Investigation into Oversheath Failure of a 275 kV Cable

A lightning event caused a 275 kV cable, which was part of a composite overhead line-cable circuit, to fail. ATP-EMTP package was used to model the network around the cable. It was found that a lightning event with current magnitude of 24 kA, striking the tip of the transmission line tower adjacent to the Cable Sealing End (CSE), can result in an increase to the sheath voltage with respect to the substation earth mat that exceeds the recommended limit. The study confirms that the Sheath Voltage Limiter (SVL) rating should not be exceeded and that the SVLs should remain intact. The finding of the study is in line with the site inspection report. Recommendations are made to reduce the sheath voltages during lightning and switching transient events. The recommendations include changing the sheath earthing arrangement by moving the SVLs to the cable entry points and earthing the sheath at the existing junction box. It is shown that the earth mat inductance around the CSE and earthing points can be reduced by using appropriate earthing rods. It is also suggested that high quality earthing straps are used to ensure a good connection to earth.