The main objective of this research is to model and analyse complex electromagnetic problems by means of a new hybridised computational technique combining the frequency domain Method of Moments (MoM), Finite-Difference Time-Domain (FDTD) method and a subgridded Finite-Difference Time-Domain (SGFDTD) method. This facilitates a significant advance in the ability to predict electromagnetic absorption in inhomogeneous, anisotropic and lossy dielectric materials irradiated by geometrically intricate sources. The Method of Moments modelling employed a two-dimensional electric surface patch integral formulation solved by independent linear basis function methods in the circumferential and axial directions of the antenna wires. A similar orthogonal basis function is used on the end surface and appropriate attachments with the wire surface are employed to satisfy the requirements of current continuity. The surface current distributions on structures which may include closely spaced parallel wires, such as dipoles, loops and helical antennas are computed. The results are found to be stable and showed good agreement with less comprehensive earlier work by others. The work also investigated the interaction between overhead high voltage transmission lines and underground utility pipelines using the FDTD technique for the whole structure, combined with a subgridding method at points of interest, particularly the pipeline. The induced fields above the pipeline are investigated and analysed. FDTD is based on the solution of Maxwell's equations in differential form. It is very useful for modelling complex, inhomogeneous structures. Problems arise when open-region geometries are modelled. However, the Perfectly Matched Layer (PML) concept has been employed to circumvent this difficulty. The establishment of edge elements has greatly improved the performance of this method and the computational burden due to huge numbers of time steps, in the order of tens of millions, has been eased to tens of thousands by employing quasi-static methods. This thesis also illustrates the principle of the equivalent surface boundary employed close to the antenna for MoM-FDTD-SGFDTD hybridisation. It depicts the advantage of using hybrid techniques due to their ability to analyse a system of multiple discrete regions by employing the principle of equivalent sources to excite the coupling surfaces. The method has been applied for modelling human body interaction with a short range RFID antenna to investigate and analyse the near field and far field radiation pattern for which the cumulative distribution function of antenna radiation efficiency is presented. The field distributions of the simulated structures show reasonable and stable results at 900 MHz. This method facilitates deeper investigation of the phenomena in the interaction between electromagnetic fields and human tissues.

There is an increasing need for accurate models describing the electrical behaviour of individual biological cells exposed to electromagnetic fields. In this area of solving linear problem, the most frequently used technique for computing the EM field is the Finite-Difference Time-Domain (FDTD) method. When modelling objects that are small compared with the wavelength, for example biological cells at radio frequencies, the standard Finite-Difference Time-Domain (FDTD) method requires extremely small time-step sizes, which may lead to excessive computation times. The problem can be overcome by implementing a quasi-static approximate version of FDTD, based on transferring the working frequency to a higher frequency and scaling back to the frequency of interest after the field has been computed. An approach to modeling and analysis of biological cells, incorporating the Hodgkin and Huxley membrane model, is presented here. Since the external medium of the biological cell is lossy material, a modified Berenger absorbing boundary condition is used to truncate the computation grid. Linear assemblages of cells are investigated and then Floquet periodic boundary conditions are imposed to imitate the effect of periodic replication of the assemblages. Thus, the analysis of a large structure of cells is made more computationally efficient than the modeling of the entire structure. The total fields of the simulated structures are shown to give reasonable and stable results at 900MHz, 1800MHz and 2450MHz. This method will facilitate deeper investigation of the phenomena in the interaction between EM fields and biological systems. Moreover, the nonlinear response of biological cell exposed to a 0.9GHz signal was discussed on observing the second harmonic at 1.8GHz. In this, an electrical circuit model has been proposed to calibrate the performance of nonlinear RF energy conversion inside a high quality factor resonant cavity with known nonlinear device. Meanwhile, the first and second harmonic responses of the cavity due to the loading of the cavity with the lossy material will also be demonstrated. The results from proposed mathematical model, give good indication of the input power required to detect the weakly effects of the second harmonic signal prior to perform the measurement. Hence, this proposed mathematical model will assist to determine how sensitivity of the second harmonic signal can be detected by placing the required specific input power.

