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Ultraschalldiagnostik als neues Verfahren zur Zustandsbewertung elastomerer Isolierstoffe in Energiekabelsystemen

  • Cornelissen, Christian
Publication Date
Jan 01, 2003
Publikationsserver der RWTH Aachen University
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Ultrasonic diagnosis is analysed as a method for condition assessment of elastomeric insulating materials for power cable accessories. It is possible to identify the most critical effects regarding a potential lowering of the insulation behaviour. In this context the basic demands on ultrasonic diagnosis for an use of condition assessment are defined and - especially regarding a possible industrial use - verified. As an important part of this project a measuring setup to conduct experiments in contact- and immersing-technique is built up. Using a micropositioning system it is possible to move the ultrasonic transceiver to any position of the sample's surface. By the development of an automation software, a scanning of the sample's surface with a minimal step increment of 50 micrometres can be realised. Furthermore there is a possibility to examine large (industrial) objects such as high voltage joints in an additional measuring device. The detection of ultrasonic signals, which are reflected by included failures like voids, depends on a large number of parameters like the size and depth of the fault, the type of the transceiver and so on. These influences are first discussed theoretically and afterwards analysed and verified in various experimental researches. Thereby the measurement resolution of the used setup is determined. For example, voids of 100 micrometres are detectable in a depth of nearly 30 millimetres, what is very interesting regarding an industrial use. A main topic of the project is the analysis of singular inhomogenities such as gas-filled microvoids. They can be detected and localised in a detailed scan process of the sample's surface. Software tools for handling the large amount of measured data are developed and so an automated evaluation of the signals regarding their amplitude, width and quantity is feasible. Furthermore the received results are used for image processing to detect existing objects in the scan. Details regarding the object's dimension, shape and position are the results of the completely automated evaluation process. Regarding aspects affecting a larger volume of the measured sample, the detection of incomplete crosslinking in industrial made elastomeric samples and laboratory made test specimen is possible due to changes in the velocity of sound. Also small parts of incomplete crosslinking with a size of only a few millimetres within a larger sample can be detected. Another application is the analysis of mechanical stress executed on coldshrinkable accessories. It can be shown, that due to this stress non reversible changes of the material's properties occur, which depend also on the storage temperature: on the one hand there is a remaining expansion of the material after the release of the samples and on the other hand a permanent reduction of the elastic modulus can be measured. Regarding an industrial use of ultrasonic, also the investigation of high voltage joints is considered. Measurements of singular inhomogenities carried out in the production plant confirm the detectibility of these failures and are confirmed by a succeeding electrical test. Additionally a special immersing measurement technique for large objects like high voltage joints is developed. It works automatically and scans the whole surface of the sample with a large increment. Afterwards areas with high echo signals are scanned again with a higher resolution. To this subject a pilot project using high voltage joints has already been carried out.

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