MEASUREMENTS OF PARTIAL DISCHARGES AND POWER LOSSES IN PET FILMS IN HIGH VOLTAGE AC FIELDS

Konstantinos Theodosiou - Dimosthenis P. Agoris - Ioannis Gialas - Isidoros Vitellas

   This paper examines the behavior of polymeric films exposed under the influence of high intensity 50 Hz, AC fields. PET films (Polyethylene terephthalate) of various thicknesses are stressed in a voltage range starting from 0,5 kV and reaching close to their breakdown voltage (approximately 70% of the previously measured V_b. During the stress time, partial discharges are measured (Q) as a function of voltage and their relation to the electric field strength F is derived. Measurements of losses (dissipation factor tan &delta) and capacitance C are also taken as a function of the applied voltage and the relative permittivity &epsilon_r of the samples was calculated. An attempt of explanation of the partial discharge activity is taking place using the void size distribution and an equivalent electric circuit, that simulates the presence of an air filled void inside the bulk of the polymer. A useful relation between partial discharge activity and the specimen thickness is derived. It was also found that the non-linear behavior that is observed in power losses measurements is due to the conduction component of loss current which is field dependent, if we assume that the polarization current remains almost linearly dependent up to near-breakdown fields and we ignore the dipole losses. The constant value of the calculated relative permittivity &epsilon_r using the measured specimens capacitance, indicates that the imaginary component is also constant as the voltage increases.

Keywords: PET, polymeric insulation, dielectric breakdown, dielectric strength, finite elements simulation

[full-paper]