Industri- och materialvetenskap (IMS) // Industrial and Materials Science (IMS)
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Browsar Industri- och materialvetenskap (IMS) // Industrial and Materials Science (IMS) efter Program "Electric power engineering (MPEPO), MSc"
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- PostCharges on polymeric insulators and their effect on flashover characteristics(2011) Semere, Mebrahtu Melake; Chalmers tekniska högskola / Institutionen för material- och tillverkningsteknik; Chalmers University of Technology / Department of Materials and Manufacturing TechnologyNowadays polymeric materials are widely applied in outdoor high voltage insulation systems gradually replacing traditionally used porcelain and glass. It is believed that polymer based insulation will become dominant at ultra high voltages, e.g. in 1000 kV ac and 800 kV dc overhead transmission lines, which are presently being developed. Although withstand performance of polymeric insulating materials has been studied over the years, there is still a lack of complete physical understanding of all important aspects related to practical situations.Thus, the behavior and performance of polymeric materials for high-voltage applications in presence of accumulated surface charges, which can be generated in practice due to different sources (e.g. corona, surface discharges, space charge build-up, etc), are not completely understood today. This phenomenon needs to be considered when designing insulation systems and when performing insulator testing, especially for high-voltage dc (HVDC) applications where presence of surface charges is the inherent property of the system. The research conducted within the thesis project focused on experimental investigations of the influence of pre-deposited surface charges on impulse flashover characteristics of polymeric model insulators in air. The study also involved development of surface charge deposition system based on dc corona discharge as well as measurements of charge dynamics and its relaxation on cylindrical polymeric insulators made of poly-dienethylsiloxane based rubber, so called Silicon Rubber (SIR) and Room Temperature Vulcanized Silicon Rubber (RTVSIR) based rubber, which are widely used in outdoor high voltage insulation systems. The results presented and discussed in the report indicate that the presence of charges on surfaces of polymeric insulators can lead to a decrease or an increase of insulator withstand voltages depending on the polarities of the deposited surface charges and applied impulse voltages.
- PostCoupled Electro-Thermal Model for Submarine HVDC Power Cables(2013) Tefferi, Mattewos Berhane; Chalmers tekniska högskola / Institutionen för material- och tillverkningsteknik; Chalmers University of Technology / Department of Materials and Manufacturing Technology
- PostEffect of surface charging on dc flashover characteristics of polymeric insulators(2011) Hoque, Imtiaz Rifanul; Alam, Shahid; Chalmers tekniska högskola / Institutionen för material- och tillverkningsteknik; Chalmers University of Technology / Department of Materials and Manufacturing Technology
- PostElectrical Discharge and Breakdowns. A Study of Carbon Dioxide(2014) Avaheden, Johannes; Chalmers tekniska högskola / Institutionen för material- och tillverkningsteknik; Chalmers University of Technology / Department of Materials and Manufacturing TechnologySulfur hexafluoride (SF6) is widely used in modern high voltage gas insulated equipment. Despite of its excellent electrical and thermal performance, it is known as an extremely strong greenhouse gas and therefore a replacement is needed. One of the potential candidates is carbon dioxide (CO2), however many questions still remain regarding the dielectric properties of CO2. Experiments were carried out in uniform and weakly non-uniform background fields to find conditions for breakdowns, together with streamer and leader inception and propagation in CO2 in various levels of the electric fields and pressures from 1 to 5 bar. Also fundamental characteristic of streamers and leaders, such as discharge channel radius, propagation length at different field conditions, etc. were determined. The gathered data allow for predictions of time lags and streamer velocities. The results are discussed based on the knowledge of streamers in air and SF6.
