This thesis deals with the modeling, control and analysis of doubly fed induction generators (DFIG) based wind turbines (DFIG-WT). The DFIG-WT is one of the mostly employed wind power generation systems (WPGS), due to its merits including variable speed operation for achieving the maximum power conversion, smaller capacity requirement for power electronic devices, and full controllability of active and reactive powers of the DFIG. The dynamic modeling of DFIG-WT has been carried out at first in Chapter 2, with the conventional vector control (VC) strategies for both rotor-side and grid-side converters. The vector control strategy works in a synchronous reference frame, aligned with the stator-flux vector, became very popular for control of the DFIG. Although the conventional VC strategy is simple and reliable, it is not capable of providing a satisfactory transient response for DFIG-WT under grid faults. As the VC is usually designed and optimized based on one operation point, thus the overall energy conversion efficiency cannot be maintained at the optimal point when the WPGS operation point moves away from that designed point due to the time-varying wind power inputs. Compared with VC methods which are designed based on linear model obtained from one operation point, nonlinear control methods can provide consistent optimal performance across the operation envelope rather than at one operation point. To improve the asymptotical regulation provided by the VC, which can't provide satisfactory performance under voltage sags caused by grid faults or load disturbance of the grid, input-output feedback linearization control (IOFLC) has been applied to develop a fully decoupled controller of the active $\&$ reactive powers of the DFIG in Chapter 3. Furthermore, a cascade control strategy is proposed for power regulation of DFIG-WT, which can provide better performance against the varying operation points and grid disturbance. Moreover, to improve the overall energy conversion efficiency of the DFIG-WT, FLC-based maximum power point tracking (MPPT) has been investigated. The main objective of the FLC-based MPPT in Chapter 4 is to design a global optimal controller to deal with the time-varying operation points and nonlinear characteristic of the DFIG-WT. Modal analysis and simulation studies have been used to verify the effectiveness of the FLC-based MPPT, compared with the VC. The system mode trajectory, including the internal zero-dynamic of the FLC-MPPT are carefully examined in the face of varied operation ranges and parameter uncertainties. In a realistic DFIG-WT, the parameter variability, the uncertain and time-varying wind power inputs are existed. To enhance the robustness of the controller, a nonlinear adaptive controller (NAC) via state and perturbation observer for feedback linearizable nonlinear systems is applied for MPPT control of DFIG-WT in Chapter 5. In the design of the controller, a perturbation term is defined to describe the combined effect of the system nonlinearities and uncertainties, and represented by introducing a fictitious state in the state equations. As follows, a state and perturbation observer is designed to estimate the system states and perturbation, leading to an adaptive output-feedback linearizing controller which uses the estimated perturbation to cancel system perturbations and the estimated states to implement a linear output feedback control law for the equivalent linear system. Case studies including with and without wind speed measurement are carried out and proved that the proposed NAC for MPPT of DFIG-WT can provide better robustness performance against the parameter uncertainties. Simulation studies for demonstrating the performance of the proposed control methods in each chapter, are carried out based on MATLAB/SIMULINK.

