Designing EHV Lines to a Given Outage Rate Simplified Techniques

A simplified method for determining the COF required for a given Switching Surge Performance Level is given. If the surge voltage varies along the line, the line is shown to be equivalent to a shorter line. This shorter line will have a number of towers proportional to the insulation strength standard deviation (¿f/CFO) and inversely proportional to (1-¿), where ¿ is the ratio of switch end to open end voltage. The method is illustrated with examples, including both normal and extreme value distributions of over-voltage. Comparison of the method with exact calculations show no significant differences.