899 research outputs found

    High Voltage DC-biased Oil Type Medium Frequency Transformer; A Green Solution for Series DC Wind Park Concept

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    The electric energy generated by remote offshore wind parks is transported to the consumers using high voltage submarine cables. On the generation site, such transmissions are realized today by collecting the energy produced by several wind turbines in a bulky and expensive transformer placed on a dedicated platform. An alternative solution has been proposed recently, which allows to reduce the installation and maintenance costs by eliminating such a platform. It is suggested to equip each wind turbine in the wind park by an individual DC/DC converter and connect them in series to reach the DC voltage level required for an efficient HVDC energy transportation to the shore. The DC/DC converter is supposed to be a Dual Active Bridge (DAB) converter, which can be made reasonably small to be placed on the wind turbine tower or even in its nacelle. The key element of the converter defining its size and mass is a special transformer, which operates at voltages comprising a high (switching) frequency component superimposed on a high DC offset voltage. DC insulation design of such a transformer and investigation of the effects of a high DC insulation level on the other electromagnetic properties of the transformer is the subject of the present research.In order to verify the concept a prototype of the transformer was built, and its evaluation presented. The unit has been manufactured for the rated power of 50 kW and rated voltages 0.4/5 kV including DC offset of 125 kV and square-shaped oscillations with the frequency of 5 kHz. The magnetic system was made of ferrite material and consisted of 10 shell-type core segments. The magnetic properties have been verified by measuring magnetization and losses at various frequencies in the range 1-10 kHz to cover the operational range of the DAB. The types and dimensions of the windings and their conductors were chosen to minimize the proximity and eddy current effects at higher frequencies. To reduce the size of the transformer and to allow for its efficient cooling, the active part was immersed in oil and cellulose-based materials (paper and pressboard) were used to build the high voltage insulation system. The principles for dimensioning the insulation of the transformer are discussed. The criteria used for selecting insulating distances were based on the consideration of the electric field strength obtained from FEM simulations and using the non-linear Maxwell-Wagner model accounting for local variations of the electric field caused by accumulation of interfacial charges induced by DC stresses. The properties of the materials needed for the calculations were obtained by measuring their dielectric constants and electric conductivities. The methodology used for the measurements conducted for conventional mineral oil and eco-friendly biodegradable transformer oils and, respectively, for oil-impregnated paper/pressboard, is presented. The methodologies used for obtaining parameters of the built transformer prototype needed for its integration in the power electric circuit of the DAB are introduced. A method developed for accurate calculations of the leakage inductance for the shell-type multi core transformers with circular windings is described. Two innovative methods for evaluations of parasitic capacitances based on high frequency equivalent circuits of the transformer are presented. The results of their verifications against performed Frequency Response Analysis measurements and FEM calculations as well as their accuracy are discussed.Thermal performance of the developed transformer prototype is analysed based on the results of computer simulations of heat transfer in its active part under rated load. Identified hot spots and solutions for their elimination are presented.Finally, the expected dimensions, weight, and efficiency of an actual DC/DC converter with the rated parameters corresponding to a 6 MW, 1.8 kV real wind turbine having a 250 kV offset DC voltage are estimated assuming that the developed transformer prototype is scalable. It is shown that the proposed solution allows for installing the full-scale converter having 2.2 Tons in weight and 1.8 m3 in volume on the bottom of the wind turbine’s tower

    Erfassung und Evaluierung von Teilentladungen in Leistungstransformatoren mit speziellen Sensoren und Diagnoseverfahren

