MECHANISM FOR IMPULSE CONVERSION OF WATER TREES TO ELECTRICAL TREES IN XLPE

A lightning impulse causes substantial capacitive current in a water t ree channel which, as a result of its small cross section, has relativ ely low conductance. Transient, nonlinear finite element computations with coupled thermal and electric fields for the geometry of a 15 kV X LPE dielectric cable indicate that an 80 kV lightning impulse can caus e the water within a water tree channel to boil over a range of four o rders of magnitude in water conductivity, from 10(-2) S/m to 10(2) S/m . boiling of the water in a water tree channel reduces substantially t he yield stress of the XLPE, raises the pressure within the water tree channel, and is likely to leave a cavity which can support partial di scharge resulting in electrical tree initiation.