Zustandsbewertung polymerer Isolierstoffe in Energiekabeln und deren Garnituren unter hauptsächlicher Anwendung der Kernspinresonanz-Messtechnik und der Ultraschalldiagnostik
Final Report Abstract
Due to manufacturing errors, mechanical, thermal, and electrical stress, polymers used for insulating cables undergo different aging processes. In order to be able to monitor changes in the properties of these materials, non-destructive methods based on the combination of ultrasound and NMR were developed during the project. In the first and second period of the project, it could be shown, that fabrication quality and state of silicon-insulators in terms of inhomogeneities like soft spots and filler material agglomerates can be evaluated with high-field NMR measurements. In cable joints two layers of different materials could be distinguished by NMR-imaging. With the help of the mobile NMR- MOUSE, the NMR-relaxation time T2 was measured in medium voltage cable joints. The measurements revealed different values for untreated, stretched, and aged samples. Stretched and relaxed samples could be identified based on an incomplete recovery of the initial T2 value, possibly caused by a stress-induced, partially irreversible crystallization process. The precision of the measurements was higher in the outer part (depth 3 mm) than in the inner layer (depth 7.5 mm). Measurements on silicon rubber test specimens showed the T2 relaxation time to be positively correlated with the shore hardness. The measurements in the outer part could be performed with a simpler NMR-MOUSE, consisting of a bar-magnet and a figure-8 coil. In order to facilitate simultaneous measurements at different depths in the sample, five NMR-MOUSE-sensors were driven by a five parallel channel spectrometer. Hence it was shown, that mobile NMR is an adequate tool to determine the state of electrical isolations. The measurements of the cross link density of the test specimens did not yield appreciable correlation between expected material properties and NMR parameters in order to detect treeing effects. Therefore these effects needed to be detected by refined imaging methods. By implementing a new and fast imaging pulse sequence originally developed in a DFG-Forschergruppe for a mobile magnet, the position and shape of glass beads embedded in silicon rubber test specimens and simulating air entrapments in elastomers were detected using shorter data acquisition times. The aging process of cable joints embedded in soil for several years is expected to be angle independent with respect to the center of the cable. Opposed to conventional NMR devices, mobile NMR sensors offer the advantage of a sensitive-volume shape customized to fit the sample geometry. During the last period of this project, a new mobile NMR-sensor was developed the sensitive volume of which is shaped to cover parts of a cylinder surface, matching the curvature of the investigated power cable insulations with 36 µm resolution. The sensor was used to measure depth profiles, showing the ingress of solvents in cylindrical elastomer layers. Due to its portability it can be moved to cables in operation to determine their state in a non-destructive way.
Publications
- NMR Methods for Strongly Inhomogeneous Magnetic Fields: Application to Elastomers Using the NMR-MOUSE. Dissertation, RWTH Aachen University (2001)
Anette Wiesmath
- Ultrasonic and Nuclear Magnetic Resonance for non-destructive diagnostics of cable systems. Proceedings of the 7th International Conference on Properties and Applications of Dielectric Materials, 2003
C. Cornelissen, A. Wiesmath, A. Schnettler, B. Blümich
- Applications of Unilateral NMR in Nondestructive Testing. Dissertation, RWTH Aachen University (2004)
Shatrugan Sharma
- Mobile NMR for Analysis of Polyethylene Pipes. Acta Physica Polonica A, 108, 13-23 (2005)
B. Blümich, F. Casanova, A. Buda, K. Kremer, and T. Wegener
- Qualitätskontrolle mit mobiler NMR: Elastomere und Kunststoffe. Dissertation, RWTH Aachen University (2005)
Kai Kremer
- Achieving Spatial Localization with singlesided NMR sensors. Applied Magn Reson, 32, 171-184 (2007)
J. Kolz, N. Goga, F. Casanova, T. Mang, and B. Blümich
- Single-Sided NMR. In: Modern Magnetic Resonance Part III: Applications in Materials Science and Food Science, edited by Graham Webb, Springer, pp. 1503-1506 (2007)
J. Perlo, F. Casanova, and B. Blümich
- Mobile single-sided NMR. Prog Nucl Magn Reson Spectroscopy 52 (4), 197-269 (2008)
B. Blümich, J. Perlo, and F. Casanova
- Shaping the Sensitive Volume of a Mobile NMR-Sensor to Profile Cylindrical Samples with High Resolution. 49th Experimental Nuclear Magnetic Resonance Conference, Pacific Grove, California, USA, March 9th-14th, 2008
Jörg Mauler, Federico Casanova, Bernhard Blümich
- Surface UV aging of elastomers investigated with microscopic resolution by single-sided NMR. J Magn Reson, 192, 1-7 (2008)
N.O. Goga, D.E. Demco, J. Kolz, R. Ferencz, A. Haber, F. Casanova, and B. Blümich
