Probabilistic reliability criterion for planning transmission system expansions

Probabilistic reliability criteria, such as LOLP, LOLE, EENS etc., have been used extensively for generation expansion planning throughout the world. Typically, only deterministic reliability criteria, such as (N - 1) or (N - 2) contingencies, have been used for grid expansion planning until now. The main reason is the difficulty in determining the optimum design of a grid expansion plan using probabilistic reliability criteria and in developing the appropriate mathematical model that incorporates the chosen reliability criterion for the transmission system. Optimum design in grid expansion planning is still an important element of overall electric power system planning in deregulated electricity markets. One of the potential improvements in generation and grid expansion planning comes from the use of a more realistic reliability level as a criterion to constrain the form of the optimum expansion problem. A methodology is proposed that uses a probabilistic reliability criterion to determine an optimum plan for transmission system expansion using a criterion that minimises the expected cost, which includes both construction and outage costs, using a probabilistic reliability criterion loss of load expectation RLOLE. The optimum value of the reliability criterion RLOLE* is determined at the minimum value of the combined total of the cost of constructing new transmission and the customer outage cost associated with supply interruptions. The characteristics and effectiveness of this methodology are illustrated by a case study using a 21-busbar test system.