Over the past ten years, Reverberation Chambers (RC) have emerged as a promising facility for the measurement of antenna characteristics for the wireless communications industry. The RC has begun to diverge from its initial purpose of performing Electromagnetic Compatibility (EMC) measurements, as conceived by H. A. Mendes back in 1968. Unlike the Anechoic Chamber (AC) however, the RC facility and measurement protocols are relatively in their infancy whose standardisation is yet to be finalised. The purpose of this thesis is to present a further study into reverberation chambers and their associated measurement procedures, aimed at smoothing the transition into a fully standardised and accepted facility within the measurement industry. This thesis is comprised of three main investigation areas. The first area under investigation concerns single port textile antennas designed for use in the on-body communications role. The purpose of this investigation is twofold: (1) to chart the efficiency and impedance matching performance of the antennas in both free space and on-body roles to completely characterise their performance, and (2) to devise and configure operational procedures for the measurement of antenna characteristics on human subjects using the RC. Two separate varieties of textile antenna are subject to investigation; the first consists of two antennas with an equally small ground plane designed for use in the Industrial Scientific and Medical (ISM) 2.45 GHz band. The second variety also consists of two separate antennas that have a larger ground plane size and are designed to offer a dual band characteristic; to operate at 2.45 GHz and 5.2 GHz respectively. The results for the smaller ground plane sized antennas show that in free space conditions, the textile antenna constructed from the higher conductivity textile material exhibits a greater level of efficiency which is expected. However, when placed on-body, the antenna with the lower conductivity textile material remarkably outperforms the antenna with the higher conductivity material which is contrary to expectations; this represents new and important knowledge. The results for the larger ground plane sized textile antennas conform completely to expectations. That is, the higher conductivity material outperformed the lower conductivity material in both free space and on-body roles. Comparing both cases, further new knowledge can be concluded in the fact that in addition to the conductivity of the textile material, the ground plane dimensions is also of crucial importance. The second area under investigation concerns multiport (array) antennas. This area is sub-divided into two sections to chart a distinction between multiport antennas designed for Multiple Input Multiple Output (MIMO) applications and more conventional array antennas that are not. The first section concerns the complete practical verification of two new dual feed Planar Inverted F Antennas (PIFAs). Results show that both antennas yield a high level of diversity gain and channel capacity (close to the theoretical maximums) and very low correlation between the two feeds despite the antennas small size. Furthermore, the antennas are also proved to be highly efficient at the desired frequency of operation. Comparing all performance results, it is possible to conclude that due to the small size and excellent performance of the new designs, they could be useful in more practical and commercial applications than larger sized elements that currently exist. The second section focuses upon more conventional larger sized array antennas used for radio astronomy applications. In this thesis, a series of power dividers is used to emulate a realistic ‘all - excited’ scenario, but the power divider approach has a consequence in that it will give rise to an external power loss that is not attributed to the antenna array. A new equation is developed in this work that allows for the accurate efficiency determination of the array and the de-embedding of the power divider in one. It is shown that the new equation can make this whole process simpler and straightforward to accomplish whilst maintaining accuracy. The final area under investigation concerns the design of reverberation chambers. The most common of the mode stirring techniques used in reverberation chambers is via the rotation or movement of electrically large metallic paddles inside the chamber known as ‘Mechanical Stirring’. In this thesis, a technique based upon a meanderline principle is used to cut slots into the mechanical stirring paddles to increase the current path length (induced when a wave hits the metallic surface) and thereby increase the electrical size of the paddle. New paddle designs for reverberation chambers are designed and verified. It is shown that the overall paddle dimensions do not need to be increased in size, meaning that the working volume of the chamber can remain as large as possible. The results show that the new designs exhibit enhanced performance over and above conventional paddle designs at lower modal numbers, meaning that any chamber will be able to better perform at frequencies where fewer modes exist. Results also show that at higher frequencies, the slot cuts do not adversely affect the chambers higher frequency performance. This work therefore has the potential to forge a new way of thinking when it comes to the design of mechanical stirrers in RC’s.

Moving object detection is an essential process before tracking and event recognition in video surveillance can take place. To monitor a wider field of view and avoid occlusions in pedestrian tracking, multiple cameras are usually used and homography can be employed to associate multiple camera views. Foreground regions detected from each of the multiple camera views are projected into a virtual top view according to the homography for a plane. The intersection regions of the foreground projections indicate the locations of moving objects on that plane. The homography mapping for a set of parallel planes at different heights can increase the robustness of the detection. However, homography mapping is very time consuming and the intersections of non-corresponding foreground regions can cause false-positive detections. In this thesis, a real-time moving object detection algorithm using multiple cameras is proposed. Unlike the pixelwise homography mapping which projects binary foreground images, the approach used in the research described in this thesis was to approximate the contour of each foreground region with a polygon and only transmit and project the polygon vertices. The foreground projections are rebuilt from the projected polygons in the reference view. The experimental results have shown that this method can be run in real time and generate results similar to those using foreground images. To identify the false-positive detections, both geometrical information and colour cues are utilized. The former is a height matching algorithm based on the geometry between the camera views. The latter is a colour matching algorithm based on the Mahalanobis distance of the colour distributions of two foreground regions. Since the height matching is uncertain in the scenarios with the adjacent pedestrian and colour matching cannot handle occluded pedestrians, the two algorithms are combined to improve the robustness of the foreground intersection classification. The robustness of the proposed algorithm is demonstrated in real-world image sequences.

