Degradation behaviour of voids in silicone rubber under applied AC electric fields

This paper is concerned with an experimental study into the degradation processes that occur when voids in solid dielectric materials experience high applied electric fields. A method has been developed for manufacturing 2mm thick samples of silicone resin that contain a single void of around 1mm diameter. Five samples are simultaneously electrically stressed under an applied ac sinusoidal voltage of 12kV for 6 hours that is then increased to 15kV until a sample fails. During the stressing period, PD data is regularly acquired. The remaining 4 samples are then inspected for signs of degradation. Degraded samples that have not suffered catastrophic failure and contain pits or evidence of electrical trees were cut open using an RMC MT-7 ultra-microtome equipped with a CR-21 cryo-system set at -110°C in order to provide a surface containing open segments of pits or trees. The experiment is repeatable and the obtained degraded samples and the degradation areas of microtomed samples have been analysed using Raman spectroscopy to identify the chemical content of the degraded areas at the voids /silicone rubber interface. Initial results indicate that the degradation areas of microtomed samples are probably pits generated from the voids, which may be evidence of the initial development of a bow-tie electrical tree. This paper will detail the experiment, contain images of the obtained degraded samples and detail the chemical changes that occur in silicone rubber due to a electrical degradation process.