Power quality problems have been identified with wind generation sites and their connection with the distribution network. The main aim of this research is to put forward and develop models for the conventional components in a power system, but with provision for the representation of wind farms. To develop the necessary tools and computational methods that can be embedded in programmes in such a way that economic and security assessments can be carried out on present and future wind-based networks that are likely to be highly decentralised in future. The goal has been accomplished using MATLAB programming and the 'power library' tools.

Tidal energy conversion devices (TECDs) are in development throughout the world to help reduce the need for fossil fuels. These devices will generally be mounted on the seabed and remain there over a period of years. Most of the previous research on TECDs has focused on their power extraction capability and efficient design. The handful of studies which have focused on the effects of the devices on the marine environment have not considered small-scale three-dimensional phenomena occurring in the flow near the rotor. These phenomena are likely to disturb the marine environment by altering the dynamics of sediment. The accurate prediction of the rapidly changing flow down-stream of a TECD and its influence on the seabed poses a challenge. The nature of the interactions between such a flow and sediment has not been experimentally established. Predictions of these interactions, as is necessary for an assessment of the effects of the devices on the seabed, need to account for the depth-dependence of the flow velocity and its changes during the tidal cycle. The difference between the typical time-scales of the development of the rotor wake and the tidal cycle represents a difficulty for the computational modelling of the interactions between the device and the tidal flow. This dissertation presents an inviscid analysis of the flow down-stream of horizontal- axis, vertical-axis and cross-flow TECDs by means of computer modelling. The Vortic- ity Transport Model, modified to simulate the flow down-stream of a TECD mounted onto the seabed, predicts the shear stress inflicted by the flow on the seabed. The shear stresses on the seabed, generated by small-scale vortical structures in the wake down-stream of the devices, cause sediment to uplift. This process along with the sub- sequent motion of the sediment is simulated by a sediment model implemented into the Vorticity Transport Model. The critical bed shear stress is known as a threshold for initiation of sediment motion, therefore the relative difference between the stress on the seabed and the critical bed shear stress, called the excess bed shear stress, is chosen here as an indicator of the impact of the TECDs on the seabed. The evolution of the instantaneous stresses on the seabed is predicted to vary with the configuration of TECD. The results suggest that the average excess bed shear stress inflicted on the seabed by the horizontal-axis device increases with the inflow velocity during the flood part of the representative tidal cycle and that the increase can be expressed by a simple algebraic expression. It is also predicted that the impact of this device on the seabed does not monotonically decrease with increasing separation between the rotor and the seabed. In addition, the relationship between the excess bed shear stress and the position of the rotor is established. Furthermore, the simulations indicate that the wake down-stream of the horizontal-axis device is lifted by the flow away from the seabed, which result in a confinement of its impact to the vicinity of the rotor. In contrast with the horizontal-axis configuration, it is concluded that the vertical-axis and cross-flow configurations of the rotor would promote the erosion of the seabed further away from the device, at a location where the wake approaches the seabed again and that this location depends on the inflow velocity. The predicted effects of these devices on the marine environment need to be con- sidered in advance of their installation on the seabed.

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Nowadays, numerous power electronics application requires operation at high temperatures. In order to address increasing change of reliability problems in power die attachments for high temperature and high reliability applications, sintering Ag nanoparticles has been used as bonding material for this work. Firstly, quantitative microstructure characterization of as-sintered Ag joints has been carried out. The resulting normalized thickness, pore size and porosity decreased, and grain size increased with increasing the sintering time. A time dependence of the form t1/n with n close to 2 or 3 can be further derived for the kinetics of the thinning, densification and grain growth within the sintered Ag joints. From the results can be seen, sintering kinetics is still in the intermediate stage, the densification had not been completed, and Ag grain would continue growing afterwards, which could further explain degradation behaviours of sintered joints during isothermal ageing tests and thermal cycling tests. Secondly, sintered Ag joints with four kinds of substrate metallization have been subjected to isothermal ageing tests at temperatures of 150°C, 200°C and 250°C for up to 32 days. The different microstructure patterns of sintered joints with four substrate finishes during isothermal ageing tests have been presented and compared, which could use the results to explain part of the degradation behaviours of the sintered Ag joints during thermal cycling tests and guide selection of suitable substrate finish for the die attachments in high temperature power electronic system. Furthermore, thermal cycling tests have been carried out to investigate the reliability of two sizes of sintered Ag joints and solder joints during temperature cycling between -55°C to 125°C and -55°C to 150°C. Microstructure evolution of sintered Ag joints was investigated by non-destructive and destructive characterization methods, which revealed the factors which could effect on the degradation during thermal cycling tests. With microstructure features of sintered joints observed from X-ray tomography and SAM, because a specific specimen can be evaluated over its lifetime, a true image of microstructure evolution of damage during operation can be obtained, and crack and degradation can be observed three-dimensionally.

