A new technique for reducing extremely low frequency magnetic field emissions affecting large building structures
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When large structures such as residential compounds or public buildings are under the influence of extremely low frequency (ELF) magnetic fields, such as the one generated by a system of railways fed by 16.67 Hz, standard methods of designing shielding structures by numerical methods usually fail. The latter can be explained by the difficulty posed in the computing process by the large aspect ratios involved due to thin layers of metal (a few millimetres or centimetres) in contrast to the large dimensions of the affected structure (several tens of meters). In some cases one has to utilize special approximations such as surface conductivity, which are not easy to handle when the designed shielding structure is clearly three -dimensional. Other alternatives such as experimentation in situ are very costly. Here, a new technique is presented of mitigating the field by using three-dimensional propagation of induced currents optimizing the field reduction factors and minimizing the cost of shielding material. The particular designing method is a hybrid of numerical simulations combined with lab experimentation using scaled models of the large structure. The method is rather cost-effective and flexible as various designs can be easily tested. Results are presented in the form of magnetic field values, at various locations in the buildings, before and after this mitigation technique is applied.
KeywordsEddy Current Large Aspect Ratio Effective Shield Mitigation Technique Train Track
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The authors are grateful to the representatives of the Swedish company NCC AB for proposing the problem and support. Also to O. Ivarsson for performing the measurement in Fig. 3. We would like to thank the Faculty of Science of the Universidad Nacional de Ingeniería, in Lima Peru, for allowing us to use of their lab facilities. Our gratitude also goes to ELFORSK for the grant No 3950, which enabled us the development of some the methods used in this study.
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