Rapidly increasing energy demand from the industrial and commercial sector, especially in the current climate of high oil prices, steadily reducing energy sources and at the same time increased concerns about environmental changes, have caused fast development of Distributed Power Generation Systems (DPGS) based on renewable energy. A recent concept is to group DPGS and the associated loads to a common local area forming a small power system called a microgrid. This small autonomous system formed by DPGS can offer increased reliability and effciency of future power system networks. Furthermore, the improvement of the control capabilities and operational features of microgrids brings environmental and economic benefits. The introduction of microgrids improves power quality, reduces transmission line congestion, decreases emission and energy losses, and effectively facilitates the utilisation of renewable energy resources. As a consequence of the fast expanding DPGS based on renewable energy sources, Transmission System Operators (TSO) have issued strict interconnection requirements (grid code compliance), e.g., on power quality control, reactive power control, fault ride-through etc. Among these different requirements issued by the grid operators, power quality have recently gained a lot of attention due to excessive non-linear and unbalanced loads over-stressing the power systems and causing system failure. As nonlinear and/or unbalanced loads can represent a high proportion of the total load in small-scale systems, the problem with power quality is a particular concern in microgrids. In this work, different control strategies are proposed and implemented for the grid and microgrid connected voltage-source inverters (VSI), based on H^inf and repetitive control techniques. The repetitive control, which is regarded as a simple learning control method, offers very good performance for voltage and current tracking as it can deal with a very large number of harmonics simultaneously. This leads to a very low Total Harmonic Distortion (THD) of the output voltage and/or the current even in the presence of nonlinear loads and/or grid distortions. Initially, a voltage controller proposed in the literature for microgrid applications is further developed and experimentally tested. The aim is to improve power quality and tracking performance, while considerably reducing the complexity of the controller design. The model of the plant is reduced for single-input-single-output (SISO) repetitive control design. As a consequence, the design becomes much simpler and the stability evaluation easier. Moreover, a frequency adaptive mechanism is proposed so that the controller can cope with grid frequency variations in the grid-connected mode. This mechanism allows the controller to maintain very good tracking performance over a wide range of grid frequencies. Then, a H^inf repetitive control strategy for the inverter current is proposed and validated with experiments. As a result, the power quality and tracking performance are considerably improved. In order to demonstrate the improvements, the proposed controller is compared with the traditional proportional-resonant (PR), proportional- integral (PI) and predictive deadbeat (DB) controllers. Finally, the advantages of the proposed voltage and current controllers based on H^inf and repetitive control techniques are put together for consideration in microgrid applications and experimentally tested. The proposed cascaded current-voltage control strategy is not a simple combination of the two control strategies, but a complete re-design after realising that the inverter LCL filter can be split into two separate partsfor the design of the controllers. As a consequence, the cascaded controller is able to maintain low THD in both the microgrid voltage and the current following into/from the grid at the same time. It also enables seamless transfer of the operation mode from standalone to grid-connected or vice versa. It turns out that the voltage controller can be reduced to a proportional gain cascaded with the internal model (in a re-arranged form), which can be easily implemented in real applications. Experiments under different scenarios (e.g. in the standalone mode or in the grid-connected mode, with linear, nonlinear or unbalanced loads etc.) are presented to demonstrate the excellent performance of the controllers.

Transformer reliability and stability are the key concerns. In order to increase their efficiency, an automatic monitoring and fault diagnosing of the power transformers are required. Dissolved Gas Analysis (DGA) is one of the most important tools to diagnose the condition of oil-immersed transformer. Agents technology as a new, robust and helpful technique, successfully applied for various applications. Integration of the Multi-Agent System (MAS) with knowledge base provides a robust system for various applications, such as fault diagnosis and automated actions performing, etc. For this purpose, the present study was conducted in the field of MAS based on Gaia methodology and knowledge base. The developed MAS followed by Gaia methodology represents a generic framework that is capable to manage agents executions and message delivery. Real-time data is sampled from a power transformer and saved into a database, and it is also available to the user on request. Three types of knowledge-based systems, namely the rule-based reasoning, ontology and fuzzy ontology, were applied for the MAS. Therefore, the developed MAS is shown to be successfully applied for condition monitoring of power transformer using the real-time data. The Roger’s method was used with all of the knowledge-based systems named above, and the accuracy of the results was compared and discussed. Of the knowledge-based systems studied, fuzzy ontology is found to be the best performing one in terms of results accuracy, compared to the rule-based reasoning and ontology. The application of the developed fuzzy ontology allowed to improve the accuracy by over 22%. Unlike the previous works in this field, that were not capable of dealing with the uncertainty situations, the present work based on fuzzy ontology has a clear advantage of successfully solving the problem with some degree of uncertainty. This is especially important, as the most of the real-world situations involve some uncertainty. Overall, the work contributes the use of the knowledge base and the multi-agent system for the fault diagnosis of the power transformer, including the novel application of fuzzy ontology for dealing with the uncertain situations. The advantages of the proposed method are the ease of the upgrade, flexibility, efficient fault diagnosis and reliability. The application of the proposed technique would benefit the power system reliability, as it would result in reduction of the number of engineering experts required, lower maintenance expenses and extended lifetime of power transformer.

