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- Hettiarachchi, Pasan, 1977-
(författare)
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Measurements of X-Ray Emission from Laboratory Sparks and Upward Initiated Lightning
- 2018
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- In 1925 Nobel laureate R. C. Wilson predicted that high electric fields of thunderstorms could accelerate electrons to relativistic energies which are capable of generating high energetic radiation. The first detection of X-rays from lightning was made in 2001 and from long sparks in 2005. Still there are gaps in our knowledge concerning the production of X-rays from lightning and long sparks, and the motivation of this thesis was to rectify this situation by performing new experiments to gather data in this subject.The first problem that we addressed in this thesis was to understand how the electrode geometry influences the generation of X-rays. The results showed that the electrode geometry affects the X-ray generation and this dependency could be explained using a model developed previously by scientists at Uppsala University. The other missing information was the distribution of energy. Using a series of attenuators, we observed how the X-ray photons were attenuated as a function of barrier thickness and using a simple model we obtained the average and the maximum energy of X-rays. All the studies conducted previously was based on the lightning impulses, but in switching impulses, the voltage waveform rises very slowly compared to lightning impulses, and according to some scientists the rate of rise is an important parameter in X-ray development. Our study showed that the switching impulses were as efficient as lightning impulses in generating X-rays even though the rate of rise of voltage in switching impulses was hundreds of times slower.All the observations on X-ray generation from lightning by other scientists were based on either natural downward lightning flashes or triggered lightning in Florida. The first experiments to study the X-ray generation from upward lightning flashes systematically was conducted within this thesis work at Gaisberg Tower in Austria. The results showed that the X-ray emissions from these flashes were much weaker than the ones produced by either natural downward or triggered lightning. An attempt was made to explain this observation by invoking the possible differences in the charge distribution of leaders associated with the triggered lightning flashes in Florida and upward initiated lightning flashes at Gaisberg tower.
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| 2. |
- Montaño, Raul, 1971-
(författare)
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The Effects of Lightning on Low Voltage Power Networks
- 2006
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- The present society is highly dependant on complex electronics systems, which have a low damage threshold level. Therefore, there is a high risk of partial or total loss of the system’s electronics when they are exposed to a thunderstorm environment. This calls for a deep understanding on the mechanism related to the interaction of lightning generated electromagnetic fields with various large distributed/scattered systems. To accurately represent the interaction of lightning electromagnetic fields with electrical networks, it is necessary to have return stroke models capable to reproduce the electromagnetic field signatures generated by a lightning flash. Several models have been developed in the recent past to study the field-to-wire coupling mechanism. The most popular, simple and accurate among the available models is the Agrawal et al. model. On the other hand, ATP-EMTP is a well-known transient simulation package widely used by power engineers. This package has various built-in line models like Semlyen, Marti and Noda setups. There is a difficulty in applying the Agrawal et al. model with the built-in line models of ATP-EMTP, as the voltage source due to the horizontal component of electric field in Agrawal et al. model is in series with the line impedance and not in between two transmission line segments. Furthermore, when the electromagnetic field is propagating over a finite conducting ground plane, the soil will selectively attenuate the high frequency content of the electromagnetic field; causing a change in the field wave shape. A finite conducting ground will also produce a horizontal field component at the ground level. Several approximations are available in the literature to obtain the horizontal electric field; namely the wave-tilt and the Cooray-Rubinstein approximation. Consequently, it is important to investigate the change on the induced voltage signature when the power line is located over a finitely conducting ground. Additionally, to provide protection from lightning induced transients it is necessary to use Surge Protective Devices (SPDs) capable of diverting the incoming transients and provide protection level necessary to avoid damage in the equipment. However, standard test procedures of the SPDs do not take into account sub-microsecond structure of the transients. Therefore, to provide the required protection level to sensitive equipments connected to the low voltage power network, it is essential to understand the response of SPDs subjected to high current derivative impulses. This thesis is aimed to investigate the research problems as addressed above. Special attention will be given to a new proposed return stroke model, a simple circuit approach for efficient implementation of Agrawal et al. model using ATP-EMTP, the effect of the soil conductivity on the lightning induced overvoltage signatures and the response of surge protective devices subjected to high current derivative impulses.
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| 3. |
- Zitnik, Mihael, 1972-
(författare)
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Numerical Modelling of Transients in Electrical Systems
- 2001
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- This thesis deals with the numerical modelling of the response of electrical systems to transient surge voltages and currents caused by lightning. Electrical systems, when subjected to high amplitude transients behave in ways very different from its behaviour under normal operating conditions. Modelling the response of the system in a computationally efficient way, while maintaining sufficient accuracy, is the challenge being addressed in this thesis.While modelling transients in a complex system it is required to reduce the complexity of the system in the model to manageable levels, at the same time retaining all the important characteristics of the system, including the non-linear responses. Transmission line model in time domain is simple and can handle non-linearities easily. Therefore transmission line method is utilised for modelling complex systems in this thesis. Transmission line parameters are deduced from the geometry of the system using field simulation methods. The model is then implemented in a circuit simulation software.To enable computer modelling of transient protection, experiments were conducted on metal-oxide varistors and the results were compared with the predictions of the existing varistor models. From these comparisons the deficiency of existing varistor models are identified and a new improved varistor model is proposed that can overcome these deficiencies. Additionally, using the transmission line method a multiconductor cable model is developed and experimentally verified.Several practical problems of industrial importance are investigated using the transmission line modelling method. The co-ordination between the primary and secondary overvoltage protectors in a low voltage power installation is investigated and it is shown that the existing practice of co-ordinating the primary and secondary protectors based only on the current and voltage ratings do not always work. In another application of the method current and voltage distributions during a lighting strike to the railway signalling and traction system is determined. Lastly, simulation of direct lightning strike to a radio communication tower is performed and the division of current between the cable system and the tower construction is determined. The parameters that influence this current division are also determined.
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