As worldwide electricity demand increases, so does the requirement for effective power conversion combining increased efficiency with minimal harmonic pollution at the lowest financial cost. For medium to high voltage grid-connected applications, multilevel converter topologies enabled the use of lower rated and more efficient self-commutated switches. Due to practical limitations, efficient operation of converters with a low number of levels is restricted to low switching frequencies which in turn becomes a limiting factor for the design of smaller passive filters that are required to limit the associated switching harmonics injected in the AC grid. This thesis investigates the use of a novel hybrid converter concept aimed at medium-voltage (MV) grid-connected applications. Hybrid converters consist of a main inverter processing the bulk of the power with poor waveform performance and a fast and versatile auxiliary inverter to correct the distortion. In this case, the main converter is a medium-voltage three-level Neutral Point Clamped (NPC) inverter and the auxiliary inverter is a low-voltage and low-current rated Current Source Inverter (CSI), fitted with a series capacitor that is used to minimise the CSI voltage stress. As a result the added installed power by the auxiliary CSI switches can remain at very low levels (theoretically < 4%), resulting in a minimal added cost, whilst offering a substantial harmonic improvement to the main VSI. Furthermore the auxiliary converter can be retro-fitted to an existing MV inverter installation to improve the current harmonic quality as required by new grid standards, at a minimal cost. The performance of the proposed hybrid solution is evaluated through simulation at 3.3 kV MV level under various grid interconnection scenarios. The feasibility of the concept is validated experimentally, scaled to 415V grid voltage level (a more realistic level for a laboratory demonstrator) while operating under more challenging conditions such as switching ripple levels of 50% peak relative to the fundamental peak, showing that the added installed power can be as low as 7% with very high output grid current quality under all grid scenarios considered.

This research focuses on the development of optical fibre sensors that can be applied during critical care. The sensors developed are: (i) an optical fibre humidity sensor based on the deposition of a hygroscopic film onto the distal tip of the fibre and forming a Fabry-Pêrot cavity; (ii) chemical functionalization of U-shaped fibres and Long Period Gratings (LPGs). Fibre Bragg Gratings (FBGs) are only used for temperature monitoring in this work. The current trends in a new generation of humidity sensors suggest that electronic-based technologies could be soon replaced by photonic sensors due to better response times. Optical fibre technology has been considered as a promising platform for providing engineering solutions to unmet clinical needs. For example, anaesthetized patients are transferred from the induction room to the MRI room and taken back, with the need for continuous monitoring of vital signs such as respiration, but some technologies lack compatibility with MRI fields. In the operating room, patients are under anaesthesia, intubated and mechanically ventilated, but some under-humidification or over-humidification of the ventilation air causes secondary effects. Anaesthesiologists rely on total intravenous anaesthesia (TIVA) and monitoring of vital signs as indicators of unconsciousness; current investigations have shown that there is a need to measure the concentration of propofol in real time directly from the blood or breath of patients. Similarly, patients in the intensive care unit (ICU) after surgery require continuous monitoring of diverse biochemical parameters. In this research, the application of optical fibre sensors is demonstrated as an engineering solution for some of these clinical challenges. The simultaneous measurement of humidity and temperature with an optical fibre sensor, a biosensor for propofol detection and another for protein detection were fabricated and demonstrated in this work. The sensitivity, response time and hysteresis of a novel optical fibre humidity sensor has been investigated over a humidity range of 5% Relative Humidity (RH) to 95% RH using a controlled bench-top set-up. The optical fibre humidity sensor proposed has a very simple and low cost porous structured sensing element obtained with poly(allylamine hydrochloride) (PAH) and silica (SiO2) using the layer-by-layer (LbL) fabrication method. The fast response of the sensor (1.13 ± 0.3 s) enabled changes in humidity in individual breaths to be resolved. After calibration, the performance of the sensor was evaluated in the breathing circuit of a mechanical ventilator and respiratory equipment where its faster response allowed monitoring of breath to breath humidity levels during different modes of ventilation. It also has the capability to provide absolute humidity (AH) measurements when an FBG is included for simultaneous temperature measurements. An LPG sensor anchored with SiO2 core and gold (Au) shell nanoparticles has been investigated as a biosensing platform for protein detection. The well-known protein interaction between biotin and streptavidin (SV) was used as a proof of concept. The proposed sensor presented Langmuir adsorption to SV concentrations with a limit of detection (LoD) down to 15.13 nM and a lowest detected concentration of 2.5 nM. In addition, U-shaped fibres and LPGs functionalized with host-guest imprinted TiO2 thin films were used to test propofol detection in aqueous solutions. These sensors also presented Langmuir adsorption in the mili-molar range in aqueous solutions and a lowest detected concentration of 0.65 uM. Detection limits of propofol were improved with the addition of mesoporous silica nanoparticles (MSNs) and inorganic molecular imprinting to allow detection down to 99 nM in aqueous solution and 30 ppb in gaseous phase. In conclusion, this research has successfully demonstrated the use of novel optical fibre sensors in laboratory and real conditions through embedding the developed sensors in critical care equipment usually used during anaesthesia.