The current industrial standard for the control of the induction motor is the so called vector control (VC) or field-orientated control (FOC) which transforms and controls the induction motor as a direct current (DC) motor. Besides its many advantages, such as fast and decoupled dynamics of speed and flux, it is well known that VC depends on the detailed system model and is very sensitive to parameter uncertainties and external disturbance (load torque). To clarify further the VC is a only a partial feedback linearising control which can achieve the decoupling of speed and flux asymptotically. The coupling still exists when flux is not kept in constant, i.e. when flux is weakened in order to operate the motor at a higher speed and keep the input voltage within saturation limits, or when flux is adjusted to maximize power efficiency of the motor with light load. The thesis will summarise research of advanced control approaches of induction motors in Chapter One. The Chapter Two starts on building a fifth-order nonlinear dynamic model of an induction motor and then recalls the principal of traditional VC of induction motors. The differential-geometric technique based nonlinear control has developed for induction motors, which can convert some intractable nonlinear problems into simpler problems by familiar linear system methods. The partial decoupled dynamic of the conventional VC has been investigated via feedback linearisation control (FLC) at first. Then input-output linearisation control is applied to design a fully decoupled control of the dynamics of speed and flux. To remove the weak robustness and the requirement of an accurate model of the VC and FLC, a novel nonlinear adaptive control of induction motor is designed based on feedback linearisation control and perturbation estimation. The induction motor will be represented as a two coupled interconnected subsystems: rotor speed subsystem and rotor flux subsystem, respectively. System perturbation terms are defined to include the lumped term of system nonlinearities, uncertainties, and interactions between subsystems and are represented as a fictitious state in the state equations. Then perturbations are estimated by designing perturbation observers and the estimated perturbations are employed to cancel the real system perturbations, assumed all internal states are measured. The designed nonlinear adaptive control doesn’t require the accurate model of the induction motor and has a simpler algorithm. It can fully decouple the regulation of rotor speed and rotor flux and handle time-varying uncertainties. The parameter estimations based on nonlinear adaptive controls can only deal with unknown constant parameters and are not suitable for handling fast time-varying and functional uncertainties. Nonlinear adaptive control based on output measurements is addressed in Chapter Five, assuming that the rotor speed and the stator volatge/currents are measurable. A sliding mode rotor flux observer has been designed based on the stator voltage and current. Moreover, two third-order state and perturbation observers are designed to estimate the unmeasured states and perturbation, based on the rotor speed and the estimated rotor flux. Simulation studies have been carried out for verifying the effectiveness of the proposed advanced controllers and compared with the conventional VC and model based FLC.

