Transmission expansion planning for hybrid AC/DC grids using a mixed-integer non-linear programming approach

Transmission expansion planning is a well-established field of research focused on modifying existing networks to accommodate increasing loads and geographically distributed generation. However, due to computational constraints, many existing approaches rely on linearised models or simplified representations, such as the DC power flow approximation, which may limit modelling accuracy. In contrast, this paper introduces non-linear models to optimise the impedances of AC/DC hybrid systems. The proposed methodology supports network planning along both existing and new corridors, incorporating parallel expansion, reconductoring of current routes, and the selection of optimal line sizing. It aims to optimise investment by balancing capital and operational expenditures while ensuring compliance with system limits and loading conditions. Case studies are presented to demonstrate the versatility and industrial relevance of the approach.