The Effect of Partial Discharge Aging on the Dielectric Response of Polymers

Abstract: This work investigated the effect of partial discharges (PD) on the complex permittivity of PVC, PC, PTFE, PE and PA6. The material samples were subjected to prolonged PD activity and the complex permittivity of the material was measured by dielectric spectroscopy (DS) before and after the sample was aged. The results showed a change in the loss factor for the tested materials, some were more affected than others and showed different behaviors when subjected to the PD aging. Hence it was not possible to find any general trend in the complex permittivity of the tested materials. Several of the materials exhibited a loss factor with -1/2 slope at low frequencies corresponding to diffusion. The -1/2 slope implies that the loss factor and the dynamic component of the real permittivity should be equal, which could not be seen. Therefore it is uncertain whether what was observed is a diffusion process or not. The results could suggest that PD aging causes a change in the trapping characteristics of the material. Furthermore, space charges deposited on the cavity wall from the PD activity could diffuse into the bulk of the material where some of the charges are trapped. If a DS measurement is performed before the trapped charges have had time to recombine or be conducted away, it could affect the complex permittivity. This was demonstrated by performing another DS measurement on an aged PC sample which had been left to rest for 35 days. The measurement showed that the complex permittivity had returned to almost the same state as before aging. Measurement problems were encountered which affected the measurements in this work. The problems are believed to be caused by the measurement electrodes used for the DS measurements not being heavy enough to eliminate small unwanted air-gaps between the electrodes and sample. Furthermore, incomplete results were obtained for some measurements which was believed to be due to the capacitance of the sample being at the limit of what the measurement instrument was able to measure.