The EFI test method for accelerated growth of water trees

The philosophy behind the development of Rogowski-type test objects and the aging and test procedures used in the Norwegian Electric Power Research Institute (EFI) test method are presented. Some of the results obtained during almost eight years of the use of this method are briefly reviewed. The great advantage of the method is that a model of a cable insulation system consisting of semiconducting screens and extruded insulating materials can be tested. After production, the test objects are screened in a high-voltage test (50 kV/mm) in order to exclude production errors. After aging, water tree analysis and breakdown tests can be performed on the same test objects. The effect of differing aging conditions and materials on water treeing and breakdown strength can easily be determined. The objects are manufactured from extruded insulation tapes. The EFI test method has been demonstrated to be well correlated with cable test results provided the same batch of materials is used for comparison