Audio-visual speech recognition and visual speech synthesisers are used as interfaces between humans and machines. Such interactions specifically rely on the analysis and synthesis of both audio and visual information, which humans use for face-to-face communication. Currently, there is no global standard to describe these interactions nor is there a standard mathematical tool to describe lip movements. Furthermore, the visual lip movement for each phoneme is considered in isolation rather than a continuation from one to another. Consequently, there is no globally accepted standard method for representing lip movement during articulation. This thesis addresses these issues by designing a transcribed group of words, by mathematical formulas, and so introducing the concept of a visual word, allocating signatures to visual words and finally building a visual speech vocabulary database. In addition, visual speech information has been analysed in a novel way by considering both lip movements and phonemic structure of the English language. In order to extract the visual data, three visual features on the lip have been chosen; these are on the outer upper, lower and corner of the lip. The extracted visual data during articulation is called the visual speech sample set. The final visual data is obtained after processing the visual speech sample sets to correct experimented artefacts such as head tilting, which happened during articulation and visual data extraction. The ‘Barycentric Lagrange Interpolation’ (BLI) formulates the visual speech sample sets into visual speech signals. The visual word is defined in this work and consists of the variation of three visual features. Further processing on relating the visual speech signals to the uttered word leads to the allocation of signatures that represent the visual word. This work suggests the visual word signature can be used either as a ‘visual word barcode’, a ‘digital visual word’ or a ‘2D/3D representations’. The 2D version of the visual word provides a unique signature that allows the identification of the words being uttered. In addition, identification of visual words has also been performed using a technique called ‘volumetric representations of the visual words’. Furthermore, the effect of altering the amplitudes and sampling rate for BLI has been evaluated. In addition, the performance of BLI in reconstructing the visual speech sample sets has been considered. Finally, BLI has been compared to signal reconstruction approach by RMSE and correlation coefficients. The results show that the BLI is the more reliable method for the purpose of this work according to Section 7.7.

This work is on the characterisation of radiated fields from electronic devices. An equivalent dipole approach is used. Previous work showed that this was an effective approach for single layer printed circuit boards where an infinite ground plane can be assumed. In this work, this approach is extended for the characterisation of more complex circuit boards or electronic systems. For complex electronic radiators with finite ground planes, the main challenge is characterising field diffracting around the edges and boundaries. It is shown that this can be satisfactorily characterised using passive dipoles located along the edge. It is also shown that the number of dipoles used to characterise a device can be further optimised by using a complex location for the dipoles. Novel optimisation approaches such as particle swarm optimisation were also investigated. It is concluded that characterisation of complex electronic devices can be achieved in 3D space using edge dipoles to represent diffraction effects and available optimisation strategies.

In recent years, electrical double layer capacitor (EDLCs) has become one of the most popular energy storage devices. This can be attributed to its high capacity, long life cycle and fast charge/discharge rates. However, it has some drawbacks – mainly it stores less amount of energy than batteries. Hence, there is a need to optimize the EDLC to increase its capacity and decrease its equivalent series resistance (ESR), resulting in a supercapacitor that is able to charge quickly and will hold a large amount of energy for a long time. This thesis presents a design, build and setup process of a supercapacitor pilot plant in the University of Nottingham Malaysia Campus for manufacturing and optimization of EDLCs. Two packages were considered, cylindrical and coin type packages. In addition, the design of a manufacturing process flow, with details on steps for fabrication, which will meet specific standards (BS EN 62391-1:2006, BS EN 62391-2-1: 2006, BS EN 62391-2-1: 2006 and DOD-C-29501/3 NOT 1) for quality and throughput for both the packaging types is discussed. Following this, significant factors of the fabrication process were identified and optimized by adopting the Taguchi design of experiment (DOE) methodology. Results of the optimization process show that the most significant factors that affect the EDLC capacitance are PVDF % (polyvinylidene diflouride) and mixing time; the optimum values are determined to be 5% and 3 hours respectively. In the case of ESR optimization, the most significant factors are PVDF % and carbon black %. The optimum values are 5% for both. Using these optimized values, a final prototype EDLC was fabricated. The capacitance value obtained for the cell was 54.70mF. The final EDLC prototypes were tested according to the international standards (ISO) and compared with the supercapacitors available in the market. Results indicated that the electrochemical performance of the prototypes has a good resemblance to the performance of the supercapacitors available in the market. A selected prototype samples were sent for benchmark testing to companies in mobile applications in Korea and the Netherlands to confirm that the prototypes meet the required standards. Finally the research work sets the basis for integrating genetic algorithms with the Taguchi technique for future research in improving the optimization process for robust EDLC fabrication.

