Advancement of the Long-Term Transmission Expansion Plan for Achieving Carbon Neutrality by 2050

Long-Term Plan), developed by the Organization for Cross-regional Coordination of

Transmission Operators (OCCTO), outlining the desired future configuration of Japan’s power grid toward 2050. Transmission reinforcement projects derived from the Long-Term Plan are also examined.

The OCCTO Long-Term Plan identifies common reinforcement candidates across multiple scenarios through cost–benefit analysis, based on merit-order simulations using a detailed nodal model. Individual projects are subsequently evaluated using refined technical assumptions, updated demand and supply projections, and detailed cost and benefit assessments—including construction and O&M costs, generation cost reduction benefits, adequacy enhancement benefits, and reduced transmission losses. Implementation decisions are made based on these evaluation outcomes.

This paper highlights key reinforcement projects already approved for implementation, including AC loop reinforcement in the central region, and a new HVDC subsea interconnection between the Chugoku and Kyushu areas. Technical considerations, the benefit-evaluation framework, and supporting decision-making processes are discussed. Through these case studies, the paper clarifies Japan’s wide-area grid evaluation methodology and identifies how uncertainty is incorporated into investment decisions, including additional quantitative assessment of risk mitigation effects. The findings provide insights into future large-scale transmission expansion initiatives required to support deep decarbonization.