With the dramatic increase in electricity demand and the need to tackle climate change by reducing greenhouse gas emissions such as CO₂, renewable generation has been increased in recent years in power systems. However, highly integrating renewable generation in distribution systems raises several issues and challenges that need to be addressed and some of these issues are investigated in this thesis. This research focuses on investigating the fault ride through, the system transient stability, the system frequency response and the grid power factor of distribution systems with high renewable energy penetration based on wind and solar-photovoltaic (PV). Several proposed control techniques for wind generators and PVs are introduced in this research to solve these issues and mitigate the negative impact of these units on distribution systems. A modified control system of DGs based on wind and PV in different distribution systems is used to help to ride through faults and meet the low voltage ride through (LVRT) grid code requirements for voltage recovery. A three phase two stage transformerless PV grid connected system is proposed with a new technique to ride through faults and protect the power electronic converters. A DC chopper is added to the DC link of wind generators to enable wind generating units to ride through faults and protect their converters from overvoltages. The Transient Stability Index method is used to assess the impact of such renewable sources on the system transient stability of various distribution networks. Different frequency control methods published in literature are used for various DGs based on wind and PV to investigate the frequency response of different distribution networks with high renewable energy penetration, and a new frequency control technique is proposed for wind generators based on Doubly Fed Induction Generator (DFIG) to improve the system frequency response. The impact of high renewable energy penetration based on wind and PV with different power factor settings on grid power factor of various distribution systems is discussed in this research. A three phase two stage transformerless PV grid connected system with reactive power capability is proposed to operate under different solar irradiance conditions and improve the utility grid power factor. Simulation results validate the proposed systems in various distribution systems, including IEEE 13 bus, IEEE 37 bus and IEEE 123 bus systems.

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In a power transformer, the electrical steel core serves as a low reluctance path for the main magnetic flux linking primary and secondary windings. It is also one of the most costly components, whose properties are vital to design an efficient and reliable transformer. Normally, power transformers are predominantly operated within the linear portion of the core steel’s magnetisation curve with the maximum flux density limited at a certain value in the knee area. Nowadays, more technical challenges from the core saturation are raised, which are caused by the geomagnetically induced currents or the normal operation of Quadrature Boosters. The substantial power losses generated at such high flux densities can lead to the core overheating and consequential thermal degradation of the surrounding insulation, and even transformer failure. The characteristics of transformer core in deep saturation, however, are not readily available from measurements, and neither are the current IEC standards applicable above 1.8 Tesla for the measurement of magnetic properties of electrical steels owing to measurement difficulties, such as magnetic flux waveform stabilization. The simulation studies often need to extrapolate the steel’s magnetisation curve to high flux densities, which brings uncertainties to the results. In addition, the industry has often adopted a conservative transformer core design due to the insufficient knowledge of core loss and temperature rise under the extreme scenario. In order to fill the knowledge gap of electrical steels and transformer cores at high flux densities, this thesis uses an improved single strip test bench developed at Wolfson Centre for Magnetics to measure the magnetic properties of modern grain- oriented electrical steels up to 2.0 T under AC magnetisation up to 400 Hz. Based on the latest measurement results, a new single explicit expression is proposed to approximate and predict the AC magnetisation curve accurately over a wide range up to 2.0 T. A simple and accurate power loss separation algorithm is also proposed to identify the percentages of hysteresis loss, eddy current loss and anomalous loss, and predict the power loss at high flux densities. The finite element computational method based on Maxwell’s equations together with the measured magnetic properties of electrical steels up to 2.0 T leads to a more accurate predication for the distribution of magnetic flux and core losses in the power transformer core at high flux densities. The effects of core joint types, overlapping techniques, air gaps on the magnetic flux distribution are investigated in both 2D and 3D core corner joints. The distributions of the main flux, the leakage flux and the power loss in the core and the clamping structures are also obtained.

This thesis utilises computational simulations to investigate the relationship between morphological structure of the active layer within organic photovoltaic devices, and its impact on the device performance. Specifically, the effects of hot charge transfer states, the mixed molecular phase and fullerene aggregation on organic photovoltaic performance, and polymer crystallinity on carrier mobility, are explored using kinetic Monte Carlo simulations. These investigations agree with experimental results and shed new light on the processes of recombination, ultrafast charge separation and the utility of the amorphous phase within the context of tie-chains. A more accurate charge separation kinetic model is proposed in order to correctly describe the biexponential carrier recombination determined from Monte Carlo investigations. The model incorporates a `quasi-free' state where charges are still Coulombically bound but sufficiently separated to prevent recombination. This is conceptually similar to the cooled remains of hot charge transfer states, the effects of which are investigated on device operation and shown to provide a benefit that is strongly dependent on the aggregation of the fullerene phase, the limitation of the molecularly mixed phase and the relative charge carrier mobilities. Crystallisation within the polymer medium is then comprehensively explored using a combination of molecular dynamics and Monte Carlo simulations, along with quantum chemical calculations to help elucidate the observed annealing temperature and molecular weight dependencies of the carrier mobility for a poly(3-hexylthiophene-2,5-diyl) test system. The annealing temperature trend can be explained by increased crystallite size and order, but the molecular weight dependence is not satisfactorily explained by the crystalline regions. Instead, mobility is shown to be limited by the availability of tie-chains in the amorphous phase of the morphology, linking together crystals and providing regions of high mobility through the amorphous material.

