Qualification of fiber-reinforced insulating materials based on PD analysis

For fiber-reinforced insulating materials used e.g. in gas-insulated system (GIS), a test procedure has been created which stresses tube shaped specimens simultaneously dielectrically and mechanically. Mechanical stress is applied by four-point bending at electric ac fields up to 12 kV/mm. To prevent surface discharges the specimens are tested in a 600 kPa SF₆ atmosphere. For diagnosis a partial discharge (PD) system with a sensitivity of 100 fC has been employed. Even for high-quality vacuum-impregnated materials internal PD was detected and a clear influence of mechanical stress was found. The study of PD patterns resulted in the proposal of a prebreakdown model, which reveals the physical failure process taking place in the fiber/matrix-interface of the specimens. The model provides suitable criteria for a clear ranking of the tested materials. Also general suggestions for the improvement of fiber-reinforced insulating materials are given. For a quantitative estimation of life expectancy accelerated multifactor aging-tests are recommended