Effect of ADLash opitical fibre cable on corona onset gradient and electric fields around the shield wire of the Apollo-Cahora Bassa HVDC line.

Abstract

ADLash® is the trade name for an optical fibre cable attached to a transmission line shield wire using aramid-reinforced bands. The installation of this type of optic fibre cable has been considered for the HVDC line from the Songo hydroelectric scheme at Cahora Bassa in Mozambique, to Apollo substation in South Africa. The impact that installation of this cable will have on the onset of corona is examined. The shield wire with and without ADLash attached is modelled for both the actual line configuration and for corona cage studies. The electric field is calculated using the Method of Images and the Boundary Element Method to predict the electric field enhancements and to estimate the corona onset gradient. Corona onset gradients and phenomena for smooth and stranded conductors are researched to aid the prediction of voltage onset magnitudes for the corona cage. Estimated values are compared with observed values for both AC and DC applied voltages. Different patterns of corona and different corona onset voltages are observed for AC and negative DC applied voltages. The relative permittivity of the ADLash is higher than the surrounding air and this is shown to lead to greatly enhanced electric fields at the air/ dielectric/ shield wire interfaces that are significant enough to cause corona at the rated operating voltage. The corona will bombard the ADLash and probably damage it. Some alternative methods of installing fibre optic cables are reviewed for further research because the use of ADLash cannot be recommended.