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This work proposes a new method for choosing the best transmission system expansion plan for the highest satisfaction level of the decision maker. The proposed method considers the permissibility and ambiguity of the investment budget (economics) for constructing new transmission lines and the delivery marginal rate (reliability criterion) of the system. This is achieved by modeling the transmission expansion problem as a fuzzy integer programming one. The method solves for the optimal strategy (a reasonable and flexible plan that would not be significantly worsened by any assumed changes in the surrounding situations) using a fuzzy set theory-based branch and bound method that utilizes a network flow approach and the maximum flow-minimum cut set theorem. When only a very limited size database is available to evaluate probabilistic reliability indices, the proposed technique provides the decision maker with a valuable and practical tool to solve the transmission expansion problem, considering problem uncertainties. Test results on an existing 21-bus system show that the proposed method is suitable for solving the transmission expansion planning problem subject to practical ambiguities.

A nodal probabilistic production cost simulation method is described for power system long-term expansion planning considering unavailability and delivery limitation constraints of the transmission system. This new nodal production cost simulation model includes capacity constraints and unavailabilities of generators as well as transmission lines. This simulation methodology comes from the nodal composite power system equivalent load duration curve (CMELDC), based on a new effective load model at load points developed by the authors. The nodal CMELDC can be obtained from convolution integral processing of the outage capacity probability distribution function of the fictitious generator and the original LDC. It is expected that the new simulation model based on the nodal CMELDC proposed here will provide solutions to many problems based on nodal and decentralised operation and control of electric power systems under a competition environment. The nodal CMELDC based on the new model at load points can extend application areas of nodal probabilistic production cost simulation, probabilistic congestion cost assessment, analytical outage cost assessment and nodal reliability evaluation and so on at load points. The characteristics and effectiveness of this new proposed methodology are illustrated by a small system case study using a network flow and enumeration method.