This doctoral thesis investigates modes of failure of the IGBT power module and how these modes can be coerced from an open circuit failure mode (OCFM) to a stable short circuit failure mode (SCFM) by using different interconnect technologies and material systems. SCFM is of great importance for a number of applications where IGBT power modules are connected in series string e.g. high voltage modular multi-level converters (M2LC) where one module failing to an OCFM can shut down the whole converter. The failure modes of IGBT samples based on wirebond, flexible PCB, sandwich and press pack structured interconnect technologies have been investigated. Destructive Type-II failure test were performed which concluded that the SCFM is dependent on the energy level dissipating in the power module and the interconnect technology. The higher thermal mass and stronger mechanical constraint of the interconnect enables module to withstand higher energy dissipation. The cross-sections of the tested samples have been characterised with the scanning electron microscope and three dimensional X-ray computed tomography imaging. It was observed that the networked conductive phases within the solidification structure and the Sn-3.5Ag filled in cracks of the residual Si IGBT are responsible for low resistance conduction paths. The best networked conductive phase with lowest electrical resistance and high stability was offered by Ag if used as an intermediate interconnect material on emitter side of an IGBT. To offer a stable SCFM, a module has to be custom designed for a particular application. Hence for the applications which demand a stable SCFM, the IGBT module design becomes an integrated part of the complete power electronics system design.

The design of novel solutions for packaging and integration of power semiconductor devices to deliver switches with advanced performance and reliability is very important aspect of power electronics technology evolution. The advancement of technology in this area is committed to bring significant improvements in the design and implementation of power converters particularly in the enhancement of efficiency, higher power density and better cooling system as compared to the state-of-the-art solutions. A power module is a combination of either multiple semiconductor or discrete devices which are connected to form an electrical circuit of certain structure. They are mainly constructed with a stack of four main parts (power semiconductor devices, insulating substrate with circuit conductor, baseplate, and interconnecting material encapsulated in a plastic case) and each of these parts is of a different material. Some of the interfaces within the module are prone to failure with thermal cycling such as wire-bond, solder die attach and substrate. Therefore reducing the number of interfaces in the assembly will greatly reduce the thermal resistance from the junction to ambient and yields noticeable increase of performance. Moreover, using solid posts as opposed to wires to connect the surface of vertical power components enable a significant improvement in power density as compared with standard modules based on wire bond technology. Additionally, the replacement of wires with such posts drastically reduces the distributed parasitic inductance, together with double-sided cooling of the devices, results in an increase of performance and reliability of the components and assemblies. In this work, 70um thick Infineon technology power devices which are rated at 600V/200A were used for the assembly of a Bi-directional switch based converter and discussing the challenges and trade-offs related to selecting processes and materials. Encapsulation is also one of the important factors in making of power module to protect the power chip and the interconnections from moisture, chemicals, dust, gases, and so on. Here, insulation process was carried out for a given prototype using silicone gel; however, it is worth to note the existing challenge on insulating a very small gap between the sandwich layers of the prototype as compared with the standard planar power module structure. A basic partial discharge test was also taken to demonstrate the performance of the insulation. This research has presented an advanced modular integration approach for power device packaging demonstrating the progress beyond the state of the art in power system assembly by proposing a solution which significantly improves electromagnetic and thermomechanical performance of the power module. In particular, fully bond wireless, double sided cooling and layout symmetry are key aspects. The proposed approach is transferable to many topologies having extra benefit of restricting the impact of single device or switch failure on the general system accessibility.

