Development of a fiber optic sensor for hydrogen monitoring in transformers

<p>Electric vehicles and photovoltaic power generation are two of factors that are increasing the demands on the electrical grid. To cope with these challenges and to improve grid stability, the development of a transformer monitoring system as a fundamental part of the smart grid is necessary. Dissolved hydrogen in the transformer oil can serve as a primary indicator of the transformer health. Depending on the hydrogen concentration and rate of increase the transformer can be diagnosed. The goal of this thesis was to develop a highly sensitive hydrogen sensor for online health monitoring of transformers. The developed sensors are based on palladium and fiber Bragg gratings (FBG). Palladium expands with hydrogen absorption and this expansion is measured with an FBG. Detailed guidance for optimizing the sensor design is given. First, the selection of the working temperature is discussed. Second, the influence of the palladium geometry on the sensitivity is elaborated: by varying the cross-sectional area ratio of palladium to fiber the sensitivity can be tuned. Two different options to attach palladium are discussed: vapour deposition of palladium and adhesive bonding of palladium foils. The sensitivity of the palladium foil sensors was improved by improved manufacturing processes. The foil sensor have a sensitivity of up to 295 pm/% hydrogen and a resolution of 0.006% hydrogen in gas atmosphere at 90◦C and 1060 mbar. To further increase the hydrogen sensitivity two concepts for amplification of the signal are presented. One relies on palladium silver foils, which have an increased hydrogen solubility and therefore expansion compared to pure palladium, which achieved an increase in sensitivity of a factor of 17. This leads to sensitivity of over 4500 pm/% hydrogen, which is the most sensitive hydrogen sensor reported so far. The other concept relies on a novel concept for strain concentration using a pre-strained palladium foil and FBG, which achieved an amplification of a factor of 2.5. The sensors were characterised in gas and oil environment in a newly developed setup which is stable in pressure, temperature and gas concentration. In gas the sensors were tested at 60, 75, 90, 105, and 120◦C and for a hydrogen concentration range of 0.01 (100 ppm) to 5%. In oil the sensor was tested at 90◦C and for a hydrogen concentration range of 5- 4000 ppm dissolved hydrogen. Furthermore, the influence of carbon monoxide (CO) on the hydrogen sensitivity was examined. A slowed response could be observed, but CO had no impact on the precision of the sensor. Finally, the hydrogen calibration of the sensor is discussed by investigating the strain transfer between expanding palladium and fiber. Three different methods are elaborated to determine the coefficient of strain transfer: (a) via hydrogen measurement, (b) via temperature measurement, and (c) via strain measurement. Methods (a) and (b) were applied directly on the hydrogen sensor, and gave similar results. Method (c) was applied on a reference structure and used to verify method (b). A hydrogen sensor suitable for transformer health monitoring has been developed and characterised, and is currently being implemented in a transformer in the New Zealand network.</p>

