Using Distributed Temperature Sensing to monitor field scale dynamics of ground surface temperature and related substrate heat flux

We present one of the first studies of the use of Distributed Temperature Sensing (DTS) along fibre-optic cables to purposely monitor spatial and temporal variations in ground surface temperature (GST) and soil temperature, and provide an estimate of the heat flux at the base of the canopy layer and in the soil. Our field site was at a groundwater-fed wet meadow in the Netherlands covered by a canopy layer (between 0-0.5 m thickness) consisting of grass and sedges. At this site, we ran a single cable across the surface in parallel 40 m sections spaced by 2 m, to create a 40×40 m monitoring field for GST. We also buried a short length (≈10 m) of cable to depth of 0.1±0.02 m to measure soil temperature. We monitored the temperature along the entire cable continuously over a two-day period and captured the diurnal course of GST, and how it was affected by rainfall and canopy structure. The diurnal GST range, as observed by the DTS system, varied between 20.94 and 35.08◦C; precipitation events acted to suppress the range of GST. The spatial distribution of GST correlated with canopy vegetation height during both day and night. Using estimates of thermal inertia, combined with a harmonic analysis of GST and soil temperature, substrate and soil-heat fluxes were determined. Our observations demonstrate how the use of DTS shows great promise in better characterising area-average substrate/soil heat flux, their spatiotemporal variability, and how this variability is affected by canopy structure. The DTS system is able to provide a much richer data set than could be obtained from point temperature sensors. Furthermore, substrate heat fluxes derived from GST measurements may be able to provide improved closure of the land surface energy balance in micrometeorological field studies. This will enhance our understanding of how hydrometeorological processes interact with near-surface heat fluxes.

Modelagem dinâmica de transformadores de corrente para uso em estudos de qualidade da energia e proteção do sistema elétrico

Pós-graduação em Engenharia Elétrica - FEB

Desenvolvimento de sistema para verificação metrológica de transformadores de corrente IN SITU em sistemas de alta tensão utilizando transdutor óptico

Este trabalho apresenta o desenvolvimento e a aplicação em campo de um sistema para verificação metrológica de transformadores de corrente em alta tensão. O sistema foi desenvolvido utilizando como premissas básicas a portabilidade e a confiabilidade metrológica, de tal forma que pudesse ser facilmente transportado e instalado, sem interrupção do fornecimento de energia elétrica nem o uso de infraestrutura complexa de transporte e montagem. O sistema utiliza como padrão de referência um transdutor ótico, cujo sensor mede a corrente elétrica do primário do transformador de corrente a ser verificado através do efeito magneto-ótico de Faraday. Ele também é composto por outros instrumentos padrão que medem a corrente elétrica do secundário do transformador de corrente sob verificação, e realizam a comparação entre esta e o sinal da saída do transdutor ótico padrão. Foram realizados ensaios em laboratório e em campo. Os ensaios em campo foram realizados em duas subestações em Belém/PA, visando avaliar a correta operação do sistema em condições de alta tensão e alta corrente. Os ensaios foram realizados em seis transformadores de corrente conectados em 230 kV, em condições médias de carga, e obtiveram resultados satisfatórios.

Traditional lesion detection aids

Lesion detection aids ideally aim at increasing the sensitivity of visual caries detection without trading off too much in terms of specificity. The use of a dental probe (explorer), bitewing radiography and fibre-optic transillumination (FOTI) have long been recommended for this purpose. Today, probing of suspected lesions in the sense of checking the 'stickiness' is regarded as obsolete, since it achieves no gain of sensitivity and might cause irreversible tooth damage. Bitewing radiography helps to detect lesions that are otherwise hidden from visual examination, and it should therefore be applied to a new patient. The diagnostic performance of radiography at approximal and occlusal sites is different, as this relates to the 3-dimensional anatomy of the tooth at these sites. However, treatment decisions have to take more into account than just lesion extension. Bitewing radiography provides additional information for the decision-making process that mainly relies on the visual and clinical findings. FOTI is a quick and inexpensive method which can enhance visual examination of all tooth surfaces. Both radiography and FOTI can improve the sensitivity of caries detection, but require sufficient training and experience to interpret information correctly. Radiography also carries the burden of the risks and legislation associated with using ionizing radiation in a health setting and should be repeated at intervals guided by the individual patient's caries risk. Lesion detection aids can assist in the longitudinal monitoring of the behaviour of initial lesions.

