The increased need for electric power combined with an aging underground cable infrastructure in a deregulated market environment have forced utilities to refocus their attention on reliability while at the same time reducing maintenance costs as much as possible. This has created a significant need for diagnostic methods and technologies to assess the condition of the underground cable systems. However, while several cable diagnostic technologies are available, they have not all yet been fully accepted in the United States. This is because the different technologies lead to different conclusions for the same cable system, and thus utilities do not completely trust the conclusions. A better understanding of the diagnostic technologies and their correct application is therefore required. The most widely used diagnostic technologies in the United States include dissipation factor (Tan-delta and partial discharge measurements; these tests are therefore, the main focus of this thesis; in particular, when applied to underground extruded cable systems. The purpose of this research is to advance the field of characterization of power cable defects by addressing a number of theoretical and practical diagnostic measurements and their interpretation issues. The discussion is based on data from laboratory experiments and field tests. This thesis consists of two major parts. The first part is devoted to the characterization by Tan-delta measurements in which the major contribution is a new approach for condition assessment using this technology. The second part is devoted to the work on characterization by partial discharge measurements, and the major contribution is a novel approach that is able to analyze, evaluate, and reduce the number of partial discharge diagnostic features.

Underground power cables encounter various problems caused by manufacturing defects and/or environmental contact. In keeping with the Smart Grid vision, researchers must develop diagnostic techniques that can be utilized to facilitate the decision making processes regarding replacement prior to failure can occur, thereby minimizing impact to customers. Due to the impact of the aging infrastructure and in particular underground polymeric cables, various offline and online methods have been developed for the detection of the remaining life of underground cables. The offline methods require power outage, which can lead to further difficulty in their implementation. Signal processing techniques hold promise to provide real time or near real time diagnostics. In this thesis, three different signal processing techniques; fast Fourier transform, short-time Fourier transform, and wavelet transform; are investigated for identifying and classifying various fault types encountered in underground power cables based on cable current and voltage measurements.

Electrical power transmission companies have been inspecting underground power cables in a time consuming and destructive way. The current methodology used by Manitoba Hydro, is to remove the conductive material in the center of the cable, cutting the cable into wafers leaving behind the insulating polymer material known as XLPE, the area where many faults occur, and inspect the wafers manually with a microscope. The main goal of this work was to find a methodology to detect these cable faults in a non-destructive way so that the quality of the cable may be assessed, and its remaining lifetime be estimated and return it to use if possible. Two XLPE power cable samples were tested. Three small holes were drilled in one XLPE cable. A capacitive transducer with center frequency of 802.8 kHz was applied for transmitting receiving signal. For each sample, 48 scans were collected. Based on ultrasound images, we were able to detect these faults in this XLPE material from the peaks of the samples corresponding to the XLPE area by setting a threshold to 0.08 volts. Also, this detection technique was improved by using fourth-order cumulants. / May 2016

