Epoxy systems for low temperature applications
Epoxy-based formulated products are finding increased utility in a wide variety of structural and electrical applications that are required to operate in low temperature environments. Cryogenic technology is accelerating the growth in these fields. This versatile thermosetting polymer offers many attractive features such as high mechanical strength, outstanding dielectric characteristics, excellent adhesive properties, very good chemical resistance and excellent thermal endurance. Such features make them materials of choice for high and medium voltage electrical apparatus that operate in below zero Celsius environments. At lower operating temperatures, many factors come into play affecting overall performance. Among these factors are: speed and effectiveness of cure, fracture toughness characteristics, and thermal expansion properties.
Electrical aging tests on epoxy insulators in GIS
Due to the relatively small insulator dimensions in gas-insulated switchgear (GIS), the electrical stressing is high by comparison to that of insulators for conventional switchgear. Aging tests on a large number of full-sized insulators in several vessels in an outdoor test station have been carried out to determine the stress limits. The tests, with voltages of several hundred kilovolts, last for several years. The aim is to work out conditions (by extrapolation) for a trouble-free switchgear operating life of at least 50 years. The most important test objects are postinsulators weighing 2 kg each. It has been confirmed that partial discharges have a major effect on the failure probability. The life curve obtained from the test results of six batches of partial-discharge-free insulators is compared with the required reliability data. >
Inferring Learning Strategies from Students ’ Errors
Student models in an Intelligent Tutoring System (ITS) aim at representing both correct and incorrect knowledge of the student in a particular domain of learning. This data serves as the basis upon which the Tutor can dispense remedial feedback. This paper presents a framework in which learners’ errors are diagnosed, classified and linked to cognitive processes associated with learning. The cognitive processes we identify are those of generalization, misattribution and ignorance. Our system applies to the domain of Second Language Acquisition and our approach uses Chomsky’s linguistic theory of Principles and Parameters. We particularly concentrate on the acquisition of the Phrase Structure component.