ANRN COMPUTATIONAL TOOL FOR COMPOSITE REUABIUTY EVALUATION

Abgbact - This paper describes the modeling assumptions and computational aspects of a new computational tool for composite reliability evaluation, CREAM. The reliability evaluation methodology is based on Monte-Carlo sampling with a variance reduction scheme which allows the incorporation of user experience or of analytical models as "regression" variables. The component of reliability indices "explained" by these regression variables is "factored out" of the Monte-Carlo sampling scheme, thus reducing computational effort. The approach is illustrated in case studies with 500-bus systems derived from Bonneville Power Administration and Georgia Powcr. l"DucTK)N There is a growing need within the electric utility industry to refine the methods used in the allocation of capital resources based on reliability considerations, and to determine what are the tradeoffs between cost and reliability. The evaluation of steady-state reliability of a bulk power system is widely recognized as a very important part of this process [l]. Generation reliability evaluation methods, in which transmission limitations are ignored, were the first to be developed. They are based on the analytical characterization of the probability distribution of the available generation capacity, which is then compared to the load to be supplied. Efficient methods to compute generation reliability have been available for several years, and the resulting probabilistic indices such as loss of load probability (LOLP) are widely used in planning studies. Later, the rapid growth of power networks and increasing interconnection among utilities led to the recognition that transmission networks play a critical role in determining overall system reliability. Including transmission makes the task of calculating system reliability much more difficult than when only generation capacity is examined. Evaluation of supply adequacy for a given scenario (where load levels, generation availability and circuit availability are predetermined) usually requires a power flow analysis, instead of simple comparisons as in the case of generation reliability evaluation. Examples of currently available transmission reIiabiIity evaluation programs include SYREL[2] and RECS[3]. As part of a recent EPRI project [4], the possible role of composite reliability evaluation, i.e. taking into account both generation and transmission outages, was assessed through utility surveys, literature search, and analysis of current planning needs. The following areas were identified as requiring composite reliability 'tools: