Development of polyethylene insulation diagnosis of high voltage cables in France

The objective of the development of polyethylene insulation diagnosis of high voltage cables in France was first to check or to determine the origin of the damage after a dielectric breakdown and to confirm hypothesis such as an accidental water penetration in the insulation or an excess temperature rise in the cable. According to our experience gained in the operation, the main cause of the dielectric breakdown of extruded cables is water penetration in the insulation, generally in high voltage extruded cables manufactured without any waterproof screen in the sixties. Other defects correlated with an excess of the temperature rise in on-line cables can also be observed : we can note the existence of a high temperature annealing in polyethylene, or eventually a modification of the mechanical properties of the insulation. Sometimes manufactured defects, for example large cavities in the insulation, can also be detected. Finally, an evolution of the insulation could be correlated with the polyethylene ageing.