- PostElectrical Insulation in a 400 V Battery Module for Hybrid Vehicles(2014) Memari, Mohammad Hassan; Nakanwagi, Victoria; Chalmers tekniska högskola / Institutionen för material- och tillverkningsteknik; Chalmers University of Technology / Department of Materials and Manufacturing TechnologyThe growing interest in the use of electric vehicles worldwide has resulted in awareness of the importance of ensuring the safety of these vehicles for users. The safety of electric vehicles is greatly determined by the state of the insulation between the high voltage system and the ground. In this thesis, the behaviour of the insulation in a high voltage traction battery used in a hybrid vehicle has been studied and evaluated under different operational conditions. The objective of this thesis was to assess how much the insulation resistance of the battery system is affected by the battery geometry and its environment of operation. The international standards applicable to insulation in electric vehicles have been studied to determine the requirements that the battery insulation system must adhere to and to determine which tests are recommended for testing the battery insulation system. In this work the results from the insulation resistance tests, partial discharge tests and environmental tests performed on a battery system are presented. The results from insulation resistance tests combined with environmental tests under varying temperature and humidity conditions show that the presence of moisture on the insulating surfaces of the battery greatly decreases the insulation resistance of the battery. The partial discharge tests show that grounding of the conducting non-current carrying parts of the battery through a path of low resistance is important to minimize discharges. At operational voltages, the weakest areas of the battery system insulation have been located inside the battery module on the edges of the cells.
- PostImproved Lightning Performance for 132 kV OHL(2017) Ingemarson, Tomas; Stelin, Johan; Chalmers tekniska högskola / Institutionen för material- och tillverkningsteknik; Chalmers University of Technology / Department of Materials and Manufacturing TechnologyLightning strokes to transmission lines causes a majority of the power quality issues that are so severe that sensitive industrial loads are disturbed. Historically Vattenfall concluded that it is too expensive to use shield wires for the 132 kV grid due to the high soil resistivity in Sweden. Today the situation is different with sensitive industrial loads and the current Vattenfall standard is to build portal tower with one shield wire. The shield wire is however not used to reduce the amount of lightning faults, it is used to ground the cross arm and reduce the amount of short circuits. In this project the lightning performance of different tower configurations used in the 132 kV grid is investigated using statistical insulation coordination in the softwares LPE and PSCAD. The optimal placement of the shield wires on towers located on flat ground is determined. Furthermore, the shield wire configuration of a new portal tower with two shield wires and typical Swedish conditions is proposed and compared to a portal tower with one shield wire. The demand is that the amount of lightning faults should be reduced by half compared to the case with a portal tower without shield wires. It is concluded that in order to achieve this decrease in the amount of faults a portal tower with one shield wire requires a tower foot resistance of 16.5 while a portal tower with two shield wires requires a tower foot resistance of 35.1. The distribution between single line to ground faults and short circuits for both configurations is estimated, where a second shield wire proved to reduce the amount of short circuits. Furthermore, it is concluded that every cross arm needs to be grounded properly in order to achieve the desired lightning performance. A cost estimation based on an average 132 kV line with average loading showed that a second shield wire could not cover its own costs. It is important to remember that the cost estimation is based on an average 132 kV line and it does not consider important industries or costs of groundings. If this would be included a second shield wire could be profitable in some cases. Therefore it is recommended to consider the local conditions when deciding if a new transmission line is built with one or two shield wires.
- PostMeasurement of ion mobility in dielectric liquids(2011) Zadeh, Mohsen Shafiee; Chalmers tekniska högskola / Institutionen för material- och tillverkningsteknik; Chalmers University of Technology / Department of Materials and Manufacturing TechnologyRising energy demand and at the same time global effort to reduce greenhouse gases are strong pushes for harvesting renewable energy sources in remote areas far away from consumption centers. Ultra High Voltage Direct Current is a highly efficient way to transfer bulk power over long distances. One key component for the reliable energy transfer in this technology is HVDC converter transformer. The insulation system of HVDC converter transformers, which basically consists of oil and paper/pressboard with different dielectric properties, needs to withstand AC and DC stresses. In particular for the UHVDC application the transient phenomenon of electric stress due to space charges needs to be further understood in order to design an insulation system for safe operation during long service life time. A physical model commonly called the ion drift model describes space charge accumulation on solid-liquid interface by ion transport in liquid. One important parameter in this model is the ion mobility. An investigation was made to evaluate the experimental set up for ion mobility measurement in COMSOL Multiphysics®. Charge transport, electric field distributions and current curves were studied in a bulk of oil under dc voltage application. After analyzing the simulation result decision concerning the set-up type was made by having two factors in mind: accuracy and large enough signal to determine the mobility. Based on the physical picture of the ion movement in the transformer oil obtained by the numerical calculation, the plane parallel electrode geometry had been chosen for experiment. The evolution of instrument and software for data acquisition during the experiment enabled us to get the desired data. The measurements were carried out for two types of dielectric liquid: mineral oil and natural ester oil. By obtaining information on the time of flight from the current curves, the mobility of ions in positive and negative voltage application for the respective liquids were determined. For mineral oil the measured mobility was around 2.61±0.175×〖10〗^(-9) m^2/Vs and for natural ester oil was 1.67±0.18×〖10〗^(-10) m^2/Vs.
