Abstract
This paper has investigated surge propagation and overvoltages on a coaxial cable when a lightning current flows into the metallic sheath based on experiments, EMTP simulations and analytical calculations. At the instance of the current flowing into the sheath, the core voltage becomes the same as the sheath voltage, which is given as a product of the sheath surge impedance and a half the current, following the theory of electrostatic shield. Thus no voltage difference appears between the core and the sheath across the insulator. When a traveling wave arrives at a transition point, i.e. cable termination, an insulating or crossbonding joint, the voltages change suddenly and a significant voltage appears across the insulator. An EMTP simulation shows that the overvoltage across the insulator exceeds 1200kV at a crossbonded node on a 400kV underground cable when an ac source voltage is in the opposite polarity of the lightning surge voltage on the sheath and the inductance of a crossbonding lead wire is taken into account. The high overvoltage appears in a narrow region of the cable between the current injected node and the crossbonded node due to traveling wave reflection between the nodes. The phenomenon investigated may occur if there exists a pinhole or a watertree on an underground cable.