The reliable performance of power electronic modules has been a concern for many years due to their increased use in applications which demand high availability and longer lifetimes. Thick Al wire bonding is a key technique for providing interconnections in power electronic modules. Today, wire bond lift-off and heel cracking are often considered the most lifetime limiting factors of power electronic modules as a result of cyclic thermomechanical stresses. Therefore, it is important for power electronic packaging manufacturers to address this issue at the design stage and on the manufacturing line. Techniques for the non-destructive, real-time evaluation and control of wire bond quality have been proposed to detect defects in manufacture and predict reliability prior to in-service exposure. This approach has the potential to improve the accuracy of lifetime prediction for the manufactured product. In this thesis, a non-destructive technique for detecting bond quality by the application of a semi-supervised classification algorithm to process signals obtained from an ultrasonic generator is presented. Experimental tests verified that the classification method is capable of accurately predicting bond quality, indicated by bonded area as measured by X-ray tomography. Samples classified during bonding were subjected to both passive and active cycling and the distribution of bond life amongst the different classes analysed. It is demonstrated that the as-bonded quality classification is closely correlated with cycling life and can therefore be used as a non-destructive tool for monitoring bond quality and predicting useful service life.

This work was motivated by the extensive research on Li-N-(H)-based materials, which have attracted increasing interest for potential applications in hydrogen storage and lithium-ion batteries due to their extraordinary properties. In this thesis, Li3N-derived materials with potential use as energy storage and conversion materials were successfully synthesised and characterised. Novel energy-efficient synthetic routes such as microwave energy and ball milling as well as conventional heating methods were employed.

