| 41. |
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| 42. |
- Abdulahovic, Tarik, 1976
(författare)
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Analysis of High-Frequency Electrical Transients in Offshore Wind Parks
- 2011
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- In this thesis, a study of high frequency electromagnetic transient phenomena is performed. Models of various components needed for simulations of high frequency transients, such as for transformers, cables and breakers are developed. Measurements were performed in Chalmers research laboratory as well as at ABB Corporate Research in Västerås, Sweden for the purpose of parameter estimation of models and for verification of simulations. Critical cases where the voltage surges of the magnitude and/or rise time above basic lightning impulse voltage level appear, are identified. Also, some transient protection schemes are analyzed and the performance of different transient mitigation devices is studied. Furthermore, the voltage distribution along the winding during very fast transients is studied in order to estimate turn-to-turn voltages and the critical voltage envelope. In the work it was found that simulations of the high frequency transients are in a very good agreement with the measurements obtained in the laboratory. Simulations predicted accurately critical surges with the highest magnitude and matched with good accuracy surge waveforms recorded during the measurements. The accuracy of the rise times is within 10%, while the magnitudes during the critical cases are within a 5% margin. During the testing and simulations, surges which exceed the basic lightning impulse voltage level of dry-type transformers specified by IEEE standards, both in magnitude and rise time are observed at the transformers even when surge arresters are used to protect the transformers. Furthermore, obtained voltage surges exceeded the proposed critical voltage envelope. It is shown that the most critical voltage strikes are obtained with dry-type transformers, where the rise time is five to ten times shorter compared to oil-insulated transformer with the same rating. It was confirmed both in simulations and measurements that the use of additional transient protections devices such as surge capacitors and RC protections, decreased the magnitude of surges to be below the critical level. The analysis of the voltage during very fast transients showed that the rise time of the transients directly influences the magnitude of the turn-to-turn voltages. Furthermore, during the breaker closing transient, turn-to-turn voltages, measured in delta connected dry-type transformers, were 2.5 times higher than the voltages obtained during the same transient with a wye connected dry-type transformer, or during the stress with a 4.4 p.u. lightning impulse voltage.
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| 43. |
- Abdulahovic, Tarik, 1976
(författare)
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Analysis of High-Frequency Electrical Transients in Offshore Wind Parks
- 2009
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Licentiatavhandling (övrigt vetenskapligt/konstnärligt)abstract
- In this thesis, a study of high frequency electromagnetic transient phenomena is performed. Models of various components needed for simulations of high frequency transients, such as transformers, cables and breakers are developed. Also, measurements of high frequency transients are performed in the cable laboratory in ABB Corporate Research in Västerås, Sweden for the purpose of parameter estimation of models and for verification of simulations. Some critical cases where the voltage surges of the magnitude and/or rise time above basic lightning impulse voltage level appear, are identified. Also, some transient protection schemes are analyzed and the performance of different transient mitigation devices is studied. Furthermore, the energizing transient of the Utgrunden wind park is analyzed and the simulation model is verified using the measurements.The energizing transient simulation predicted accurately the magnitude and the frequency of the transient voltages and currents. Simulations of the high frequency transients are in a very good agreement with the measurements obtained in the cable lab. Simulations predicted accurately critical surges with the highest magnitude and matched with good accuracy surge waveform recorded during the measurements. During the testing and simulations, surges which exceed the basic lightning impulse voltage level of dry-type transformers specified by IEEE standards, both in magnitude and rise time are observed even when surge arresters are used. It is confirmed both in simulations and measurements that use of additional transient protections devices such as surge capacitors and RC protection, decreased the magnitude of surges below the critical level.
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| 44. |
- Abdulahovic, Tarik, 1976, et al.
(författare)
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Comparison of switching surges and basic lightning impulse surges at transformer in MV cable grids
- 2009
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Ingår i: Nordic Wind Power Conference, Bornholm, Danmark.
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Konferensbidrag (övrigt vetenskapligt/konstnärligt)abstract
- The electric transients generated during the switching of a breaker placed near the transformer in a system with a substantial amount of cables can have a very short rise time and can reach very high magnitudes. In practice, the magnitude of the voltage surges is usually limited using the surge arresters while the rise time of the surge to which the apparatus is exposed, remains unchanged.In this paper, the characteristic of the switching transients generated in the MV cable grids are analyzed. For this purpose, cases with both dry-type and oil insulated transformers are studied. These cases are typical for a MV industrial grid and a wind park (WP) collection grid. Furthermore, the case with and without surge arrester protection is analyzed in order to observe the impact of the surge arresters on the voltage surges.Simulations show that the magnitude of the voltage surges exceeds the basic lightning impulse insulation level (BIL) defined by present standards. Furthermore, the rise time of the voltage surges is much shorter compared to the rise time of the lightning impulse especially in the case of the dry-type transformers and the WP collection grid. When surge arresters are used, the magnitude of the surges is limited to the BIL but the rise times of the surges can be twenty five times shorter compared to the BIL. Only when surge arresters are used with other fast transient mitigation equipment such as RC protection or surge capacitor protection, the magnitude of very fast voltage surges are brought down.
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| 45. |
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| 46. |
- Abdulla, Mouhamed, 1979, et al.
