High voltage DC cables are used for long distance transmission of electric power where overhead lines are either not possible or for other reasons not attractive. Presently, cables made of extruded polymer materials are being introduced for this purpose. There is still limited service experience for this type of cable, and international standards for testing and verification of these cables are lacking. Manufacturers follow recommendations for long-term testing, type tests and routine tests issued by the CIGRE organisation. To a large extent the screening methods described in these recommendations are based on current practice for testing of AC cables. The failure modi may however be different for a DC cable compared with an AC cable, and it is therefore important to develop testing metods and criteria that are relevant for DC applications. In particular, there is need for improved test methods that can be employed to perform screening tests on cables as they are produced in the factory. The presently ap plied AC testing techniques which are based on the CIGRE recommendations become impractical and economically hardly acceptable as the lengths of and voltage of the cables increase.
The project will aim to identify and if possibly quantify the various fac tors that are critical for the performance of an extruded polymer cable under DC stress. At least three different polymer materials will be studied in order to investigate whether material-specific properties are important or whether the failure modi are generic for this type of extruded cable. Based on the knowledge of critical failure mod(es), methods will be developed that can be used to test cables as they are produced.