Effects of Corona Discharges on Silicone Rubber Samples under Severe Environmental Conditions

Abstract

Silicone rubber (SiR) material has been utilized as a replacement to ceramic insulators. The impetus for increasing SiR applications in outdoor insulators is its unique hydrophobicity. However, SiR insulators are subjected to different stresses that may lead to their aging, hence the loss of their hydrophobic properties. The aging of SiR insulators is dependent on several factors. Some of these factors are related to weather, while others associated with system conditions, including electrical discharges generated from corona and/or dry band arcing. These factors also may possess synergistic impact, which can aggravate the aging process. Studying the SiR resistance to corona can be considered of great interest because the corona is one of the critical aging factors affecting the SiR insulators. Moreover, severe environmental conditions like ultraviolet (UV) radiation and high humidity can accelerate the aging process caused by corona. Hence, extensive research has been conducted to understand the performance of silicone rubber material exposed to corona discharges under varied weather conditions. However, the literature to analyze the synergetic effect of corona and various weather conditions on the aging of silicone rubber materials is still insufficient. This motivates to investigate the impact of corona along with other weather conditions on the aging of SiR material. In the present research, experiments are conducted by generating corona discharges using a high AC voltage of 10 kV applied to sharp needle electrodes on the surface of SiR samples for 24 hours. The gap distance between the surface of samples and the tip of the needle was set at 6 mm. The experiment is performed inside a testing chamber where both the humidity level and UV radiation can be controlled. The UV radiation with 1 mW/cm2 intensity is generated from UVA-340 lamps. The humidity level inside the test chamber is increased through creating mist from deionized water using an ultrasonic humidifier. Three humidity levels are considered, i.e. high humidity (80 to 90%), medium humidity (65 to 75%), and low humidity (30 to 40%). At each testing conditions, three silicone samples are tested simultaneously. After the experiment of this study, the silicone rubber degradation is assessed using the static contact angle measurements. The measurement is taken for 100 hours to quantify the hydrophobicity loss and recovery. It has been found that considerable hydrophobicity loss in all SiR samples is originated by corona aging test. Furthermore, the outcomes show that both the UV radiation and humidity possess considerable effect on the hydrophobicity loss and recovery. For further understanding, the scanning electron microscopy (SEM) and energy-dispersive X-ray spectroscopy (EDX) have been used to investigate the intensity of surface damage. Results of SEM analysis show signs of aging in the SiR samples such as craters and cracks. Based on EDX findings, it has been found that considerable reduction in aluminum trihydrate (ATH) content, which plays important role in erosion and tracking resistance, on the SiR samples surface. Overall, it has been shown that samples tested at both high humidity and UV radiation conditions suffered from the worst corona effects, whereas the low humidity with no UV condition has the least corona impacts.