High-Sensitivity Fiber-Optic Sensor for Hydrogen Detection in Gas and Transformer Oil

This paper reports on the characterization of a palladium (Pd)-based fiber-optic hydrogen (H-2) sensor for health monitoring of distribution and power transformers in the electrical grid. The sensor consists of a Pd foil, which expands due to H-2 absorption, and a fiber Bragg grating that measures this expansion. Fifteen sensors were manufactured and characterized in gas and oil environments at various H-2 concentrations and temperatures. In gas, the sensors were evaluated at 60 degrees C, 75 degrees C, 90 degrees C, 105 degrees C, and 120 degrees C and H-2 concentrations from 0.01% to 5%. In oil, the same sensors were evaluated at 90 degrees C and dissolved H-2 concentrations from S to 2700 ppm. Furthermore, the influence of carbon monoxide (CO), which is often present in transformers and can impact H-2 response of the sensor, was investigated. At 90 degrees C in gas, the response to 0.01% and 5% H-2 was on average 28 and 719 pm, respectively. At 90 degrees C in oil, the response to S and 2700 ppm dissolved H-2 was on average 11 and 763 pm, respectively. The average, relative accuracy is better than 20% over the whole measurement range in gas and for measurements above 100 ppm in oil. These specifications make the sensor a promising candidate for online monitoring of H-2 in transformers.