INSTITUTE OF PHYSICS PUBLISHING JOURNAL OF OPTICS A: PURE AND APPLIED OPTICS J. Opt. A: Pure Appl. Opt. 4 (2002) S318–S326 PII: S1464-4258(02)39413-3 Comparative analysis of optical-fibre interferometric sensors versus accelerometers: application to vibrations inside high-power transformers J A Garc´ ıa-Souto and H Lamela-Rivera Optoelectronicsand Laser Technology Group, Department of Electronics, Universidad Carlos III de Madrid, Av. Universidad, 30, 28911 Legan´ es, Madrid, Spain E-mail: horacio@ing.uc3m.es Received 11 July 2002, in final form 3 October 2002 Published 4 November 2002 Online at stacks.iop.org/JOptA/4/S318 Abstract We report the results of calibration of an optical-fibre (OF) probe designed for interferometric measurement of vibrations inside high-power transformers. The sensor is highly sensitive in this harsh environment of electromagnetic fields, wide temperature range and oil immersion. A comparative analysis with commercial piezoelectric accelerometers is also presented, focusing on two experiments: the common calibration with a free-space Michelson interferometer that has a controlled moving arm and calibration by obtaining the transference function of magnetostriction in the magnetic laminates by dynamic means. Both methods confirm the extremely high performance of the OF interferometric sensor and the limitations of accelerometers with characteristic low-amplitude vibrations (submicron) and a large range of measurement, as well as for a typical low frequency (100 Hz) but wide bandwidth for the harmonics. The OF probe detects the dynamic strain of deformed elements and the displacement between them. Proper design of the sensor improves its sensitivity through magnification, reaching a resolution of better than 1 nm. Finally, we demonstrate on-site measurements inside two power transformers. Keywords: Interferometry, optical-fibre sensors, mechanical vibration measurements, dynamic strain/displacement, harsh environments, power transformers (Some figures in this article are in colour only in the electronic version) 1. Introduction Technological advances in optical fibre (OF) sensing deal with new applications where the intrinsic properties of the glass- based materials offer great benefits, as for example under harsh conditions of electromagnetic fields and temperature [1]. Some research has been focused on OF sensing applied to monitoring and diagnostics of electric power plants and power transmission systems. In the case of applications inside power transformers, temperature and vibration are the most interesting parameters which need to be controlled. The behaviour of transformers as they age is closely related to the temperature of the cooling oil at different points (hot-spot) and detection of failure can be achieved through measurements of vibrations of the magnetic laminates and windings [1]. Sensors developed for the environmental conditions inside power transformers must withstand high electric and magnetic fields and a wide range of temperatures (150 ◦ C in load). The technology of OF sensors satisfies these conditions well; optical fibres are also able to access the sensing region maintaining conditions of isolation. 1464-4258/02/060318+09$30.00 © 2002 IOP Publishing Ltd Printed in the UK S318