This thesis focuses on the rational design of fast and low temperature synthetic routes for the preparation of energy storage nanostructures with potential applications as electrode materials for Li-ion batteries. The materials synthesised in this work have been fully investigated by powder X-ray diffraction, electron microscopy and potentiodynamic measurements. Where possible, high resolution powder X-ray and neutron diffraction, X-ray and neutron pair distribution function (PDF) analysis and muon spin relaxation (µ+SR) studies have been conducted in order to have a better understanding of the structure-property relationship and have a complete and detailed characterisation of these battery materials. Chapter 1 includes a general introduction about Li-ion batteries and a brief analysis of the most promising electrode materials used in Li-ion batteries. Furthermore, a short description about different synthetic methodologies such as solid state, microwave-assisted and solvothermal syntheses is included. In particular, the benefits of single source precursor processes are highlighted. Finally, the main aims of this thesis are also discussed. The objective of Chapter 2 is to provide detailed experimental procedures of all materials synthesis and also to briefly describe the main characterisation techniques employed during this research, exploring in more detail those not commonly used, such as pair distribution function analysis and muon spin relaxation. In Chapter 3, a microwave-assisted solvothermal approach for the preparation of a family of LiFe1-xMnxPO4 (x=0, 0.25, 0.5, 0.75 and 1) olivines using commercial starting materials is presented. To fully characterise and have a deeper insight of the structure-property relationship of these nanocrystalline phases, high resolution powder neutron diffraction and neutron PDF analyses of these phases are conducted, allowing the examination of the local structure, cation distribution, presence of defects and Li content. Moreover, muon spin relaxation is used for the first time to investigate the lithium diffusion in this series of olivine mixed metal phosphate phases. By understanding how this double transition metal system operates, it may be possible to synthesise high performing electrode nanomaterials with higher energy density than LiFePO4 with no significant increase in cost and exhibiting charge/discharge rates acceptable for commercial applications. Chapter 4 covers a fast and energy-efficient synthetic route to olivine nanostructured LiFe1-xMnxPO4 cathodes and Mn3O4 hausmannite conversion anodes for Li-ion batteries using a new class of metal alkoxides containing one or two transition metals. The main advantage of metal alkoxides over commercially available inorganic salt mixtures is that the different metals of the final product are already present in a single precursor, which significantly reduces the energy required for reaction of a multicomponent precursor mixture employed in conventional synthesis. Furthermore, thermal decomposition of these metal alkoxide compounds can be performed at relatively low temperatures, allowing decreased temperatures during synthesis and making the process more energy efficient. This work intends to emphasise the versatility of metal alkoxide precursors in the preparation of nanostructured Li-ion battery materials for both positive and negative electrodes through relatively fast and low temperature microwave and ultrasound-assisted methods. In Chapter 5, having confirmed the suitability of employing transition metal alkoxide precursors for the preparation of nanostructured electrodes via microwave or ultrasound assisted methods, efforts have been directed to develop the synthesis of a series of heterometallic alkoxide complexes containing both Li and a transition metal (Fe, Mn). These heterometallic alkoxide precursors are then used for the generation of highly crystalline LiFe1-xMnxPO4 olivine nanostructures exhibiting an outstanding electrochemical performance. Co-location of all the required metals in these metallorganic precursors could bypass the need of diffusional mixing and allow the reactions to proceed faster and at lower temperatures generating better crystallised materials. X-ray PDF analyses of these LiFe1-xMnxPO4 olivine nanophases are conducted in an effort to examine the local structure, defect chemistry and show that microwave processes produce highly crystalline materials even after short reaction times. Finally, a ionothermal microwave-assisted synthesis of LiFePO4 nanoparticles using heterometallic alkoxide precursors has been examined in order to study the influence of the solvent in the resulting electrochemical performance. Chapter 6 explores the preparation of olivine LiFe1-xMnxPO4 nanostructures through conventional solvothermal processes using the same single source heterometallic alkoxide precursors. A reduction in particle size and an enhancement in the electrochemical behaviour are achieved when using single source precursor metallorganic compexes compared to commonly used commercial starting materials. Moreover, the fabrication of Fe3O4 magnetite nanoparticles by the room temperature hydrolysis of the [FeLi2Br(OtBu)4(THF)2]n heterometallic alkoxide precursor and its application as anode material for Li-ion batteries is presented. Chapter 7 further develops this family of heterometallic precursors by examining the preparation of olivine nanostructured Ni-doped LiFePO4 cathodes via microwave processes. The effect of the addition of polyvinylpyrrolidone (PVP) in the reaction mixture, which could act as a capping and dispersing agent to prevent particle growth and agglomeration as well as a possible carbon source for all-in-one carbon coating procedures, is investigated. The preparation of a Li and Ni containing metal alkoxide and its utilisation as a Ni precursor for the preparation of nanostructured LiFe1-xNixPO4 olivine cathodes and NiO conversion anodes is presented, demonstrating again the versatility of single source precursor synthesis using heterometallic alkoxides in the preparation of both Li-ion battery cathode and anode materials. Finally, Chapter 8 includes some general conclusions and an outlook for future work including some preliminary investigations on microwave syntheses of non-olivine β-LiFe1-xMxPO4 (M=Fe, Co, Ni) and maricite NaFe1-xMnxPO4 nanostructures for Li and Na-ion battery applications.

This thesis is concerned with the surface discharge behaviour at the oil-pressboard interface. For large transformers this is classified as a serious failure mode because it can lead to catastrophic failure under normal AC voltage operating conditions. To increase understanding on this failure mode, a surface discharge experiment at the oil-pressboard interface has been conducted on different moisture levels in pressboard by applying a long period of AC voltage stress. The processes in the surface discharge at the oil-pressboard interface until the appearance of a first full discharge have been recognised and correlated with the measured data. The results show that the different moisture levels within the pressboard play an important role on the partial discharge (PD) activity of certain processes. The decreasing trend in the PD data during the surface discharges cannot be treated as a reliable condition monitoring measure of health because it is the key indicator of white marks propagation toward the earth point. The characteristics of full discharge events have been analysed to develop knowledge for condition monitoring of surface discharge at the oil-pressboard interface. Full discharges are corona-like events in which their random occurrences are dominated by accumulated charges on the pressboard surface along the white marks rather than the polarity of applied AC voltage. A 2-D axial symmetry surface discharge model has also been developed using COMSOL Multiphysics, a finite element analysis (FEA) software package. The model considers the pressboard region near the interface (a transition region) as porous, whilst in the bulk region of pressboard as a perfect insulator. The model is developed using continuity equations and coupled with the Poisson’s equation to study the problem in terms of charge transport mechanisms and electric field distributions. The thermal conduction equation is included to study the thermal effects of surface discharge activity at the oil-pressboard interface. The behaviour of surface discharge is studied by validating the simulated surface discharge current pulse with the measured current. The simulation results show that a field dependent molecular ionisation mechanism plays an important role in the streamer propagation during the period of the rising front of the current pulse, whilst during the period of decaying tail of the current pulse, the contribution of an electron attachment process is dominant. The modelling results suggest that degradation marks (white and black marks) are due to high energy over long periods of partial discharge events that lead to thermal degradation at the oil-pressboard interface.

