AC Measurement by Using an Optical Fiber Sensor

Abstract

In this work the implementation of a sensor of optical fiber current is reported by means of a magnetic structure. The sensing element (magnetic structure) consists of a magnet placed in the center of a plastic membrane. The variation in the intensity of the light caused by a mirror adapted to the membrane allowed to detect the changes in the presence of a magnetic field produced by an alternating current. Its changes were detected optically. We design a circuit to convert the light to equivalent in data to be process by a PC. The data are read and by means of a program in software Labview®, is the equivalent of current according to the intensity of detected magnetic field. INTRODUCTION Accelerated changes in technology of systems of communications and arrival of systems optical that of way they replace to systems electronic that is used in this type of systems, have influenced to a great extent, so that we takes to the optic and in special case optical communications, like the door that opens new breaches and produces a great field of the investigation [2,3]. Many electronics devices like logic gates, elements of commutation and sensors, they have been replaced by optical fiber devices that have revolutionized of considerable way and which they surpass some restrictive of the electronic systems. The optical fibers are an average ideal to be used in atmospheres of high voltage of equal way, are ideal for the information transmission and taking advantage of this property also it is possible to be created devices that help us to take measurements. The conventional electrical systems are often inadequate in surroundings of high tensions and zones with interferents, local fields of industries with emanations, the optical fiber does not create east problem to us. The sensors of current based on optical fibers, are devices that are being studied and that have obtained good results [1]. ISSN 0040-2508 © 2005 Begell House, Inc. 691 J.M. ESTUDILLO-AYALA, R. ROJAS-LAGUNA,J.A. MARTIN-VELA E. ALVARADO-MENDEZ... The electronics devices to measure current discharges but unfortunately they are very expensive and that can well be alternative that they little by little allow to develop and to innovate sensors us of current done with optical fiber designed in schools of engineering for applications in electrical substations in the country, and measurements in the laboratory with greater exactitude. The sensors of current based on optical fiber have received a special attention in the recent years. This must to its immunity to the electromagnetic fields mainly; its hostile, low atmosphere robustness lost by the transmission of signals in long distances, possibility the multiplex of several signals, adjustable length of the sensible element, great bandwidth and dynamic range. They provide complete galvanic isolation when being an average dielectric; do not circulate around them electrical signals. DESIGN AND OPERATION Theory The sensor element consisted of a magnet placed in the center of a plastic membrane. The change in the intensity of the light caused by the mirror adapted to the membrane allowed to detect the changes in the presence of a magnetic field produced by an alternating current. If the implemented structure this located to a certain distance D of an electrical line driver, an electrical intensity Ielec, would be a force of Lorentz in the magnet. This force can be expressed like: m F MV H = ∇ . (1) With M and Vm being the magnetization and the volume of the magnet, respectively, and being the gradient of associated magnetic field to Ielec, the position of the magnet. If an ideal load line is considered, magnetic field H generated by Ielec to a distance D can be expressed in cylindrical coordinates like: ˆ 2 elec I B H u D θ μ π = = (2) from this expression, H ∇ it can be obtained like: 2 ˆ 2 elec r I H u D π ∇ = (3) In the equation 1 sample which the force that acts in the magnet indicates the direction of , that this throughout the radial axis. H ∇