- PostMeasurement techniques for identifying polarity of ions in transformer oil(2012) Ramesh, Deepthi Kubevoor; Chalmers tekniska högskola / Institutionen för material- och tillverkningsteknik; Chalmers University of Technology / Department of Materials and Manufacturing Technology
- PostMeasurements of Air Ion Concentrations and Electric Field Strengths for HVDC Applications(2017) Mikiver, Anders; Chalmers tekniska högskola / Institutionen för material- och tillverkningsteknik; Chalmers University of Technology / Department of Materials and Manufacturing TechnologyHigh Voltage Direct Current (HVDC) enables transfer of electric power over long distances with high efficiency. In order to further increase the efficiency, the voltage levels are increasing. In turn, this means that for the design of both robust and cost effective electric insulation, the importance of understanding specific DC voltage related phenomena also increase. For this thesis, specific DC voltage phenomena were studied by means of simultaneous measurements of air ion concentrations, DC electric field strength and ion current. The measurements were performed at ABB Corporate Research in Västerås from January to May of 2017, utilizing a data acquisition system denoted the ion box. Measurements both with and without applied voltage were carried out. The measurements without voltage were implemented in order to observe how the equilibrium ion concentrations varied with respect to location, and with other parameters such as humidity. The high voltage measurements were performed with different voltage polarities and electrode configurations, resulting in different electric field distributions. Additionally, the level of humidity was also varied during the high voltage measurements. The results of the measurements demonstrate that equilibrium ion concentration correlates strongly with concentrations of radon. In turn, the concentrations of both ions and radon were strongly affected by the air ventilation system for the indoor measurements. Furthermore, due to the tendency of radon gas to accumulate indoors, the measured indoor ion concentrations were larger than the ion concentrations outdoors. From the high voltage measurements the different regions of the general current-voltage characteristics for air were observed. The measurements also confirm that positive corona results in the injection of positive ions, whereas negative corona injects negative ions. In turn, the corona injected ions are shown to disrupt the linear relationship between applied DC voltage and DC electric field strength.
- PostMethodology for measurements of electrical conductivity of doped silicon carbide(2014) Frisk, Nils-Bertil; Nilsson, Douglas Jutsell; Chalmers tekniska högskola / Institutionen för material- och tillverkningsteknik; Chalmers University of Technology / Department of Materials and Manufacturing TechnologyThis work aims to develop a method for measurements of electric conductivity of doped silicon carbide in powder form. A general purpose measurement cell is used to produce an array of initial results and the experience and conclusions drawn from those are used to refine both the hardware and methodology. The main part of the refinements are focused on how force is applied to the powder and temperature control. The developed method is used to compare and evaluate produced batches of the material. The work concludes with a series of measurements that quantifies the repeatability of the final system together with ideas and suggestions for how the result could be improved.
- PostModelling and Simulations of Corona Discharge Currents in a Large Scale Coaxial Geometry with a Dielectric Barrier due to Low Frequency Triangular Voltages(2014) Macken, Martin; Chalmers tekniska högskola / Institutionen för material- och tillverkningsteknik; Chalmers University of Technology / Department of Materials and Manufacturing TechnologyWhen a high voltage is applied to electrodes immersed in air and provides a strongly non-uniform electric field, charged species such as free electrons and positive and negative ions can be created in gas due to corona discharge. Charged species will drift in the field from one electrode to another and eventually will be collected on dielectric surfaces if solid insulating elements are present in the discharge volume. The accumulated surface charges may strongly alter the electric field distribution in the entire system intensifying insulation ageing and increasing risk for flashovers. Such charge accumulation is an inherent phenomenon for DC applications and is also essential in cases of varying voltages when the dimension of the air gap is smaller than the travelling distances of the charged species. Recently, experiments have been carried out at ABB Corporate Research in Västerås (Sweden), where a large scale coaxial electrode arrangement was used to measure the level of the corona discharge current when triangular voltages of different frequencies were applied. The experiments were carried out both for free air and when a dielectric barrier was introduced in the discharge gap. In the thesis, the experimental results obtained for the case of coronas with dielectric barrier are analyzed by means of computer simulations. A model was developed that couples partial differential equations describing drift and diffusion of charged species with Poisson’s equation for computing space charge controlled electric field. The model accounts for field dependent generation and loss of free charges in gas and their accumulation of solid surfaces. The model was implemented in COMSOL Multiphysics. Simulations were conducted for conditions as close as possible to those used in the experiments including electrode system geometry, environmental parameters (temperature, pressure) and shapes of the applied voltages. Special attentions was paid to correct representations of boundary conditions that was found to be a key for reproducing experimental results for voltages of low frequencies when the travelling length of ions was large. The results obtained from the performed simulations are in agreement with the experimental corona characteristics acquired by ABB. The performed computational study allows for analyzing experimental data and provides insight on physical mechanisms leading to experimentally observed phenomena.
