Influence of insulation thickness on the electric Properties of unfilled µm-sized thermosets

Through the ongoing miniaturization of electric components, insulation systems with microscopic dimensions become more significant. Furthermore, the increasing use of HVDC and power electronics in energy distribution and transport results in a varied electrical stress situation for the used insulation materials composed of AC-, DCand mixed voltages. In this context, the paper contains experimental examinations analyzing the dielectric properties and the conducting behavior of unfilled, unsaturated polyester resin with thicknesses s≈3 .. 1000 μm at electric field strengths E≈1 .. 9 kV/mm. For the realization of test specimens a vacuum process and an optimized micro-milling process are used. This enables the production of samples with insulation thicknesses in the lower μm-range and surface roughnesses of a few nanometers. According to IEC 60250 and IEC 60093 the test specimens are equipped with a three-electrode system. The dielectric properties, relative permittivity and dissipation factor, are measured at frequencies f = 20 Hz .. 20 kHz and different sample thicknesses. The apparent conductivity is determined by polarization and depolarization current analyses. All measurements are carried out at a controlled temperature ϑ = 20°C± 2°C. As a result, the electric properties of unfilled, unsaturated polyester resin show significant thickness dependencies as they are known for polymer´s permeability and breakdown strength. The measured relative permittivity, dissipation factor and apparent conductivity decrease with decreasing sample thickness. For the interpretation of this behavior a model based on space charges in microscopic areas close to the electrodes is proposed.