The Time Line Method for Assessing Galloping Exposure

The design of double circuit transmission structures is often determined by the need to allow sufficient electrical clearance between phases under galloping span conditions. In the past such designs have been arrived at according to certain "galloping ellipse" criteria in which the ellipse geometry is based on mid-span sag. The new method, disclosed herein, starts with the statistical history of the weather in the particular region, as to wind, wind direction, temperature, and ice leading to an exposure rate (Hrs./Yr.) for the normal component of wind speed. These data are combined with estimates of galloping motion including amplitude dependence on wind speed, gusting, and frequency mis-match between galloping and horizontal (swinging) movement at mid-span. Illustrative examples are worked out showing the comparison of galloping exposure rates and flashover between two typical double circuit structure designs for a 345kV transmission line. A comparison is also included between untreated and treated spans, the latter having galloping control devices with only 50% amplitude reduction capability. A range of span lengths between 750 ft. (227M) and 1,500 ft. (454M) is considered. The new method, for the first time, provides a means to assess the benefits of alternative designs in quantitative terms.