The relay based cooperative network is one of the promising techniques for next generation wireless communications, which can help extend the cell coverage and enhance the diversity. To deploy relays efficiently with limited power and bandwidth under certain performance requirements, resource allocation (RA) plays an increasingly important role in the system design. In recent years, with the fast growth of the number of mobile phone users, great portion of CO2 emission is contributed by wireless communication systems. The combination of relay techniques and RA schemes reveals the solution to green communications, which aims to provide high data rate with low power consumption. In this thesis, RA is investigated for next generation relay based green wireless systems, including the long-range cellular systems, and the short-range point-to-point (P2P) systems. In the first contribution, an optimal asymmetric resource allocation (ARA) scheme is proposed for the decode-and-forward (DF) dual-hop multi-relay OFDMA cellular systems in the downlink. With this scheme, the time slots for the two hops via each of the relays are designed to be asymmetric, i.e., with K relays in a cell, a total of 2K time slots may be of different durations, which enhances the degree of freedom over the previous work. Also, a destination may be served by multiple relays at the same time to enhance the transmission diversity. Moreover, closed-form results for optimal resource allocation are derived, which require only limited amount of feedback information. Numerical results show that, due to the multi-time and multi-relay diversities, the proposed ARA scheme can provide a much better performance than the scheme with symmetric time allocation, as well as the scheme with asymmetric time allocation for a cell composed of independent single-relay sub-systems, especially when the relays are relatively close to the source. As a result, with the optimal relay location, the system can achieve high throughput in downlink with limited transmit power. In the second contribution, the power consumption in relay based 60 GHz cooperative networks is studied, which is based on three-terminal diversity amplify-and-forward (DAF) and diversity DF (DDF) relaying strategies. A total power consumption model including drive power, decoding power, and power consumption of power amplifier (PA) is proposed, excluding the transmit power, as it is relatively small compared to decoding power and PA power in the indoor environment. This model is formulated as a function of drive power, which gives an easy access to the system level power allocation. To minimise the system total power consumption, the optimal drive power can be allocated to the source node by numerical searching method while satisfying the data rate requirement. The impact of relay locations on the total power consumption is also investigated. It is shown that, with the same data rate requirement, in the small source-relay separation case, DAF consumes slightly less power than DDF; while with larger source-relay separation, DAF consumes much more power than DDF. In the future work, multiuser relay-based short-range communication systems will be considered for the 60 GHz communication in the fading channel scenario, which extends the proposed power consumption model in a more practical way. The power consumption model of other components, such as analog-to-digital converter, data buffer, modulation/demodulation could also be considered to provide more details about green P2P communications.

Water-based liquid antennas are a new type of antennas, which have attracted increasing attention in recent years. They have emerged as promising alternatives to traditional antennas for many applications. The purpose of this thesis is to present a comprehensive study into water-based liquid antennas, aiming at gaining a better understanding of water-based liquid antennas from the liquids used to the antenna designs. This thesis is comprised of two main research areas. The first area under investigation focuses on water-based liquid property characterisation. In water-based liquid antenna designs, a precise knowledge of the complex permittivity of the liquid is essential. Three water-based liquids, namely pure water, water with propylene glycol (PG) and salty water, are carefully studied from an antenna design point of view. A liquid measurement software package is developed to automatically record the liquid complex permittivity data under different temperatures, and measurements are conducted. The experimental data are processed to obtain accurate mathematical expressions for the complex permittivity of these liquids over a temperature range 0 ~ 70oC (for pure water and salty water) and -10oC ~ 70oC (for water with PG), frequency range 0 ~ 18 GHz, PG concentration 0 ~ 70% and salinity 0.1 ~ 50 ppt. Water with PG is proposed as an alternative candidate for pure water in cold climates. It is demonstrated that the performance of the antenna will not be changed significantly by using water with PG. The second area concerns water-based liquid antenna designs and is divided into three sections: The first section deals with the water antenna working as a conducting antenna. A water monopole antenna with a dielectric layer is designed. Salty water is used to replace the conducting material (usually copper) in traditional designs. A comprehensive parametric study is performed and the physical insights behind the design are studied. A close relationship between the salty water conductivity and antenna radiation efficiency is explored. The second section investigates the hybrid water antenna for hand-portable applications. By combining the resonance from the water dielectric resonator antenna (DRA) and that from the feeding structure, a wideband response can be achieved. Three hybrid water antennas are developed with low profiles and high efficiency. The unique features of water, namely liquidity and transparency are effectively utilised. A complex feeding structure is placed inside the water dielectric resonator (DR) to feed the water DR and also work as a radiating element. The third section relates to the water loaded reconfigurable antennas. Two water loaded reconfigurable antennas with special 3D folded structures are designed. Different technologies are applied in the reconfigurable designs such as the special folded 3D monopole structure, the use of water and its holder as a transparent dielectric loading, and the integration of an active component. The results show that the designs have compact sizes, reasonable efficiency and bandwidths. This thesis has successfully demonstrated the attractive features and great potential of water-based liquid antennas. The knowledge gained in this work is very valuable for future water-based liquid antenna development.