Neste trabalho, estuda-se o problema de estabilidade de tensão do ponto de vista dinâmico, enfocando a análise nas instabilidades originadas por grandes perturbações. A modelagem utilizada nestes estudos envolve dinâmicas que possuem múltiplas escalas de tempo, mas na prática, os problemas de instabilidade de tensão estão geralmente associados às dinâmicas de longo prazo. Nesse sentido, o método de análise QSS fornece muitas vantagens do ponto de vista computacional, reduzindo significativamente o tempo de processamento, mediante a substituição das equações de dinâmica rápida por suas correspondentes equações de equilíbrio. As contribuições deste trabalho são duas: a primeira consiste no estudo da importância da região de estabilidade das dinâmicas lentas do SEP e sua relação com a região de estabilidade do sistema original. A segunda consiste em estudar a aplicação do método QSS no SEP oleando para as condições existentes na literatura e analisando principalmente as desvantagens de sua aplicação. Os sistemas de potência apresentados na literatura são utilizados como exemplos. Com base nestas simulações e na teoria existente da análise QSS, são estudadas algumas condições sob as quais o método QSS é válido. A teoria de região de estabilidade para estes sistemas é explorada para fornecer indicativos de margem de estabilidade e controle preventivo. / In this work, we study the problem of voltage stability of the dynamic point of view, focusing the analysis on the instability caused by large disturbances. The modeling used in these studies involves dynamics that have multiple time scales, but in practice, the problems of voltage instability are generally associated with long-term scale. Thus, the QSS method of analysis provides many advantages in terms of computational resource, significantly reducing the processing time, by replacing the equations by their corresponding fast dynamic equilibrium equations. The contributions of this work are two: the first is to study the importance of the stability region of the slow dynamics of the power system and its relation to the stability region of the original system. The second is to study the application of the QSS method in power systems considering the current theory in the literature and analyzing the main disadvantages of its application. The power system presented in the literature are used as examples. Based on these simulations and the existing theory of the QSS method, we study some conditions under which this method is valid. The theory of stability region for these systems is exploited to provide indicative of margin stability and preventive control.

Neste trabalho, estuda-se o problema de estabilidade de tensão do ponto de vista dinâmico, enfocando a análise nas instabilidades originadas por grandes perturbações. A modelagem utilizada nestes estudos envolve dinâmicas que possuem múltiplas escalas de tempo, mas na prática, os problemas de instabilidade de tensão estão geralmente associados às dinâmicas de longo prazo. Nesse sentido, o método de análise QSS fornece muitas vantagens do ponto de vista computacional, reduzindo significativamente o tempo de processamento, mediante a substituição das equações de dinâmica rápida por suas correspondentes equações de equilíbrio. As contribuições deste trabalho são duas: a primeira consiste no estudo da importância da região de estabilidade das dinâmicas lentas do SEP e sua relação com a região de estabilidade do sistema original. A segunda consiste em estudar a aplicação do método QSS no SEP oleando para as condições existentes na literatura e analisando principalmente as desvantagens de sua aplicação. Os sistemas de potência apresentados na literatura são utilizados como exemplos. Com base nestas simulações e na teoria existente da análise QSS, são estudadas algumas condições sob as quais o método QSS é válido. A teoria de região de estabilidade para estes sistemas é explorada para fornecer indicativos de margem de estabilidade e controle preventivo. / In this work, we study the problem of voltage stability of the dynamic point of view, focusing the analysis on the instability caused by large disturbances. The modeling used in these studies involves dynamics that have multiple time scales, but in practice, the problems of voltage instability are generally associated with long-term scale. Thus, the QSS method of analysis provides many advantages in terms of computational resource, significantly reducing the processing time, by replacing the equations by their corresponding fast dynamic equilibrium equations. The contributions of this work are two: the first is to study the importance of the stability region of the slow dynamics of the power system and its relation to the stability region of the original system. The second is to study the application of the QSS method in power systems considering the current theory in the literature and analyzing the main disadvantages of its application. The power system presented in the literature are used as examples. Based on these simulations and the existing theory of the QSS method, we study some conditions under which this method is valid. The theory of stability region for these systems is exploited to provide indicative of margin stability and preventive control.

