisions that are of primary interest.