Estudio de la adherencia de armaduras corroídas y su influencia en la capacidad resistente de elementos de hormigón armado

La verificación de la seguridad estructural, tanto de estructuras que permitan un cierto grado de deterioro en su dimensionado como de estructuras existentes deterioradas, necesita disponer de modelos de resistencia que tengan en cuenta los efectos del deterioro. En el caso de la corrosión de las armaduras en las estructuras de hormigón armado, la resistencia depende de múltiples factores tales como la sección del acero corroído, el diagrama tensión-deformación del acero corroído, la adherencia hormigón-acero corroído, la fisuración o desprendimiento del hormigón debido a la expansión de los productos de corrosión. En este sentido, la transferencia de las fuerzas a través de la superficie de contacto entre el hormigón y el acero, la adherencia, es uno de los aspectos más importantes a considerar y es la base del comportamiento del hormigón armado como elemento estructural. La adherencia debe asegurar el anclaje de las armaduras y transmitir la tensión tangencial que aparece en las mismas como consecuencia de la variación de las solicitaciones a lo largo de un elemento estructural. Como consecuencia de la corrosión de las armaduras, el desarrollo de la adherencia se altera y, por tanto, la transferencia de la tensión longitudinal. Esta Tesis Doctoral aborda el comportamiento en estado límite último de la adherencia en el hormigón estructural con armaduras corroídas. El objetivo principal es la obtención de un modelo suficientemente realista y fiable para la evaluación de la adherencia con armaduras corroídas en el marco de la verificación de la seguridad estructural de elementos de hormigón armado con armaduras corroídas. Para ello se ha llevado a cabo un programa experimental de ensayos tipo pull-out excéntricos, con diferentes probetas, unas sin corrosión y otras sometidas tanto a procesos de corrosión natural como a procesos de corrosión acelerada, con diferentes grados de deterioro. Este tipo de ensayo de adherencia representa de forma realista y fiable realista los esfuerzos de adherencia en la zona de anclaje. Por otra parte, para la realización de estos ensayos se ha puesto a punto, además del procedimiento de ensayo, un sistema de adquisición de datos entre los que se incluye el empleo de sensores de tipo fibra óptica con redes de Bragg embebidos en la armadura para determinar los parámetros representativos de la adherencia en el hormigón estructural con armaduras corroídas. Por otra parte, la recopilación de los datos de los estudios de adherencia con armaduras corroídas procedentes de la literatura científica, además de los resultados de la presente investigación, junto con la identificación de las variables relevantes en el comportamiento de la adherencia con armaduras sanas y corroídas ha servido para la obtención de una formulación realista y fiable para la evaluación conjunta de la adherencia con armaduras sanas y corroídas a partir de modelos de regresión múltiple. La formulación propuesta ha sido validada mediante criterios estadísticos y comparada con otras formulaciones propuestas en la literatura científica. Además se ha realizado un análisis de las variables influyentes de la formulación propuesta. También se ha obtenido un modelo numérico simple y eficiente, validado con alguno de los ensayos realizados en esta tesis, para simular la adherencia con armaduras sanas y corroídas. Finalmente, se presenta un procedimiento para realizar la evaluación de vigas deterioradas por corrosión mediante el método de los campos de tensiones que incluye la evaluación de la adherencia mediante la formulación sugerida en esta Tesis Doctoral. Las conclusiones alcanzadas en este trabajo han permitido evaluar la adherencia con armaduras corroídas de forma realista y fiable. Asimismo, se ha podido incluir la evaluación de la adherencia en el marco de la verificación de la seguridad estructural en elementos de hormigón armado deteriorados por corrosión. ABSTRACT Structural safety verification of both structures allowing a certain degree of deterioration in design and deteriorated existing structures needs strength models that factor in the effects of deterioration. In case of corrosion of steel bars in reinforced concrete structures, the resistance depends on many things such as the remaining cross-section of the corroded reinforcement bars, the stress-strain diagrams of steel, the concrete-reinforcement bond and corrosion-induced concrete cracking or spalling. Accordingly, the force transfer through the contact surface between concrete and reinforcement, bond, is one of the most important aspects to consider and it is the basis of the structural performance of reinforced concrete. Bond must assure anchorage of reinforcement and transmit shear stresses as a consequence of the different stresses along a structural element As a consequence of corrosion, the bond development may be affected and hence the transfer of longitudinal stresses. This PhD Thesis deals with ultimate limit state bond behaviour in structural concrete with corrode steel bars. The main objective is to obtain a realistic and reliable model for the assessment of bond within the context of structural safety verifications of reinforced concrete members with corroded steel bars. In that context, an experimental programme of eccentric pull-out tests were conducted, with different specimens, ones without corrosion and others subjected to accelerated or natural corrosion with different corrosion degrees. This type of bond test reproduces in a realistic and reliable way bond stresses in the anchorage zone. Moreover, for conducting these tests it was necessary to develop both a test procedure and also a data acquisition system including the use of an embedded fibre-optic sensing system with fibre Bragg grating sensors to obtain the representative parameters of bond strength in structural concrete with corroded steel bars. Furthermore, the compilation of data from bond studies with corroded steel bars from scientific literature, including tests conducted in the present study, along with the identification of the relevant variables influencing bond behaviour for both corroded and non-corroded steel bars was used to obtain a realistic and reliable formulation for bond assessment in corroded and non-corroded steel bars by multiple linear regression analysis. The proposed formulation was validated with a number of statistical criteria and compared to other models from scientific literature. Moreover, an analysis of the influencing variables of the proposed formulation has been performed. Also, a simplified and efficient numerical model has been obtained and validated with several tests performed in this PhD Thesis for simulating the bond in corroded and non-corroded steel bars. Finally, a proposal for the assessment of corrosion-damaged beams with stress field models including bond assessment with the proposed formulation is presented. The conclusions raised in this work have allowed a realistic and reliable bond assessment in corroded steel bars. Furthermore, bond assessment has been included within the context of structural safety verifications in corrosion-damaged reinforced concrete elements.