yes / There is an increasing need for accurate models describing the electrical behaviour of individual biological cells exposed to electromagnetic fields. In this area of solving linear problem, the most frequently used technique for computing the EM field is the Finite-Difference Time-Domain (FDTD) method. When modelling objects that are small compared with the wavelength, for example biological cells at radio frequencies, the standard Finite-Difference Time-Domain (FDTD) method requires extremely small time-step sizes, which may lead to excessive computation times. The problem can be overcome by implementing a quasi-static approximate version of FDTD, based on transferring the working frequency to a higher frequency and scaling back to the frequency of interest after the field has been computed. An approach to modeling and analysis of biological cells, incorporating the Hodgkin and Huxley membrane model, is presented here. Since the external medium of the biological cell is lossy material, a modified Berenger absorbing boundary condition is used to truncate the computation grid. Linear assemblages of cells are investigated and then Floquet periodic boundary conditions are imposed to imitate the effect of periodic replication of the assemblages. Thus, the analysis of a large structure of cells is made more computationally efficient than the modeling of the entire structure. The total fields of the simulated structures are shown to give reasonable and stable results at 900MHz, 1800MHz and 2450MHz. This method will facilitate deeper investigation of the phenomena in the interaction between EM fields and biological systems. Moreover, the nonlinear response of biological cell exposed to a 0.9GHz signal was discussed on observing the second harmonic at 1.8GHz. In this, an electrical circuit model has been proposed to calibrate the performance of nonlinear RF energy conversion inside a high quality factor resonant cavity with known nonlinear device. Meanwhile, the first and second harmonic responses of the cavity due to the loading of the cavity with the lossy material will also be demonstrated. The results from proposed mathematical model, give good indication of the input power required to detect the weakly effects of the second harmonic signal prior to perform the measurement. Hence, this proposed mathematical model will assist to determine how sensitivity of the second harmonic signal can be detected by placing the required specific input power.

The main objective of this research is to model and analyse complex electromagnetic problems by means of a new hybridised computational technique combining the frequency domain Method of Moments (MoM), Finite-Difference Time-Domain (FDTD) method and a subgridded Finite-Difference Time-Domain (SGFDTD) method. This facilitates a significant advance in the ability to predict electromagnetic absorption in inhomogeneous, anisotropic and lossy dielectric materials irradiated by geometrically intricate sources. The Method of Moments modelling employed a two-dimensional electric surface patch integral formulation solved by independent linear basis function methods in the circumferential and axial directions of the antenna wires. A similar orthogonal basis function is used on the end surface and appropriate attachments with the wire surface are employed to satisfy the requirements of current continuity. The surface current distributions on structures which may include closely spaced parallel wires, such as dipoles, loops and helical antennas are computed. The results are found to be stable and showed good agreement with less comprehensive earlier work by others. The work also investigated the interaction between overhead high voltage transmission lines and underground utility pipelines using the FDTD technique for the whole structure, combined with a subgridding method at points of interest, particularly the pipeline. The induced fields above the pipeline are investigated and analysed. FDTD is based on the solution of Maxwell¿s equations in differential form. It is very useful for modelling complex, inhomogeneous structures. Problems arise when open-region geometries are modelled. However, the Perfectly Matched Layer (PML) concept has been employed to circumvent this difficulty. The establishment of edge elements has greatly improved the performance of this method and the computational burden due to huge numbers of time steps, in the order of tens of millions, has been eased to tens of thousands by employing quasi-static methods. This thesis also illustrates the principle of the equivalent surface boundary employed close to the antenna for MoM-FDTD-SGFDTD hybridisation. It depicts the advantage of using hybrid techniques due to their ability to analyse a system of multiple discrete regions by employing the principle of equivalent sources to excite the coupling surfaces. The method has been applied for modelling human body interaction with a short range RFID antenna to investigate and analyse the near field and far field radiation pattern for which the cumulative distribution function of antenna radiation efficiency is presented. The field distributions of the simulated structures show reasonable and stable results at 900 MHz. This method facilitates deeper investigation of the phenomena in the interaction between electromagnetic fields and human tissues. / Ministry of Higher Education Malaysia and Universiti Tun Hussein Onn Malaysia (UTHM)