Evaluation of live-line maintenance techniques used on BPA reduced clearance 500 kV lines

One way to increase transmission availability is to perform maintenance while the line is energized. The proposed practices for performing live-line maintenance on the Bonneville Power Administration (BPA) compact 500 kV designs have not been tested. The purpose of this project is to determine the electrical safety margin of these live-line techniques. Full-scale tower models based on BPA´s compact 500 kV line design were constructed and tested at the Carey High Voltage Lab in Vancouver, Washington. Switching impulse tests were conducted to determine the critical flashover (CFO) level of each configuration. Several variables were evaluated including insulator type, number of broken units, position of broken units, and presence of tools. A team of experts from maintenance, safety, laboratories, conductor design, and structural design participated in this project from the planning stage to the review of test results. The following conclusions resulted from this collaboration: there is adequate electrical safety margin to replace as many as four broken insulators in a given V-string; the maximum degradation of the tower dielectric strength due to the presence of tools is 9%; the porcelain insulators used for these tests exhibited the same dielectric strength as the glass insulators, even when broken; and the single circuit tower window has 9% higher dielectric strength than the double circuit structure.