Infoscience

Thesis

Locating transient disturbance sources and modelling their interaction with transmission lines: use of electromagnetic time reversal and asymptotic theory

This thesis deals with the application of electromagnetic time reversal to locating transient disturbance sources and the use of the asymptotic theory for the modelling of their interaction with transmission lines. We demonstrate that the time-of-arrival, which is one of the most commonly used methods to locate lightning discharges, can be seen as a special case of time reversal. The problem of a lossy ground that affects the propagation of electromagnetic transient fields generated by a lightning strike is addressed by proposing three different back-propagation models and comparing their performances in terms of location accuracy. Two sets of simulations are carried out to evaluate the accuracy of the proposed approaches. The first set of simulations is performed using numerically-generated fields and the proposed algorithm is shown to yield very good results even if the soil is not perfectly conducting. In particular, it is shown that considering a model in which losses are inverted in the back-propagation yields theoretically exact results for the source location. We also show that a lack of access to the complete recorded waveforms may lead to higher location errors, although the computed errors are found to be within the range of performance of the present LLSs. A second set of simulations is performed using the sensor data reported by the Austrian Lightning Location System (ALDIS). The locations obtained by way of the EMTR method using only the available sensor data (amplitude, arrival time and time-to-peak), are observed to be within a few kilometres of the locations supplied by the LLS. The possible sources of this discrepancy are discussed in the thesis. The second part of this document deals with the computation of the current induced in a line due to an external electromagnetic field. We derive high-frequency expressions for the current induced along a multiconductor line by an external plane wave, in which the effects of the terminals of the line are modelled by matrices of scattering and reflection coefficients. Different approaches are proposed to compute the coefficients that feed the analytical expression for the current induced along the line. Using an iterative method, mathematical expressions are derived, for the particular case of open-circuit lines. For the general case of arbitrary line terminations, an approach using auxiliary short lines, solved with a numerical solver is proposed. At low frequencies, the proposed three-term formulation can be adapted to lossy lines and analytical expressions for the coefficients, providing a new and elegant formulation for the classical transmission line theory. The proposed theory is validated through numerical simulations and experiments and is found to be much more effective than the traditional full-wave approaches in terms of memory requirements and computational times. The asymptotic theory is also applied to a lumped source excitation, according to a procedure analogous to the one for a plane wave excitation. A method for the determination of matrices of coefficients is also presented.

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