This paper describes an optical fiber sensor and temperature sensor-based instrumentation system to measure the moisture content in transformer oil. The sensor system consists of (i) Diode Laser Source, (ii) a bare and bent multimode fiber as sensor probe, (iii) an LDR as detector, (iv) LM35-based temperature sensor, and (v) microcontroller system having a trained ANN for processing and calibration. The bare and bent optical fiber sensor and the temperature sensor LM35 are used to provide the measures of refractive index (RI) and temperature of a transformer oil sample. An ATmega32-microcontroller-based system with trained ANN algorithm has been developed to determine the moisture content of the transformer oil sample by sampling the readings of the bare bent optical fiber sensor and the temperature sensor. 1. Introduction Fiber optic sensors offer unique advantages, such as immunity to electromagnetic interferences, stability, repeatability, durability against harsh environment, and fast response. Therefore, they are used for the measurement of physical parameters such as temperature [1], pressure [2], acceleration [3], curvature measurement [4], hydrocarbon monitoring [5], and host of other applications. Power transformers are one of the most expensive investments in electric power systems [6]. They are fundamental components of electric power systems, and their reliability is an important factor in the operation of the system [7]. The transformer oil is a good insulating material. However, due to some inevitable factors, some unwanted particles like water and gas can contaminate the oil. As a result, insulation strength gets reduced that may result in partial discharge of transformer oil [8]. Model-based online detection method [6], partial discharge method [7], chromatography, radiofrequency method, infrared spectroscopy, neural network method [9], and so forth are some of the techniques developed for detection of moisture content of transformer oil. The dielectric parameter ( , relative permittivity) of transformer oil indicates the quality of the oil as the insulation property is concerned. Again, square of refractive index is represented as relative permittivity of an optical material. Therefore, the moisture content in transformer oil sample can be related to the change in refractive index of the sample. In fact, the measure of refractive index provides information about the turbidity of a liquid [10]. An optical fiber sensor for measuring the refractive index of liquid, based on the measurement of the input and output power of a multimode
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