Partial Discharge Diagnostics of Micro Cavities in Epoxy Insulating Materials and their Modelling

The state evaluation of an electric insulation using diagnostic measuring technique generally takes place with integral procedures or with partial discharge measuring and analysis technology, which can detect local and limited defects. With such diagnostic measurements different goals are pursued: new insulating systems are examined in order to judge their suitability in the context of development processes regarding the adopted new materials, production technologies or construction principles. For the condition-oriented maintenance and service partial discharge measurement and analysis got more and more important, because evaluation of the measurement results allows focussing the maintenance costs directly at the concerned operating facilities. Thus the life cycle costs can be substantially reduced with increased process security. In this contribution exemplary specimens of mineral filled epoxy resin are measured and evaluated by partial discharge intensities. Thereby possible cavity sizes are related to the partial discharge intensity. The gas discharge process, which appears during a partial discharge, is well-known for larger cavities (starting from 1 mm in diameter). This knowledge must be examined regarding their transferability for micro cavities. Here the question arises, starting from what size on micro cavities with modern partial discharge measuring methods are to be detected and starting from which order of magnitude (height of apparent charge, diameter) they are relevant for aging and/or destruction of the insulator. The negative effects of humidity-induced aging concern all material properties, e.g. dielectric values (dielectric loss factor, relative permittivity) degrade and enforce the field displacement from the epoxy resin into the micro cavities with less permittivity. The measurements of artificial micro cavities pursue the aim to show similar PD levels like the ones mentioned before. Additionally, the process of field displacement by humidity-ag- ing is also shown.