Discharge Characteristics of Trapped Charge in Power Lines with Underground Cable and Overhead Line Segments

The use of underground cables as power lines, either alone or in combination with overhead lines, is steadily increasing. After a cable is isolated from the system, its trapped charge starts to slowly discharge. We may need to determine the discharge time for multiple reasons such as safety, and limitations on circuit breaker capabilities to withstand capacitive inrush currents. This paper will modify the existing method of discharge time calculation considering the fact that the ends of the cable remain connected to the switchgear bus after cable isolation. A procedure is also presented to calculate the discharge time for the combination (series connection) of underground cables and overhead lines. Sensitivity studies are performed to investigate the effects of different factors such as the type and conditions of the line insulator and design voltage level on the discharge characteristics. All the studies are conducted on a real transmission system. However, the procedure and findings are general and can be used in any application such as in a distribution system or any industrial power system.