BPA´s Probability Based Clearance Buffers, Part III: Application to the Analysis of Existing Lines

A procedure to develop clearance buffers for the analysis of existing lines, based on probabilistic techniques, as used by BPA is described. A statistical clearance model for existing lines, Thermal Analysis Statistical Clearance Model, tailors the expected scatter in field clearance about a profile clearance for an existing line. This expected field clearance scatter results from measurement tolerances in the design and construction of overhead lines. The paper discusses the use of assurance levels for maintaining regulatory clearance requirements in the field and relates those assurance levels to the statistical clearance model. Using the statistical clearance model and assigned assurance levels, a process is described which establishes the thermal capacity of an existing line based on present regulatory clearance requirements. Finally, an existing line is analyzed to demonstrate the steps to implement the probabilistic clearance analysis process.