The ageing of composite insulating materials-new possibilities to detect and to classify ageing phenomena with dielectric diagnostic tools

The application of composite insulations based on minerally filled and fibre reinforced polymers has increased drastically during the last twenty years. The present paper summarizes the most critical ageing phenomena and gives a survey about new and traditional dielectric diagnostic tools and their applicability for the detection of material ageing phenomena. frequency and time domain test procedures within the range of 10^-6 Hz to 100 GHz will be summarized. Especially the extreme low frequency (ELF) range will be investigated experimentally in detail for a detection and classification of ionic conductivity problems and internal delamination processes in composite insulations like fibre reinforced and minerally filled polymeric matrix systems. The ELF method will also be used for the detection of material damages due to electrical overstresses. Ageing problems in composite materials can often be detected by an intensively pronounced low frequency dispersion (LFD). In this context LFD phenomena will be discussed using Cole plots and simplified mathematical approaches. Results from experimental tests with artificially aged material test samples will be presented. For this purpose simplified and model-like thin electrical insulations will be used. Finally, a classification and evaluation of the estimated dielectric responses during the ageing process will be done