This thesis proposes novel blind and semi-blind receivers for Multiple-Input Multiple-Output (MIMO) wireless communication systems employing Orthogonal Frequency Division Multiplexing (OFDM) or Single-Carrier Frequency Domain Equalisation (SCFDE) modulation. MIMO systems can achieve higher data rates than Single-Input Single-Output (SISO) systems while the use of OFDM and SC-FDE is motivated by their simple equalisation of frequency selective channels. Furthermore, the blind or semi-blind operation increases the bandwidth efciency compared to traditional training based receivers, which is particularly interesting in MIMO systems since they require more training data than SISO systems. Blind and semi-blind equalisation in this work is performed using the statistical tool of Independent Component Analysis (ICA), which relies on the independence of the source streams. First, novel blind receivers for MIMO OFDM systems are proposed. Two linear receivers, which have different complexity / performance tradeoffs are presented. The indeterminacies of ICA estimates are handled by post-processing of the estimates, which relies on precoding of the source streams. Subsequently, these receivers are combined with Layered Space-Frequency Equalisation (LSFE), which results in performance close to the case with perfect Channel State Information (CSI) at the receiver. Reduced-complexity versions are also considered, which are essential for OFDM systems employing a large number of subcarriers. Furthermore, performance and computational complexity comparisons with other published blind receivers show the superiority of the proposed methods. Next, semi-blind receivers for MIMO OFDM systems are proposed, which are derived from the blind methods above. The advantage of operating semi-blindly is that the indeterminacies of the ICA estimates are avoided. Furthermore, better performance than in the blind case is obtained at low to moderate Signal to Noise Ratios (SNRs) and Doppler frequency shifts, using a training overhead of 2%. However, for high SNRs or Doppler frequency shifts as well as with reduced-complexity versions convergence to a solution extracting the sources in the wrong order can occur, which results in an error floor, since no post-processing to handle these ICA indeterminacies is used. Finally, semi-blind receivers for MIMO SC-FDE systems using linear equalisation and LSFE are investigated. To the author's knowledge this is the first published application of ICA to MIMO SC-FDE systems. Since in SC-FDE systems the frequency domain signals are Gaussian, ICA is applied in time domain while a computationally efficent frequency domain equalisation is employed. The resulting receiver is shown to achieve performance close to the perfect CSI case for moderate to high SNRs, using a training overhead of only 0:05%. Furthermore, convergence to a wrong solution as in the semi-blind OFDM case has not been observed, which can be explained by the larger solution space in SC-FDE systems than in OFDM systems.

A method for automatic detection of mammographic masses is presented. As part of this method, an enhancement algorithm is proposed that improves image contrast based on local statistical measures of the mammograms. After enhancement, regions are segmented via thresholding at multiple levels, and a set of features is computed from each of the segmented regions. A region-ranking system is also presented that identifies the regions most likely to represent abnormalities based on the features computed. The method was tested on 57 mammographic images of masses and achieved a sensitivity of 80% at 2.3 false-positives per image (average of 0.32 false-positives). Two new algorithms for segmentation of masses are presented. These are based on the Dynamic Programming-based Boundary Tracing (DPBT) algorithm proposed in: Timp and Karssemeijer, Med. Phys. 31 (5), pp. 958-971, (2004). The DPBT algorithm contains two main steps: 1) construction of a local cost function, and 2) application of dynamic programming to the selection of the optimal boundary. Modifications to the computation of the local cost function are proposed and produce the Improved-DPBT (IDPBT) algorithm. A procedure for the dynamic selection of the strength of the components of the local cost function is also presented that makes these parameters independent of the image dataset, and produces another new algorithm, ID2PBT. Both of the new algorithms outperform the original DPBT. Four new features for the analysis of breast masses are presented. These features are designed to be insensitive to the exact shape of the contour of the masses, so that an approximate contour, such as one extracted via an automated segmentation algorithm, can be employed in their computation. Two of the features, SpSI and SpGO, measure the degree of spiculation of a mass and its likelihood of being spiculated. The last two features, Fz1 and Fz2, measure the local fuzziness of the mass margins based on points defined automatically. The features were tested for characterisation and diagnosis of breast masses using a set of 319 masses and three different classifiers, and obtained approximately 90% and 76% correct classification, respectively.