The insulation lifetime of power cables is determined by several factors. One of the most important of these is the occurrence of partial discharge (PD) at the dielectric. The ability to detect and locate a PD source is limited by attenuation of the high frequency PD pulses as they propagate through the cable to the sensor. Therefore it is necessary to understand the high frequency response of such cables. The ultimate aim of this thesis is to develop an accurate frequency-dependent cable model for detecting and locating degraded insulation regions on power cables, caused by partial discharge activities. Numerical methods can calculate field distribution in the vicinity of a cavity of non-standard shape which generates PDs, and is difficult to calculate by analytical methods. The simulated results show the important influence of the shape of cavity on the electric stress within it. The cavity stress enhancement increases as the permittivity of the dielectric increases. The increase is greater for cavities with large diameter to thickness ratios. A cavity with its axis parallel to the applied field direction has a higher stress enhancement. In addition the stress distribution in the cavity is smaller for spherical cavities than for cylindrical types. The research results show that the semi-conducting layers response voltage increases as frequency increases. This indicates that the semi-conducting layers can have high sensitivity for detection of partial discharge signals and this may be a useful feature to incorporate in the design of cables and in the application of cable models. By using ATPDraw, FEM and EMTP-RV techniques, three different types of cable models are developed. The simulated results give a good agreement with the measured results on the single and three phase power cable. The developed cable model can use for reconstruction of PD source signal by using the receiving signal captured at the cable ends. It is important to use the true pulse shape because it is characteristic of the PD type and location. An investigation into the possibility of detecting different PD patterns and signals when conducting PD tests using different sensor bandwidths is also presented in this thesis. The occurrence of discharge activity was created by an artificial defect manufactured in the single core cable insulation. The artificial defect generated internal discharge and was used to investigate the PD signal propagation on cross-linked polyethylene (XLPE) cable. Capacitance coupled external sensors have been applied for the PD detection measurements and the results show that these external sensors have a number of advantages compared to high frequency current transformer (HF-CT) sensors for the detection of PD pulses. In addition, development of a method to detect cross-coupling of PD signals between phases of a three core cable and location of the PD source on the three phase cable. In order to visualize recorded data gained by PD measurement of three phase cable under test, the 3PARD diagram was used. Each data pulse is assigned to a single dot in the (scatter plot) diagram. The measured results show that the 3PARD diagram allowed the user to identify the fault between phases with PD location. The model used for reconstruction which includes the effect of semicon material in the losses provides accurate reproduction of the propagation characteristics of high frequency PD pulses and the thesis work had used such a model to reconstruct PD waveforms of site PDs from the measured signal for the first time. The use of the original waveform is important for PD identification and location in the practical situation.

This thesis reports on a novel approach to cable thermal field and ampacity computations using a newly proposed concept of perturbed finite-element analysis, which involves the use of derived sensitivity coefficients associated with various cable parameters of interest. It uses the sensitivity coefficients to achieve optimal cable performance. The proposed model provides a quick methodology, based on the finite element model, to assess the cable thermal performance subject to variations in the cable thermal circuit parameters. Furthermore, an optimization model for an underground power cable thermal circuit, based on generated gradients was developed, where subsequent utilization of the derived sensitivities as gradients of objective functions in a general framework of power cable performance optimization is presented. This comprehensive model uses the more accurate perturbed finite element method, which enables calculation of the objective function value and its gradients, without sacrificing the model accuracy. The algorithm developed was applied to various benchmark cable systems with their actual configurations, for different practical cable performance optimization objectives of interest to power utilities operators. The thermal field of an underground power cable sample directly buried in the soil was observed in the laboratory using a developed full size experimental setup. The investigation involves all parts of the thermal circuit parameters including cables composition, surrounding soil and boundaries phenomena. This experimental set was used to validate the developed simulation model by comparing the simulation results with the real laboratory measurements. Such experimental verification confirmed the accuracy of the newly introduced finite element sensitivity methodology. / This thesis reports on a novel approach to cable thermal field and ampacity computations using a newly proposed concept of perturbed finite-element analysis, which involves the use of derived sensitivity coefficients associated with various cable parameters of interest. It uses the sensitivity coefficients to achieve optimal cable performance. The proposed model provides a quick methodology, based on the finite element model, to assess the cable thermal performance subject to variations in the cable thermal circuit parameters. Furthermore, an optimization model for an underground power cable thermal circuit, based on generated gradients was developed, where subsequent utilization of the derived sensitivities as gradients of objective functions in a general framework of power cable performance optimization is presented. This comprehensive model uses the more accurate perturbed finite element method, which enables calculation of the objective function value and its gradients, without sacrificing the model accuracy. The algorithm developed was applied to various benchmark cable systems with their actual configurations, for different practical cable performance optimization objectives of interest to power utilities operators. The thermal field of an underground power cable sample directly buried in the soil was observed in the laboratory using a developed full size experimental setup. The investigation involves all parts of the thermal circuit parameters including cables composition, surrounding soil and boundaries phenomena. This experimental set was used to validate the developed simulation model by comparing the simulation results with the real laboratory measurements. Such experimental verification confirmed the accuracy of the newly introduced finite element sensitivity methodology. / Thesis / Doctor of Engineering (DEng)

