Characterization of the volume conductivity and the polarization behavior of biaxially-oriented polyester films for use in HVDC bushings

摘要

The electric Field distribution inside a bushing is given by capacitive and resistive components of the insulation materials. Capacitive components can be controlled by conductive foils which form a grading. In this case, the layers of a bushing core represent metal-insulator arrangements. The DC field distribution is dominated by the resistive components. They can be subdivided into volume and surface resistances. In the case of metal-insulator-metal contacts, the used metal and its contact to the insulator may influence the observed resistances. In order to calculate the DC electric field within a bushing core, the specific volume and surface conductivity must be known. For DC applications, solid-gas insulated bushings with polymeric insulation materials are an alternative to oil or resin impregnated paper bushings because of the significant lower resistive losses. Therefore, polarization and depolarization current (PDC) measurements of polyester films will be presented. By using a guard ring arrangement, volume and surface effects can be separated. The presented measurements are taken at different temperatures (25 °C up to 75 °C) and controlled humidity (< 1%). Aluminum foils are used for contacting the test sample as they are often used in capacitive graded bushings. Massive electrodes (contact pressure of 2 kPa) are put on to guarantee entirely contact to the sample. The test arrangement is shielded in order to reduce background noise. Due to measure very low currents (< 10 pA), the arrangement is also shielded against vibrations and controlled against offset shifts. The measurements show a decreasing noise level caused by the measurement data preparation. The maximum standard deviation of the current was less than 2 pA. The results from the PDC measurement will be compared with measurements taken by the recovery voltage method. The Curie-von Schweidler law for the depolarization current as well as the Arrhenius law for the temperature dependency of the volume conductivity will be taken into account to discuss the measurement results.