Methodology based on FBG sensors for monitoring of FRP-strengthened beams

Advanced composite materials are increasingly used in the strengthening of reinforced concrete (RC) structures. The use of externally bonded strips made of fibre-reinforced plastics (FRP) as strengthening method has gained widespread acceptance in recent years since it has many advantages over the traditional techniques. However, unfortunately, this strengthening method is often associated with a brittle and sudden failure caused by some form of FRP bond failure, originated at the termination of the FRP material or at intermediate areas in the vicinity of flexural cracks in the RC beam. Up to date, little effort in the early prediction of the debonding in its initial instants even though this effect is not noticeable by simple visual observation. An early detection of this phenomenon might help in taking actions to prevent future catastrophes. Fibre-optic Bragg grating (FBG) sensors are able to measure strains locally with high resolution and accuracy. Furthermore, as their physical size is extremely small compared with other strain measuring components, it enables to be embedded at the concrete-FRP interface for determining the strain distribution without influencing the mechanical properties of the host materials. This paper shows the development of a debonding identification methodology based on strains experimentally measured. For, it a simplified model is implemented to simulate the behaviour of FRP-strengthened reinforced concrete beams. This model is taken as a basis to. develop an model updating procedure able to detect minor debonding at the concrete-FRP interface from experimental strains obtained by using FBG sensors embedded at the interface

A clustering technique for partial discharge and noise sources identification in power cables by means of waveform parameters

On-line partial discharge (PD) measurements have become a common technique for assessing the insulation condition of installed high voltage (HV) insulated cables. When on-line tests are performed in noisy environments, or when more than one source of pulse-shaped signals are present in a cable system, it is difficult to perform accurate diagnoses. In these cases, an adequate selection of the non-conventional measuring technique and the implementation of effective signal processing tools are essential for a correct evaluation of the insulation degradation. Once a specific noise rejection filter is applied, many signals can be identified as potential PD pulses, therefore, a classification tool to discriminate the PD sources involved is required. This paper proposes an efficient method for the classification of PD signals and pulse-type noise interferences measured in power cables with HFCT sensors. By using a signal feature generation algorithm, representative parameters associated to the waveform of each pulse acquired are calculated so that they can be separated in different clusters. The efficiency of the clustering technique proposed is demonstrated through an example with three different PD sources and several pulse-shaped interferences measured simultaneously in a cable system with a high frequency current transformer (HFCT).

Simulation of power station protection equipment

Computer programs have been developed to enable the coordination of fuses and overcurrent relays for radial power systems under estimated fault current conditions. The grading curves for these protection devices can be produced on a graphics terminal and a hard copy can be obtained. Additional programs have also been developed which could be used to assess the validity of relay settings (obtained under the above conditions) when the transient effect is included. Modelling of a current transformer is included because transformer saturation may occur if the fault current is high, and hence the secondary current is distorted. Experiments were carried out to confirm that distorted currents will affect the relay operating time, and it is shown that if the relay current contains only a small percentage of harmonic distortion, the relay operating time is increased. System equations were arranged to enable the model to predict fault currents with a generator transformer incorporated in the system, and also to include the effect of circuit breaker opening, arcing resistance, and earthing resistance. A fictitious field winding was included to enable more accurate prediction of fault currents when the system is operating at both lagging and leading power factors prior to the occurrence of the fault.

