Condition assessment of porcelain and toughened glass insulators from residual strength tests
Transmission and distribution networks of most utilities worldwide have a substantial quantity of porcelain and toughened glass insulators that have been in service for over 30 years; a number that has been traditionally considered as useful life for planning purposes. Many utilities are implementing programs to replace older or defective insulators for improving reliability and also for upgrading their lines. In order to help prioritize line maintenance it is important to identify the condition of insulators and replace those insulators whose performance has reduced significantly from their new condition, hence threatening reliability. In this project, porcelain and toughened glass insulators that were in service for periods ranging from 10-30 years were removed from several locations. Electrical (puncture strength, flashover voltage) and mechanical (tensile strength) tests were performed to assess any reduction in performance. Diagnostic tests such as analysis of bonding agents, identification of internal failure paths were performed to understand failure causes. A sufficiently large number of samples were evaluated and this permitted condition assessment with statistically significant confidence limits
Experimental Study of Electrical Breakdown Voltage of a Glass Insulator Strings with Different Numbers of Broken Units
This work presents results of a comparative study conducted on breakdown voltage measurements in a glass insulators string containing several units with broken glass part. String with 16 insulator units were tested simulating a transmission line of 230 kV. Two arrangements have been adopted: (i) seven good insulators and nine broken units and, (ii) six good insulators and ten broken units. The electrical breakdown voltage measurements have been carried out in dry and without pollution, in five different configurations. The experimental works have been conducted in industrial frequency and with switching impulse with positive and negative polarities. As a result, depend on the distribution and localization of the good insulators in the chain, the breakdown voltage values of the chain with six good insulators was higher than the chain with seven good insulators. A string with six good insulators in configuration #4 showed higher value of the electrical breakdown voltage than a string with seven good insulators in configurations #1 and #2.
X-rays and photocarcinogenesis in hairless mice
It is well known that excessive X-ray radiation can cause non-melanoma skin cancers. With the increased incidence of sun-related skin cancer there is a need to investigate the combination of sunlight and X-rays. Immunocompetent C3.Cg/TifBomTac mice (n = 298) were divided into 12 groups. Mice were irradiated with 12, 29 or 50 kV X-rays. The mice received a total dose of 45 Gy. They were irradiated with 3 SED simulated solar radiation (SSR) either before or after irradiation with X-rays. The groups irradiated with X-rays alone, 0, 3, 9 and 10 mice (0, 12, 29 and 50 kV, respectively) developed squamous cell carcinoma. In the groups irradiated with SSR after X-rays the development of tumours was significantly faster in the 50 kV group than in the corresponding control group (175 vs. 194 days, p < 0.001). In the groups irradiated with SSR prior to the X-ray radiation the development of tumours was significantly faster in the 29 and the 50 kV groups than in the corresponding control group (175 vs. 202 days, p < 0.001 and 158 vs. 202 days, p < 0.001, respectively). In conclusion, X-ray radiation alone is a weak carcinogen in hairless mice. There is an added carcinogenic effect if X-ray radiation is given on prior sun-exposed skin or if the skin is sun-exposed after X-rays. We still believe that X-ray radiation is a safe and effective therapy for various dermatological diseases but caution should be observed if a patient has severely sun-damaged skin or has a high-risk sun behaviour.