Over two decades of research has led to numerous low-power design techniques being reported. Two popular techniques are supply voltage scaling and power gating. This thesis studies the impact of these two design techniques on the reliability of embedded processor registers and memory systems in the presence of transient faults; and with the aim to develop and validate efficient mitigation techniques to improve reliability with small cost of energy consumption, performance and area overhead. This thesis presents three original contributions. The first contribution presents a technique for improving the reliability of embedded processors. A key feature of the technique is low cost, which is achieved through reuse of the scan chain for state monitoring, and it is effective because it can correct single and multiple bit errors through hardware and software respectively. To validate the technique, ARMR Cortex TM -M0 embedded microprocessor is implemented in FPGA and further synthesised using 65-nm technology to quantify the cost in terms of area, latency and energy. It is shown that the presented technique has a small area overhead (8.6%) with less than 4% worst-case increase in critical path. The second contribution demonstrates that state integrity of flip-flops is sensitive to process, voltage and temperature (PVT) variation through measurements from 82 test chips. A PVT-aware state protection technique is presented to ensure state integrity of flip-flops while achieving maximum leakage savings. The technique consists of characterisation algorithm and employs horizontal and vertical parity for error detection and correction. Silicon results show that flip-flops state integrity is preserved while achieving up to 17.6% reduction in retention voltage across 82-dies. Embedded processors memory systems are susceptible to transient errors and blanket protection of every part of memory system through ECC is not cost effective. The final contribution addresses the reliability of embedded processor memory systems and describes an architectural simulation-based framework for joint optimisation of reliability, energy consumption and performance. Accurate estimation of memory reliability with targeted protection is proposed to identify and protect the most vulnerable part of the memory system to minimise protection cost. Furthermore, L1-cache resizing together with voltage and frequency scaling is proposed for further energy savings while maintaining performance and reliability. The contributions presented are supported by detailed analyses using state-of-the-art design automation tools, in-house software tools and validated using FPGA and silicon implementation of commercial low power embedded processors

Heteropolyacids (HPAs) are a group of chemicals that have shown promising results as catalysts during the last decades. Since HPAs have displayed encouraging performance as electrocatalysts in acidic environment, in this project their redox activity in acid and alkaline aqueous electrolytes and their electrocatalytic performance as additives on a bifunctional gas diffusion electrode in alkaline aqueous electrolyte are tested. The results from the electrochemical characterisation of two different HPAs, the phosphomolybdic acid (PMA) and the phosphotungstic acid (PWA) dissolved in acidic and alkaline environment showed that both heteropolyacids demonstrate a redox activity but they also suffer from low stability issues. A series of gas diffusion electrodes were manufactured having PMA and PWA incorporated in their catalyst layer. The electrode support was carbon Toray paper and each heteropolyacid was mixed with Ni to create the catalyst layer of the electrode. From the electrochemical characterisation oF these electrodes in alkaline electrolyte, it was shown that the addition of HPAs enhances the activity of the nickel towards OER and ORR. During the constant current measurements on the manufactured gas diffusion electrodes it was noticed that the electrodes fail after a period of time which could be attributed to the corrosion of the carbon support. In order to find alternative, non-carbon materials to be used as the electrode support, electrochemical characterisation on Magneli phase bulk materials, Magneli spray coated electrodes and PVD coated electrodes was performed. The results from this investigation showed that Magneli phase materials can support electron transfer reactions but their electron conductivity is rather low and it needs to be enhanced. Additionally, it was presented that the Magneli coating protects the substrate over the potential region where OER and ORR take place. Hence, Magneli materials could be used as a support for the bifunctional HPA gas diffusion electrodes.