In the framework of developing greenhouse gas mitigation strategies, we propose an efficient modelling assessment for planning optimal transmission network expansion considering the market competition between the generation investors, while stimulating the further expansion of renewable energy sources. The proposed approach accounts for centralised and decentralised electricity industry structure considering the power market modelled as either perfect competition or Cournot oligopoly in the latter case. We apply the aforementioned modelling approach to implement an illustrative case study for the Northern European energy system.