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    Transformers are key elements of the power grid. Due to their importance and high initial cost, asset managers utilize monitoring and diagnostic tools to optimize their operation and extend their service life. The main objective of this thesis is to develop new methods in the field of monitoring and diagnosis of transformers in order to reduce maintenance costs and decrease the frequency of forced outages. For this purpose, two concepts are proposed. Small generator step-up transformers are essential in wind and photovoltaic parks. The first presented concept entails an online fault gas monitoring system for these transformers, specially hermetically-sealed transformers. The developed compact, maintenance-free and cost-effective monitoring system continuously tracks the level of the key leading indicators of transformer faults in the gas cushion. The second presented concept revolves around partial discharge (PD) assessment by the UHF measurement technique, which is based on capturing the electromagnetic (EM) waves emitted in case of PD in the insulation of a transformer. In this context, the complex EM system established when probes are introduced into the tank of a transformer and with PD as the excitation source is analyzed. Drawing on this foundation, a practical approach to the detection and classification of PD with the focus on the selection of the optimal frequency range for performing UHF measurements depending on the device under test is presented. The UHF measurement technique also offers the possibility of PD localization. Here, the determined arrival time (AT) of the captured signals is critical. A PD localization algorithm, based on a multi-data-set approach with a novel AT determination method, is proposed. The methods and algorithms proposed for the detection, classification and localization of PD are validated by means of practical experiments

    Federated Learning with Uncertainty-Based Client Clustering for Fleet-Wide Fault Diagnosis

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    Operators from various industries have been pushing the adoption of wireless sensing nodes for industrial monitoring, and such efforts have produced sizeable condition monitoring datasets that can be used to build diagnosis algorithms capable of warning maintenance engineers of impending failure or identifying current system health conditions. However, single operators may not have sufficiently large fleets of systems or component units to collect sufficient data to develop data-driven algorithms. Collecting a satisfactory quantity of fault patterns for safety-critical systems is particularly difficult due to the rarity of faults. Federated learning (FL) has emerged as a promising solution to leverage datasets from multiple operators to train a decentralized asset fault diagnosis model while maintaining data confidentiality. However, there are still considerable obstacles to overcome when it comes to optimizing the federation strategy without leaking sensitive data and addressing the issue of client dataset heterogeneity. This is particularly prevalent in fault diagnosis applications due to the high diversity of operating conditions and system configurations. To address these two challenges, we propose a novel clustering-based FL algorithm where clients are clustered for federating based on dataset similarity. To quantify dataset similarity between clients without explicitly sharing data, each client sets aside a local test dataset and evaluates the other clients' model prediction accuracy and uncertainty on this test dataset. Clients are then clustered for FL based on relative prediction accuracy and uncertainty

    2022 comprehensive permanent improvement plan for the plan years 2023-2027 statewide

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    This planning document tells the costs and funding sources for capital improvements of state agencies for the plan years 2023-2027. Each agency has a summary of proposed permanent improvement projects including funding source, functional group and business area

