A new diagnosis of the XLPE insulation with water treeing by space charge measurements with the thermal step method and water nuclear magnetic resonance imaging

In presence of water, the rehabilitees of XLPE are greatly influenced by the formation of water trees. In this paper the XLPE insulation degraded by water trees by water inception through external cracks in the PVC sheath has been studied by different techniques. Classical techniques has been used as the optical microscope examination of water trees after coloration, FT-IR mapping of cut thin films but also with other methods of investigation as space charges measurement by the thermal step method and Magnetic Resonance Imaging (MRI) for water penetration. A 2 D MRI transversal mapping of the water in the insulation show three regions: an external one full of water trees till the third of the depth explaining the electrical breakdown, thereafter a region of voids due to an old steam extrusion and a third without water trees. The degradation of the cables arises from a simultaneous presence of both dipoles and injected space charges which create a non linear internal field. Therefore the space charge profile in function of the depth has been studied on a wet cable from the net, after thermal treatment and after comparison with a non aged insulation as reference. The density of charges depends of the examined region in the insulation. The TSM technique has been also applied on 70 m cables using a positive thermal step by joule effect (Inner Heating Technique "IHT"). In the laboratory, 50 cm cable samples were studied with a cold thermal step (outer cooling technique "OCT"). The interpretation of the TSM results is in accordance with the observed morphology identified by optical microscopy and MRI.