Material erosion and build-up at the high voltage electrode during PD testing of subsea insulation materials using a modified Cigré II test method

The main aim of this work has been to test the long term durability of insulation materials subjected to surface partial discharges (PD), as a part of a larger study on the performance of materials used in subsea high voltage apparatus. Electrical equipment used in subsea installations are subjected to high hydrostatic pressures and seawater, often combined with high temperatures and even reactive chemical environments, making new challenges for the materials involved. The test method used for PD-testing is based on the Cigré II method. The method has been used in dry and humid conditions to identify the effect of humidity on the degradation of insulation materials subjected to PD relevant for subsea applications. The insulation materials samples were 2 millimeter thick disks of POM (polyoxymethylene) and XLPE (cross-linked homo-polymer polyethylene). The samples where subjected to surface discharges in a 1 mm sphere-to-plane electrode gap. The results show that the humidity during testing had a significant impact on material erosion and material transport in the electrode gap. POM was more rapidly degraded by PD than XLPE.