abstract: The utilization of power cables is increasing with the development of renewable energy and the maintenance replacement of old overhead power lines. Therefore, effective monitoring and accurate fault location for power cables are very important for the sake of a stable power supply. The recent technologies for power cable diagnosis and temperature monitoring system are described including their intrinsic limitations for cable health assessment. Power cable fault location methods are reviewed with two main categories: off-line and on-line data based methods. As a diagnostic and fault location approach, a new passive methodology is introduced. This methodology is based on analyzing the resonant frequencies of the transfer function between the input and output of the power cable system. The equivalent pi model is applied to the resonant frequency calculation for the selected underground power cable transmission system. The characteristics of the resonant frequencies are studied by analytical derivations and PSCAD simulations. It is found that the variation of load magnitudes and change of positive power factors (i.e., inductive loads) do not affect resonant frequencies significantly, but there is considerable movement of resonant frequencies under change of negative power factors (i.e., capacitive loads). Power cable fault conditions introduce new resonant frequencies in accordance with fault positions. Similar behaviors of the resonant frequencies are shown in a transformer (TR) connected power cable system with frequency shifts caused by the TR impedance. The resonant frequencies can be extracted by frequency analysis of power signals and the inherent noise in these signals plays a key role to measure the resonant frequencies. Window functions provide an effective tool for improving resonant frequency discernment. The frequency analysis is implemented on noise laden PSCAD simulation signals and it reveals identical resonant frequency characteristics with theoretical studies. Finally, the noise levels of real voltage and current signals, which are acquired from an operating power plant, are estimated and the resonant frequencies are extracted by applying window functions, and these results prove that the resonant frequency can be used as an assessment for the internal changes in power cable parameters such as defects and faults. / Dissertation/Thesis / Doctoral Dissertation Electrical Engineering 2015

Le travail de cette thèse porte sur l'étude des pertes AC dans les supraconducteurs pour des applications tels que les câbles ou les aimants. Les modélisations numériques rapportées sont de type éléments-finis et méthode intégrale. Toutes ces méthodes visent à résoudre à calculer les distributions de densité de courant et de champ magnétique en prenant en compte différents loi de comportement pour le supraconducteur. Deux conducteurs sont introduits dans ce mémoire. Tout d'abord, les supraconducteurs à haute température critiques sont étudiées avec l'introduction d'une nouvelle forme de conducteur (fils cylindriques) et sont envisagés pour des câbles fort courant de 3~kA. Dans un second temps, des simulations numériques 3-D sont réalisés sur un conducteur MgB2. Le chapitre suivant traite des contraintes de calculs des pertes dans le but de dimensionner l'ensemble des pertes d'un câble complet. Enfin, les modèles numériques développés précédemment sont utilisé sur un exemple concret : le démonstrateur 10~kA fait à l'aide du conducteur MgB2 dans le projet BEST-PATHS / The work reported in this PhD deals with AC losses in superconducting material for large scale applications such as cables or magnets. Numerical models involving FEM and integral methods have been developed to solve the time transient electromagnetic distributions of field and current density with the peculiarity of the superconducting constitutive E-J equation. Two main conductors have been investigated for two ranges of superconducting cables. First, REBCO superconductors working at 77 K are studied and a new architecture of conductor (round wires) for 3~kA cables. Secondly, for very high current cables, 3-D simulations on MgB2 wires are approach and solved using FEM modeling. The following chapter introduced new development used for the calculation of AC losses in DC cables. The thesis ends with the use of the developed numerical model on a practical example in the BEST-PATHS project: a 10 kA MgB2 demonstrator.

