Performance analysis of the RC clampers in switching off no load arc furnace transformers
This paper presents an over-voltage attenuation performance analysis of RC clampers and surge arresters during transients caused by switching off a no load transformer applied to an arc furnace. A Brazilian Industry actual system was simulated using the Matlab/Simulink software. The simulation results indicate that the adequate use of RC clampers can reduce the arresters operations and practically eliminates the re-strikes and repetitive transient over-voltages when using vacuum circuit breakers to switch the arc furnace transformer.
Detailed study of fast transient phenomena in transformers and substations leading to an improved system design
In recent years several unusual failures were reported in medium voltage distribution systems. Some conventional dry-type and oil-filled transformers had clearly been exposed to a specific type of overvoltage. During thorough lab and factory investigations, no design or manufacturing faults could be found. Internal winding resonances were generally accepted to be the cause of the failure. This paper describes typical symptoms, critical factors and some ideas for appropriate protection and total system design, based upon experimental results from laboratory and on-site measurements. Laboratory tests are performed to investigate the high frequency behaviour of layer windings in distribution transformers. It is shown that multiple reignitions in vacuum circuit breakers can initiate internal winding resonances. Impulse tests confirm the resonance patterns at the natural frequencies. Long term high frequency measurements are performed in a medium voltage substation where problems have been detected before. The main purpose of the paper is to make system designers more aware of fast transient phenomena and to show that they represent a larger problem than generally accepted.
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.