The microgrid concept integrates Renewable Energy Systems (RES) to the Electrical Power System (EPS) as a means to produce clean energy, meet consumer energy demands and preserve the depleting fossil fuels reserves. These RES are usually interfaced to the grid using power electronic converters (such as Voltage Source Converters) to achieve the required control and conversion of power. Nevertheless, Voltage Source Converters (VSCs) produce both current and voltage harmonics which negatively impact on the Power Quality (PQ) of a microgrid and may cause damage or malfunctions of equipment. This thesis focuses on the impact of VSC harmonics on the power quality of a microgrid. It also investigates various factors that affect the harmonics generated by VSCs with the aim of predicting their impact on the PQ of the microgrid. The PQ of the microgrid is represented as a measure of the level of harmonic distortion of the voltage and current at the Point of Common Coupling (PCC) to the grid. The harmonic mean was used as a measure to determine if the VSCs harmonic level meets the IEEE Standard 519 harmonic limits. The level of harmonic distortion of many VSCs can be significantly affected and difficult to predict in the presence of uncertainties, which may arise due to design parameter choice or system parameter changes. This necessitates the use of statistical techniques to quantify VSC harmonic distortion level in the presence of uncertainties. A common statistical approach is to employ Monte Carlo Simulation (MCS), although accurate it is time consuming and burdensome for systems containing a large number of variables. This thesis utilizes the Univariate Dimension Reduction (UDR) technique formulated from an enhanced Unscented Transform (UT) equation in predicting the harmonic distortion level of large numbers of VSCs in a microgrid, when some system or design parameters are only known within certain constraints. The UDR technique drastically reduce the computation time and burden associated with the MCS approach and avoids assumptions that leads to system simplification required to implement other analytical methods. Various microgrid configuration and statistical distributions similar to practical system variations of RES are considered in order to achieve a good evaluation of the UDR performance in predicting the VSC harmonics. The UDR performance was also evaluated experimentally using a practical microgrid lab containing 3 VSCs. The MCS approach was used as a benchmark for the predicted UDR results. In all cases the UDR predicted results were obtained with significant time saved as compared to the MCS approach and the UDR results showed a good match with the MCS approach.

The in-service reliability of power electronics modules during their normal operation in their work environment is a major concern for application developers. Failure mechanisms act on power modules limiting their lifetime and leading to unpredictable interruptions of power converter operation. That reduces the availability of power converters and can have large financial and safety implications in applications such as in wind turbines and railway traction. Therefore, many attempts are made to use Physics-of-Failure models to estimate the lifetime of power modules while in service utilizing the rainflow counting algorithm. However, large uncertainty in the lifetime estimate given by Physics-of-Failure methods limits the usefulness of that estimate and cannot help improving the availability of power converters. Condition Monitoring on the other hand provides information about the current health state of power modules based on online measurements of failure indicators that can be obtained from the power modules. This information can be used to inform the prognostics stage to provide an estimate of lifetime based on PoF models and online measurements in a Fusion-based approach such that uncertainty in the resulting lifetime estimate can be reduced. In this thesis, the main emphasis is to use online measurement data of failure indicators that can be obtained during the normal operation of power modules to infer the health status of the power module. Failure indicators such as the on-state voltage and junction temperature are estimated or measured online from the power converter. They are indicative of the two dominant failure mechanisms of power modules which are wire-bond lift-off and solder fatigue. Therefore, different simultaneous failure mechanisms can be discriminated. However, in order to infer the health information from the online measurement and discriminate between failure mechanisms, the measurement noise and the effects of operating conditions should be removed from the measurement. The approach proposed in this thesis is based on combining online measurements with pre-determined models of the power module in its original state. Comparing the online measurements with the models reveals the deviation of the power module from its original state. To achieve this, Kalman filter is used to estimate junction temperature based on a noisy estimate from a thermo-sensitive electrical parameter. In addition, measurement circuits are developed to realize the online measurements during normal operation of power modules. The health information inferred from the online measurement of failure indicators can be used in the future to estimate the remaining useful lifetime of the power modules and to inform the Physics-of-Failure models in a fusion framework in order to reduce the uncertainty in the lifetime estimates.

Pulse width modulation (PWM) inverters convert a direct current (DC) power supply to an alternating current (AC) supply by means of high frequency switching between two DC sources. Undesirable high-frequency components are generated in the frequency spectra of the voltages and currents of PWM inverters. The high-frequency components are ultimately removed from the input and output waveforms by filters. PWM inverters are used in a wide variety of electrical devices, ranging from microwave ovens to the electrical parts of aircraft. In many of these devices, minimising the size and weight of the electrical parts is important, and, consequently, it is desirable to design efficient filters for PWM inverters. Identification of the unwanted high-frequency components allows for optimal filter design. In this thesis we use alternative methods to calculate the voltages and currents of PWM inverters. Mathematical models are developed for several PWM inverter designs, and Fourier analysis of the mathematical expressions for the currents and voltages allow us to determine frequency spectra. The methods used in this thesis are shown to be more suitable to the calculation of spectra for complex inverter designs, compared to conventional techniques. In particular, input current spectra are calculated for PWM inverters that incorporate dead time and space vector modulation (SVM) inverters for the first time here.