    Enhancement of fault current contribution from inverter-based resources

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    The reduction in levels of fault current infeed as inverter-based resources (IBR) displace synchronous machines undermines the ability of a conventional protection system to identify and isolate faults in an effective manner and is therefore a concern for system operators (SOs). This observation provided the motivation to investigate the limitations of IBRs when injecting fault current and to explore how these limitations might be overcome. This thesis investigates techniques aimed at significantly increasing Fault Current Contribution (FCC) from an IBR system so that renewable energy resources can continue to be deployed without compromising the protection system. The techniques for enhancing FCC are at three different levels of an IBR system: at semiconductor or device level, circuit level and system level. The first study uses phase change materials (PCM) to provide a short-term overload rating to insulated-gate bipolar transistors (IGBTs) and found them to have very limited potential to provide FCC. A Finite Element Analysis (FEA) of heat-flow concluded that, although the PCM was useful for dealing with short over-load currents, it was unsuitable for facilitating large fault currents of several times normal load current. The view was that if the fault current cannot be created at device level through better thermal management, then a circuit level innovation would be required. The second study investigates series/parallel switching of submodules in modular converters. This takes advantage of the fact that during a fault, the line voltage is reduced, and if it falls below 0.5 pu then half of the sub-modules (SMs) can be put into parallel with the other half to double the FCC (2 pu) at half the voltage (0.5 pu). Similarly, if the voltage drops below 0.25 pu, parallel connection of four groups of SMs would enable 4 pu current capability. A model of a static synchronous compensator (STATCOM) was developed, inspired by the alternate arm converter (AAC), with the director switch of the AAC used as part of the reconfiguration circuit. The conclusion of this study was that the penalty paid in power losses in the additional semiconductor devices used for reconfiguration is reasonable for the 2 pu FCC case but not at the 4 pu FCC case. The third study was based on circuit reconfiguration but beyond the converter itself and in this case the windings of the coupling transformer of a STATCOM. Sections of winding were switched using thyristors to tap-change the transformer by a large factor. Using the proposed thyristor-based electronic tap-changer (eTC), the number of turns of the grid-side winding was reduced during a voltage dip, so that larger current can be delivered to the network for the same converter current. The STATCOM was controlled in the natural frame (abc frame) and this control is used to actively drive the currents in the tap-changer thyristors to zero when needed so that they can be commuted rapidly. The transformer was configured to give a normal ratio of 1:4 and be able to tap-down to 1:2 and 1:1 to increase FCC to 2 pu or 4 pu. Theoretical analysis of, and operating principles for, the proposed eTC, together with their associated control schemes, are verified by time-domain simulation at full-scale. The case-study circuit demonstrates delivery of substantial fault current contribution (FCC) of up to 4 pu at the point of common coupling (PCC) in less than half a cycle (10 ms) after detection of three- and single-phase faults. The results demonstrate that the proposed eTC is a good candidate for the enhancement of fault current from IBR systems that employ coupling transformers, allowing them thereby to make a contribution to future electricity networks dominated by IBR.Open Acces

    Life Cycle Cost Analysis of a Floating Wind Farm Located in the Norwegian Sea

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    This thesis aims to investigate the levelized cost of energy of an offshore floating wind farm, as well as evaluate its financial feasibility. Thus, the research question is as follows: How to estimate the life cycle costs of a floating wind farm off the coast of Norway? The investigated wind farm is located off the coast of Norway, more specifically in the Troll field area west of Bergen. This area has a water depth of 325 m and a distance to shore of 65 km. The wind farm is set to consist of 50 wind turbines and has a lifespan of 25 years. The OC4 Deepwind semisubmersible floater developed by the National Renewable Energy Laboratory, complemented with a 15 MW turbine, is used as the research model. To find the capital expenditures of the planned wind farm, the Offshore Renewables Balance-of-system and Installation Tool is used, while the operational expenditures are calculated based on the theoretical energy output. The total levelized cost of energy of the wind farm is calculated to be 100.69 /MWh.Capitalexpenditureisthemostprominentcostandconstitutes63.1expendituresconstitutetheremaining36.9lifespan,capacityfactor,andprojectdiscountratearethefactorswiththemostpotentialtoinfluencethelevelizedcostofenergy.Thefinancialcalculationsshowthatthewindfarmisnoteconomicallyfeasibleasithasacomputednetpresentvalueofnegative/MWh. Capital expenditure is the most prominent cost and constitutes 63.1 % of the total cost, thus, operational expenditures constitute the remaining 36.9 %. Further, sensitivity analyses show that the lifespan, capacity factor, and project discount rate are the factors with the most potential to influence the levelized cost of energy. The financial calculations show that the wind farm is not economically feasible as it has a computed net present value of negative 561 900 000. Finally, novel offshore wind energy solutions involving the utilization of shared substructures and mooring lines have been studied, and the findings suggest the possibility of a diminished levelized cost of energy

    ELECTRIC STRESS AND INSULATION FAILURE MECHANISM OF MEDIUM VOLTAGE MEDIUM FREQUENCY TRANSFORMER

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    Power Semiconductors for An Energy-Wise Society

