A method for determining probability based allowable current ratings for BPA´s transmission lines

Allowable current ratings for Bonneville Power Administration´s (BPA) transmission lines are presently based on conservative deterministic criteria. This conservative criteria has served BPA well for many years with minimal cost penalty as most lines have historically been stability limited. Beginning in the mid-70s, a number of BPA´s lines have shifted from stability limited to thermally limited, suggesting the conservative assumptions in the present allowable current rating model may be associated with significant cost penalties. A new method, based on probabilistic techniques, is described which diminishes the conservative assumptions in the present deterministic method promising substantial cost savings. General probabilistic equations are developed and the overall technique´s philosophy discussed. The probabilistic method revolves around statistical ambient weather models which are reported. Research work has been performed to characterize weather regions in BPA´s service area assembling limited statistical weather models for each region. Planned future work is outlined including the development and deployment of a data acquisition system capable of collecting ambient weather data in support of probabilistic line ratings