Evaluation of the leakage current performance of RTV silicone rubber-coated glass samples, energized under AC, DC+, and DC- in a controlled local laboratory

Abstract

Insulators are of the most important components of electrical transmission and distribution networks. Therefore, it is necessary to find ways to improve insulator performance and save maintenance costs. Various materials have been used in the manufacture of power line insulators. Originally, porcelain and glass were used in the industry. Later materials such as epoxy and silicone rubber have been used. Silicone rubber possesses the property of hydrophobicity i.e. it is water repellent. Silicone rubber is also used as a coating on conventional insulators. This research project evaluates the performance of RTV-SR coated borosilicate glass and HTV-SR extruded onto glass fibre, rod test samples, and compared them with each other under HVAC, HVDC+, and HVDC-, in a controlled laboratory using the Rotating Wheel Dip Test (RWDT) method. The study was conducted in an existing Rotating Wheel Dip Test (RWDT) facility located in the Eskom Stikland substation in Cape Town. Six (6) × RTV-SR and six (6) HTV-SR test samples were used to provide a measured creepage distance of 275 mm. The tests were carried out using a modified version of the test methods described in the IEC 62730:2012 standard. The IEC / TR 62730:2012 standard for wheel test specifies the total test duration of 30 000 cycles. In the current study, 2 700 cycles were used for each of the three tests (AC, DC+, and DC-), for a total of 8 100 cycles. This means that instead of 30 000 cycles for each test, only 2 700 cycles were completed. Each subtest was conducted over six days or 144 hours. Thus, the total hours for the entire test (AC, DC+, and DC-) were 432 hours or 18 days. Further, the study used the Jarrar et al. (2014) hydrophobicity classification method to determine whether each insulator at the end of the test cycle retained its hydrophobicity and, therefore, it’s insulating properties. Visual inspection was also used to determine the ageing, cracking, erosion, discolouration, pollution build-up etc. on the surface of the test samples. The following results were obtained: Under AC conditions, the cumulative electric charge values for the HTV-SR samples were slightly lower than that of the RTV-SR. But this result is not conclusive, further testing is required over longer periods to determine which of the two performs better.

The hydrophobicity test indicated that the RTV-SR performed better, and the ageing test also indicated that the RTV-SR performed better. Under DC+ conditions, the RTV-SR performed better than the HTV-SR in all four categories (leakage current, cumulative electric charge, hydrophobicity, and insulator ageing). Under DC- conditions, the HTV-SR samples performed slightly better than the RTV-SR samples, in the categories of (leakage current, cumulative electric charge, and hydrophobicity). The RTV-SR performed better in the insulator-ageing category. Due to the short duration of the tests and other limitations, further testing is recommended, particularly in an outdoor environment for a longer duration.