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    This IEC White Paper establishes the critical role that power semiconductors play in transitioning to an energy wise society. It takes an in-depth look at expected trends and opportunities, as well as the challenges surrounding the power semiconductors industry. Among the significant challenges mentioned is the need for change in industry practices when transitioning from linear to circular economies and the shortage of skilled personnel required for power semiconductor development. The white paper also stresses the need for strategic actions at the policy-making level to address these concerns and calls for stronger government commitment, policies and funding to advance power semiconductor technologies and integration. It further highlights the pivotal role of standards in removing technical risks, increasing product quality and enabling faster market acceptance. Besides noting benefits of existing standards in accelerating market growth, the paper also identifies the current standardization gaps. The white paper emphasizes the importance of ensuring a robust supply chain for power semiconductors to prevent supply-chain disruptions like those seen during the COVID-19 pandemic, which can have widespread economic impacts.The white paper highlights the importance of inspiring young professionals to take an interest in power semiconductors and power electronics, highlighting the potential to make a positive impact on the world through these technologies.The white paper concludes with recommendations for policymakers, regulators, industry and other IEC stakeholders for collaborative structures and accelerating the development and adoption of standards

    Utsira Nord Floating Wind farm – Optimalisation of marine operations related to inter-array cable installation.

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    The offshore wind industry is experiencing significant growth, with wind farms expanding in size, capacity, and moving to more challenging locations. However, cable faults remain a major concern, particularly during the installation of inter-array and export cables. This thesis aims to research and analyse the installation steps of inter-array cables at the Utsira Nord offshore wind farm. It further seeks to enhance the installation process by identifying functional sea states through a combination of previous work and new data. The research focuses on answering key questions related to the installation of inter-array cables. This includes identifying the main steps, defining critical phases, and analysing the typical loads that impact cable installation to determine the parameters affecting the process. Additionally, the thesis seeks to determine the optimal environmental conditions for cable installation. The methodology employed in this thesis includes literature studies, experience from employees at DeepOcean regarding offshore operations, and the use of Orcaflex and Excel. The analysis, primarily conducted using Orcaflex, concentrates on the installation of 1st and 2nd end dynamic section. The floating windmill is treated as a fixed structure in the Orcaflex model, neglecting its movement due to the ongoing development of the foundation concept and limited comprehensive information on weather and environmental factors. The study primarily focuses on tension and curvature acting on the cable during installation, disregarding boat and floating structure considerations that are not directly relevant to the research objectives. From the knowledge gained in the literature review, an analysis was carried out to investigate crucial parameters in the cable laying process and a conclusion for when it is optimal to do inter-array cable installation operations at Utsira Nord was given. The focus was on tension loads, minimum bending radius, and potential clashes with the Moonpool. This was conducted with both Gumbel and Rayleigh analysis with irregular vessel motions. The result of the analysis illustrated the extent of impact the various environmental wave conditions had on the system. Throughout the study, certain wave conditions were found to be more favourable than others. Wave directions ranging from 90-135° and 225-270°, simulated with a current velocity of 0.3 m/s, offered the most optimal conditions for cable installation. In contrast, the analysis showed that sea states with wave directions of 0° and 180°, as well as current at 1.0 m/s, presented a higher risk of cable damage. To enhance the findings of this master's thesis, further analysis of different current velocities is recommended to identify operable sea states under higher currents. Additionally, analysing the system with a moving floating wind turbine, given that sufficient data is available

    Selected Papers from 2020 IEEE International Conference on High Voltage Engineering (ICHVE 2020)

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    The 2020 IEEE International Conference on High Voltage Engineering (ICHVE 2020) was held on 6–10 September 2020 in Beijing, China. The conference was organized by the Tsinghua University, China, and endorsed by the IEEE Dielectrics and Electrical Insulation Society. This conference has attracted a great deal of attention from researchers around the world in the field of high voltage engineering. The forum offered the opportunity to present the latest developments and different emerging challenges in high voltage engineering, including the topics of ultra-high voltage, smart grids, and insulating materials
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