Study on Sheath Voltage and Earth Potential Rises of the Hybrid Overhead-cable Lines

The breakdown accidents of cable outer sheath and sheath voltage limiters frequently occur due to the unreasonable sheath bonding design. Unlike pure cable lines, the sheath voltage of hybrid overhead-cable lines could exceed the power frequency withstand voltage of outer-sheath under some circumstances. Its amplitude is closely related to the earth potential rise at the connection point of overhead lines and cables. This paper employed PSCAD-EMTDC to study the power frequency sheath overvoltage of a hybrid overhead-cable line type in the case of single-phase-to-ground fault of the overhead line terminal, where the overhead line was inserted between the cable lines applied with various sheath bonding methods. And the influence of the grounding resistance of the terminal tower and the earth continuity cable on the sheath induced voltage was also discussed. Simulation results indicated that, the earth potential rise and sheath voltage of single-point bonded cables exceed 25 kV in serious cases. Compared with the cables with sheath single-point bonded, the earth potential rise at the connection point was much lower, below 10 kV, when the overhead line was connected to the cables with sheath cross-bonded, and the grounding resistance of the terminal tower had little influence on the earth potential rise. Besides, the installation of earth continuity conductor had a markedly positive effect on the power frequency withstand performance of cable outer-sheath and sheath voltage limiters for this hybrid line type.