Development of a fiber optic sensor for hydrogen monitoring in transformers

<p>Electric vehicles and photovoltaic power generation are two of factors that are increasing the demands on the electrical grid. To cope with these challenges and to improve grid stability, the development of a transformer monitoring system as a fundamental part of the smart grid is necessary. Dissolved hydrogen in the transformer oil can serve as a primary indicator of the transformer health. Depending on the hydrogen concentration and rate of increase the transformer can be diagnosed. The goal of this thesis was to develop a highly sensitive hydrogen sensor for online health monitoring of transformers. The developed sensors are based on palladium and fiber Bragg gratings (FBG). Palladium expands with hydrogen absorption and this expansion is measured with an FBG. Detailed guidance for optimizing the sensor design is given. First, the selection of the working temperature is discussed. Second, the influence of the palladium geometry on the sensitivity is elaborated: by varying the cross-sectional area ratio of palladium to fiber the sensitivity can be tuned. Two different options to attach palladium are discussed: vapour deposition of palladium and adhesive bonding of palladium foils. The sensitivity of the palladium foil sensors was improved by improved manufacturing processes. The foil sensor have a sensitivity of up to 295 pm/% hydrogen and a resolution of 0.006% hydrogen in gas atmosphere at 90◦C and 1060 mbar. To further increase the hydrogen sensitivity two concepts for amplification of the signal are presented. One relies on palladium silver foils, which have an increased hydrogen solubility and therefore expansion compared to pure palladium, which achieved an increase in sensitivity of a factor of 17. This leads to sensitivity of over 4500 pm/% hydrogen, which is the most sensitive hydrogen sensor reported so far. The other concept relies on a novel concept for strain concentration using a pre-strained palladium foil and FBG, which achieved an amplification of a factor of 2.5. The sensors were characterised in gas and oil environment in a newly developed setup which is stable in pressure, temperature and gas concentration. In gas the sensors were tested at 60, 75, 90, 105, and 120◦C and for a hydrogen concentration range of 0.01 (100 ppm) to 5%. In oil the sensor was tested at 90◦C and for a hydrogen concentration range of 5- 4000 ppm dissolved hydrogen. Furthermore, the influence of carbon monoxide (CO) on the hydrogen sensitivity was examined. A slowed response could be observed, but CO had no impact on the precision of the sensor. Finally, the hydrogen calibration of the sensor is discussed by investigating the strain transfer between expanding palladium and fiber. Three different methods are elaborated to determine the coefficient of strain transfer: (a) via hydrogen measurement, (b) via temperature measurement, and (c) via strain measurement. Methods (a) and (b) were applied directly on the hydrogen sensor, and gave similar results. Method (c) was applied on a reference structure and used to verify method (b). A hydrogen sensor suitable for transformer health monitoring has been developed and characterised, and is currently being implemented in a transformer in the New Zealand network.</p>

Codiversification of gut microbiota with humans

Some gut microbes have cospeciated with hominids, but whether they further codiversified with human populations is unclear. Here, we identify predominant gut microbial species sharing a parallel evolutionary history with human populations. Patterns of strain transfer between populations are generally consistent with an African origin, and suggest long-term vertical transmission over thousands of generations. We show the same strains also faithfully transmit between mothers and their children. Consistent with the development of intimate symbiosis, species with strongest patterns of codiversification have the smallest genomes. This study reveals long-term fidelity of gut microbiota with human populations through transmission among individuals living in close proximity. Dominance of specific strains in different populations is based in part on vertical transmission and they may provide population-specific health benefits.

An Improved Calibration Method to Determine the Strain Coefficient for Optical Fibre Sensing Cables

The strain coefficient of an optical fibre sensing cable is a critical parameter for a distributed optical fibre sensing system. The conventional tensile load test method tends to underestimate the strain coefficient of sensing cables due to slippage or strain transfer loss at the fixing points during the calibration procedure. By optimizing the conventional tensile load test setup, the true strain of a sensing cable can be determined by using two sets of displacement measuring equipment. Thus, the strain calculation error induced by slippage or strain transfer loss between a micrometre linear stage and sensing cable can be avoided. The performance of the improved calibration method was verified by using three types of sensing cables with different structures. In comparison to the conventional tensile load test method, the strain coefficients obtained by the improved calibration method for sensing cables A, B, and C increase by 1.52%, 2.06%, and 1.86%, respectively. Additionally, the calibration errors for the improved calibration method are discussed. The test results indicate that the improved calibration method has good practicability and enables inexperienced experimenters or facilities with limited equipment to perform precise strain coefficient calibration for optical fibre sensing cables.

A General Solution to Determine Strain Profile in the Core of Distributed Fiber Optic Sensors under Any Arbitrary Strain Fields

Despite recent publications, the strain transfer in distributed optical fiber sensors is still often overlooked and poorly understood. In the first part of this paper, strain transfer is shown to be driven by a second-order differential equation, whether the optical fiber is embedded into the host material or surface-mounted. In this governing equation, only the value of a key parameter, called strain lag parameter, varies according to the attachment configuration and the type of optical fiber used as a sensor. Then, a general solution of the governing equation is proposed. It is an analytical expression established from new boundary conditions that are more adequate than those used previously in the literature and allows the determination of the strain profile in the core of a distributed optical fiber sensor under any arbitrary strain fields. This general solution has been validated by two experiments presented in the third part of the paper. A very good agreement between the analytical solutions and measured strain profiles using a high spatial resolution optical interrogator for both uniform and non-uniform strain fields has been obtained. These results highlight the importance of the strain lag parameter which must be taken into account for a correct interpretation of measurements, especially in the case of important strain gradients.