Numerical modelling of advanced modulation formats and WDM transmission

This thesis presents a theoretical investigation of the application of advanced modelling formats in high-speed fibre lightwave systems. The first part of this work focuses on numerical optimisation of dense wavelength division multiplexing (DWDM) system design. We employ advanced spectral domain filtering techniques and carrier pulse reshaping. We then apply these optimisation methods to investigate spectral and temporal domain characteristics of advanced modulation formats in fibre optic telecommunication systems. Next we investigate numerical methods used in detecting and measuring the system performance of advanced modulation formats. We then numerically study the combination of return-to-zero differential phase-shift keying (RZ-DPSK) with advanced photonic devices. Finally we analyse the dispersion management of Nx40 Gbit/s RZ-DPSK transmission applied to a commercial terrestrial lightwave system.

All-optical phase and amplitude regenerator for next-generation telecommunications systems

Fibre-optic communications systems have traditionally carried data using binary (on-off) encoding of the light amplitude. However, next-generation systems will use both the amplitude and phase of the optical carrier to achieve higher spectral efficiencies and thus higher overall data capacities(1,2). Although this approach requires highly complex transmitters and receivers, the increased capacity and many further practical benefits that accrue from a full knowledge of the amplitude and phase of the optical field(3) more than outweigh this additional hardware complexity and can greatly simplify optical network design. However, use of the complex optical field gives rise to a new dominant limitation to system performance-nonlinear phase noise(4,5). Developing a device to remove this noise is therefore of great technical importance. Here, we report the development of the first practical ('black-box') all-optical regenerator capable of removing both phase and amplitude noise from binary phase-encoded optical communications signals.

High stability multiplexed fiber interferometer and its application on absolute displacement measurement and on-line surface metrology

We propose a self-reference multiplexed fibre interferometer (MFI) by using a tunable laser and fibre Bragg grating (FBG). The optical measurement system multiplexes two Michelson fibre interferometers with shared optical path in the main part of optical system. One fibre optic interferometer is used as a reference interferometer to monitor and control the high accuracy of the measurement system under environmental perturbations. The other is used as a measurement interferometer to obtain information from the target. An active phase tracking homodyne (APTH) technique is applied for signal processing to achieve high resolution. MFI can be utilised for high precision absolute displacement measurement with different combination of wavelengths from the tuneable laser. By means of Wavelength-Division-Multiplexing (WDM) technique, MFI is also capable of realising on-line surface measurement, in which traditional stylus scanning is replaced by spatial light-wave scanning so as to greatly improve the measurement speed and robustness.

Spiral attractor created by vector solitons

Mode-locked lasers emitting a train of femtosecond pulses called dissipative solitons are an enabling technology for metrology, high-resolution spectroscopy, fibre optic communications, nano-optics and many other fields of science and applications. Recently, the vector nature of dissipative solitons has been exploited to demonstrate mode locked lasing with both locked and rapidly evolving states of polarisation. Here, for an erbium-doped fibre laser mode locked with carbon nanotubes, we demonstrate the first experimental and theoretical evidence of a new class of slowly evolving vector solitons characterized by a double-scroll chaotic polarisation attractor substantially different from Lorenz, Rössler and Ikeda strange attractors. The underlying physics comprises a long time scale coherent coupling of two polarisation modes. The observed phenomena, apart from the fundamental interest, provide a base for advances in secure communications, trapping and manipulation of atoms and nanoparticles, control of magnetisation in data storage devices and many other areas. © 2014 CIOMP. All rights reserved.

Study on fatigue and energy-dissipation properties of nanolayered Cu/Nb thin films

Energy dissipation and fatigue properties of nano-layered thin films are less well studied than bulk properties. Existing experimental methods for studying energy dissipation properties, typically using magnetic interaction as a driving force at different frequencies and a laser-based deformation measurement system, are difficult to apply to two-dimensional materials. We propose a novel experimental method to perform dynamic testing on thin-film materials by driving a cantilever specimen at its fixed end with a bimorph piezoelectric actuator and monitoring the displacements of the specimen and the actuator with a fibre-optic system. Upon vibration, the specimen is greatly affected by its inertia, and behaves as a cantilever beam under base excitation in translation. At resonance, this method resembles the vibrating reed method conventionally used in the viscoelasticity community. The loss tangent is obtained from both the width of a resonance peak and a free-decay process. As for fatigue measurement, we implement a control algorithm into LabView to maintain maximum displacement of the specimen during the course of the experiment. The fatigue S-N curves are obtained.