This thesis is based on a research project to monitor the temperature profile along a power cable using the fibre optic Distributed Temperature Sensing (DTS) technology. Based on the temperature measured by a DTS system, real time condition monitoring of power cables can be achieved. In this thesis, there are three main research themes. 1. Develop a DTS system for industrial applications. The entire hardware system and measuring software are developed to be an industrial product. Multiple functions are provided for the convenience of users to conduct temperature monitoring, temperature history logging and off-line simulation. 2. Enhance the robustness of the DTS system. An algorithm for signal compensation is developed to eliminate the signal fluctuation due to disturbance from the hardware and its working environment. It ensures robustness of the system in industrial environments and applicability to different system configurations. 3. Improve the accuracy of the DTS system. A calibration algorithm based on cubic spline fitting is developed to cope with non-uniform fibre loss in the system, which greatly improved the accuracy of the temperature decoding in real applications with unavoidable nonlinear characteristics. The developed DTS system and the algorithms have been verified by continuous experiments for about one year and achieved a temperature resolution of 0.1 degree Celsius, a spatial resolution of 1 meter, and a maximum error of 2 degree Celsius in an optic fibre with the length of 2910 metres.

No description available.

Le travail présenté vise à mettre en place une méthodologie pour le suivi en service de la fatigue mécanique pour l’ombilical dynamique d’un système EMR flottant. L’approche envisagée consiste à simuler à l’aide d’outils numériques la réponse de l’ombilical aux cas de chargement observés sur site. Le post-traitement des résultats de ces simulations devant permettre d’accéder à différentes quantités d’intérêt en tout point du câble. Pour quantifier et réduire l’incertitude sur la réponse calculée de l’ombilical ce dernier doit être instrumenté. Un certain nombre de paramètres du modèle numérique feront alors l’objet d’une calibration régulière pour suivre l’évolution des caractéristiques de l’ombilical susceptibles d’évoluer. Dans ce contexte ce manuscrit présente et compare différentes méthodes pour analyser la sensibilité de la réponse de l’ombilical aux paramètres susceptibles d’être suivis. L’objectif est notamment d’orienter le choix des mesures à mettre en oeuvre. L’analyse en composantes principales permet pour cela d’identifier les principaux modes de variation de la réponse de l’ombilical en réponse aux variations des paramètres étudiés. Différentes approches sont également envisagées pour la calibration des paramètres suivis,avec en particulier le souci de quantifier l’incertitude restante sur le dommage. Les méthodes envisagées sont coûteuses en nombre d’évaluations du modèle numérique et ce dernier est relativement long à évaluer. L’emploi de méta-modèles en substitution des simulations numériques apparait donc nécessaire, et là encore différentes options sont considérées. La méthodologie proposée est appliquée à une configuration simplifiée d’ombilical dans des conditions inspirées du projet FLOATGEN. / The present work introduces a methodology to monitor fatigue damage of the dynamic power cable of a floating wind turbine. The suggested approach consists in using numerical simulations to compute the power cable response at the sea states observed on site. The quantities of interest are then obtained in any location along the cable length through the post-treatment of the simulations results. The cable has to be instrumented to quantify and to reduce the uncertainties on the calculated response of the power cable. Indeed some parameters of the numerical model should be calibrated on a regular basis in order to monitor the evolution of the cable properties that might change over time. In this context, this manuscript describes and compares various approaches to analyze the sensitivity of the power cable response to the variations of the parameters to be monitored. The purpose is to provide guidance in the choice of the instrumentation for the cable. Principal components analysis allows identifying the main modes of power cable response variations when the studied parameters are varied. Various methods are also assessed for the calibration of the monitored cable parameters. Special care is given to the quantification of the remaining uncertainty on the fatigue damage. The considered approaches are expensive to apply as they require a large number of model evaluations and as the numerical simulations durations are quite long. Surrogate models are thus employed to replace the numerical model and again different options are considered. The proposed methodology is applied to a simplified configuration which is inspired by the FLOATGEN project.