Optimal location and sizing of HVDC converters for same-tower AC-DC overhead lines

Using one circuit of multi-circuit AC overhead lines for HVDC transmission has been proposed to increase the transmission capacity, to minimize system stability problems, and to avoid opening new transmission corridors. This paper presents a formulation to determine the optimal location and power rating of HVDC converters to convert one circuit of a multi-circuit AC line to DC. These configurations, where AC and DC circuits share the same tower, can also be referred to as ‘AC-DC hybrid lines.’ The proposed method is similar to the formulation of one-stage transmission expansion planning. This problem is transformed to a mixed-integer linear programming by exploiting ‘disjunctive modelling.’ In addition, the proposed formulation takes into account the investment cost of HVDC converters, cost of reconfiguration of existing AC circuits to DC, energy and reserve provision costs, and N-1 security criterion. This optimization model allows the system operator to investigate the impacts of HVDC lines on transmission capacity increase, and power flow controllability. The advantages of two different hybrid configurations are investigated. The proposed deterministic optimization model is effectively applied to the IEEE RTS 24-bus system.