Abstract-- An important component to be considered in electric power system expansion planning is the security of service that the system is able to provide. Under the market environment the effect of expansion on the market conditions also has to be regarded during the planning. This paper proposes a new method for transmission expansion planning under market environment which considers global welfare, construction and security enhancement cost. Finally a proper method to share the cost of expansion fairly between all network agents is suggested. To investigate the validity of the method, it is applied to the modified “Garver 6-bus test system for expansion”. Index Terms--Transmission expansion, electricity market, expansion planning, cost of security enhancement, cost allocation

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