- PostMultiscale Modelling of a Metallized Film Capacitor for HVDC Applications(2017) Hjert, Andreas; Chalmers tekniska högskola / Institutionen för material- och tillverkningsteknik; Chalmers University of Technology / Department of Materials and Manufacturing TechnologyMetallized dielectric films used for high-voltage direct current (HVDC) Light capacitors are subjected to voltage ripples in operation. This leads to heat generation and a temperature rise in the capacitor. The operating temperature, which affects the function and the lifetime of the capacitor. Designs of capacitor units are evaluated by thermal stability tests to ensure thermal stability under overload conditions. It is therefore important for the designer to know locations of hot spots, the average temperature and the maximum temperature. This thesis presents a multiphysics model of a HVDC Light capacitor unit simulating electric currents and heat transfer under the conditions of a thermal stability test. A capacitor unit consists of capacitor elements made of metallized films with large difference in scales. To overcome this, a multiscale approach have been used to model the capacitor unit. Effective properties and losses are computed on a small scale (several layers of metallized film) and utilized on a large scale (capacitor unit). The performed calculations yield the impedance of the capacitor unit corresponding to the rated impedance with the effective properties implemented. The loss computed on the small scale is lower than average measured loss of a capacitor element. Additionally, the loss computed on the large scale matches the average measured loss of a capacitor unit. The hot spot of the capacitor unit is located at the front. The elements located there are hotter than the elements at the rear. A multiscale modelling approach is utilized to model metallized film capacitors and to simulate thermal stability tests to estimate hot spots, average and maximum temperatures. The simulation results are compared to the results from a thermal stability test in a laboratory environment. This comparison shows that the simulated results are close to the test results. However, there are uncertainties related to heat transfer coefficients at different exterior boundaries of the unit and comparison with additional tests may improve the model.
- PostSimulation and Analysis of Corona Currents in Large Scale Coaxial Geometry due to Triangular Voltages(2013) Georgii, Karman; Chalmers tekniska högskola / Institutionen för material- och tillverkningsteknik; Chalmers University of Technology / Department of Materials and Manufacturing TechnologyCharged species can be generated in air due to corona discharges from sharp electrodes. They are of great concern in practical applications due to their ability to drift in the electric field and to accumulate on dielectric surfaces modifying electric field distributions. To understand better regularities of dynamic behavior of charge carriers produced by corona, series of experiments have been conducted at ABB Corporate Research Center in Västerås, Sweden. For this, a coaxial large scale electrode arrangement was utilized. Corona discharges were initiated by applying triangular voltages that allowed for studying drift of ionic species under controlled conditions. The experimentally obtained data required physical interpretation and this was the main objective of the present MSc thesis work. In the thesis, a computer model of corona discharges in air is presented and its implementation in COMSOL Multiphysics is described. The developed model is utilized for simulations of corona discharges in the set-up used in the experiments. The computed current-voltage characteristics are compared with the experimental results. Influences of different model parameters, including the effect of boundary conditions, on corona characteristics are studied and a set of parameters providing the best fit between experiments and simulations is identified. An analysis of physical processes in corona discharges in air under triangular voltages at frequencies 1-50 Hz is presented focusing on interpretation of experimentally observed phenomena.