(författare)
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5G Connected Vehicles: the Missing Link to Highly Autonomous Vehicles
- 2017
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Ingår i: Routes et transports Magazine. - 0319-3780. ; 46:2, s. 122-127
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Tidskriftsartikel (övrigt vetenskapligt/konstnärligt)abstract
- Today, there is a clear convergence between technologists, visionaries, corporations, academics, and policymakers on the expected trend of Intelligent Transportation Systems (ITS). The related infrastructure and vehicles of the future will essentially have four key attributes, which are (a) electric (i.e. sustainable), (b) autonomous (i.e. convenient), (c) connected (i.e. safe), and (d) shared (i.e. rely on smart mobility utilization).
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| 47. |
- Abdulla, Mouhamed, 1979, et al.
(författare)
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Fine-Grained Reliability for V2V Communications around Suburban and Urban Intersections
- 2017
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Rapport (övrigt vetenskapligt/konstnärligt)abstract
- Safe transportation is a key use-case of the 5G/LTE Rel.15+ communications, where an end-to-end reliability of 0.99999 is expected for a vehicle-to-vehicle (V2V) transmission distance of 100-200 m. Since communications reliability is related to road-safety, it is crucial to verify the fulfillment of the performance, especially for accident-prone areas such as intersections. We derive closed-form expressions for the V2V transmission reliability near suburban corners and urban intersections over finite interference regions. The analysis is based on plausible street configurations, traffic scenarios, and empirically-supported channel propagation. We show the means by which the performance metric can serve as a preliminary design tool to meet a target reliability. We then apply meta distribution concepts to provide a careful dissection of V2V communications reliability. Contrary to existing work on infinite roads, when we consider finite road segments for practical deployment, fine-grained reliability per realization exhibits bimodal behavior. Either performance for a certain vehicular traffic scenario is very reliable or extremely unreliable, but nowhere in relatively proximity to the average performance. In other words, standard SINR-based average performance metrics are analytically accurate but can be insufficient from a practical viewpoint. Investigating other safety-critical point process networks at the meta distribution-level may reveal similar discrepancies.
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| 48. |
- Abdulla, Mouhamed, 1979, et al.
(författare)
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Vehicular Transmission Reliability over Blind Intersections
- 2017
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Ingår i: IEEE Swedish Communication Technologies Workshop (Swe-CTW'17). ; , s. 1-2
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Konferensbidrag (övrigt vetenskapligt/konstnärligt)abstract
- Vehicle-to-vehicle (V2V) communication can improve road safety and traffic efficiency, particularly around critical areas such as intersections. We analytically derive V2V success probability near an urban intersection, based on empirically supported line-of-sight (LOS), weak-line-of-sight (WLOS), and non-line-of-sight (NLOS) channel models. The analysis can serve as a preliminary design tool for performance assessment over different system parameters and target performance requirements. The most interesting outcome of this research is the ability to design the network and explicitly quantify the tolerated number of simultaneous transmissions that could occur at the same time-frame of the wanted transmission, while still meeting the predetermined target reliability. Meanwhile, we will also discuss means to determine the fraction of vehicular traffic realizations that achieve the target reliability. This is a more granular finely detailed analysis, and it will basically build on the results presented earlier.
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| 49. |
- Abdulwahab, Fethi, 1964
(författare)
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Postprocessing Techniques and Adaptivity in the Finite Element Method
- 1995
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- The Finite Element (FE) method is an approximate method and as such the accuracy of its solutions must be controlled to allow for a reliable application. This thesis deals with error estimation and improvements of the FE solutions based on superconvergent properties of the FE solution in certain points. Two new postprocessing procedures are developed for elliptic problems. The procedures are based on local patches of elements and developed in such a way that the methods function at local level. Being local methods, the cost involved in implementing the procedures is small. The derivatives of the FE solutions are discontinuous across element boundaries. The primary purpose of the proposed postprocessing procedures is to smooth the discontinuous derivatives of finite element solutions like stresses and fluxes. The smoothed solution is used both as a new improved solution and in an error estimation. The presented postprocessing methods do not only smooth the stress discontinuities but also improve the satisfaction of the governing differential equation. This is in contrast to the fact that the FE solutions fulfill equilibrium conditions only in a weak and global sense. Similarly, the FE solutions do not generally satisfy the prescribed natural boundary conditions satisfactorily. On the other hand, the solutions obtained by the proposed postprocessing procedure satisfy the prescribed boundary conditions. Satisfaction of both the equilibrium and boundary conditions is achieved in the sense of the least squares. The proposed methods yield superconvergent solutions meaning that the accuracy of the postprocessed solutions are at least one order higher than the finite element solutions. This in its turn means that use of these postprocessed solutions in error estimations gurantees that the error estimate is asymptotically exact or that the estimated discretization error converges towards the true error in the limit. Several numerical test examples are given to validate the accuracy and effectiveness of these new methods. The presented methods are also implemented in an adaptive finite element procedure of the h-version or mesh enrichment. The postprocessing procedures proposed in this thesis can be regarded as a step forward towards an accurate and automated adaptive analysis in which the ultimate goal is to provide finite element solutions of high quality in an economical manner. Starting from some arbitrary mesh, such an automated adaptive procedure should provide an improved finite element model that meets a user specified level of accuracy. To accomplish this goal, every FE program package should be equipped with efficient postprocessing procedures.
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| 50. |
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