The modern grid system or the smart grid is likely to be populated with multiple distributed energy sources, e.g. wind power, PV power, Plug-in Electric Vehicle (PEV). It will also include a variety of linear and nonlinear loads. The intermittent nature of renewable energies like PV, wind turbine and increased penetration of Electric Vehicle (EV) makes the stable operation of utility grid system challenging. In order to ensure a stable operation of the utility grid system and to support smart grid functionalities such as, fault ride-through, frequency response, reactive power support, and mitigation of power quality issues, an energy storage system (ESS) could play an important role. A fast acting bidirectional energy storage system which can rapidly provide and absorb power and/or VARs for a sufficient time is a potentially valuable tool to support this functionality. Battery energy storage systems (BESS) are one of a range suitable energy storage system because it can provide and absorb power for sufficient time as well as able to respond reasonably fast. Conventional BESS already exist on the grid system are made up primarily of new batteries. The cost of these batteries can be high which makes most BESS an expensive solution. In order to assist moving towards a low carbon economy and to reduce battery cost this work aims to research the opportunities for the re-use of batteries after their primary use in low and ultra-low carbon vehicles (EV/HEV) on the electricity grid system. This research aims to develop a new generation of second life battery energy storage systems (SLBESS) which could interface to the low/medium voltage network to provide necessary grid support in a reliable and in cost-effective manner. The reliability/performance of these batteries is not clear, but is almost certainly worse than a new battery. Manufacturers indicate that a mixture of gradual degradation and sudden failure are both possible and failure mechanisms are likely to be related to how hard the batteries were driven inside the vehicle. There are several figures from a number of sources including the DECC (Department of Energy and Climate Control) and Arup and Cenex reports indicate anything from 70,000 to 2.6 million electric and hybrid vehicles on the road by 2020. Once the vehicle battery has degraded to around 70-80% of its capacity it is considered to be at the end of its first life application. This leaves capacity available for a second life at a much cheaper cost than a new BESS Assuming a battery capability of around 5-18kWhr (MHEV 5kWh - BEV 18kWh battery) and approximate 10 year life span, this equates to a projection of battery storage capability available for second life of >1GWhrs by 2025. Moreover, each vehicle manufacturer has different specifications for battery chemistry, number and arrangement of battery cells, capacity, voltage, size etc. To enable research and investment in this area and to maximize the remaining life of these batteries, one of the design challenges is to combine these hybrid batteries into a grid-tie converter where their different performance characteristics, and parameter variation can be catered for and a hot swapping mechanism is available so that as a battery ends it second life, it can be replaced without affecting the overall system operation. This integration of either single types of batteries with vastly different performance capability or a hybrid battery system to a grid-tie 3 energy storage system is different to currently existing work on battery energy storage systems (BESS) which deals with a single type of battery with common characteristics. This thesis addresses and solves the power electronic design challenges in integrating second life hybrid batteries into a grid-tie energy storage unit for the first time. This study details a suitable multi-modular power electronic converter and its various switching strategies which can integrate widely different batteries to a grid-tie inverter irrespective of their characteristics, voltage levels and reliability. The proposed converter provides a high efficiency, enhanced control flexibility and has the capability to operate in different operational modes from the input to output. Designing an appropriate control system for this kind of hybrid battery storage system is also important because of the variation of battery types, differences in characteristics and different levels of degradations. This thesis proposes a generalised distributed power sharing strategy based on weighting function aims to optimally use a set of hybrid batteries according to their relative characteristics while providing the necessary grid support by distributing the power between the batteries. The strategy is adaptive in nature and varies as the individual battery characteristics change in real time as a result of degradation for example. A suitable bidirectional distributed control strategy or a module independent control technique has been developed corresponding to each mode of operation of the proposed modular converter. Stability is an important consideration in control of all power converters and as such this thesis investigates the control stability of the multi-modular converter in detailed. Many controllers use PI/PID based techniques with fixed control parameters. However, this is not found to be suitable from a stability point-of-view. Issues of control stability using this controller type under one of the operating modes has led to the development of an alternative adaptive and nonlinear Lyapunov based control for the modular power converter. Finally, a detailed simulation and experimental validation of the proposed power converter operation, power sharing strategy, proposed control structures and control stability issue have been undertaken using a grid connected laboratory based multi-modular hybrid battery energy storage system prototype. The experimental validation has demonstrated the feasibility of this new energy storage system operation for use in future grid applications.

The most abundant form of ceramic capacitor is the Multilayer Ceramic Capacitor (MLCC) and the most commonly used dielectric material is barium titanate (BaTiCT). BaTiCL is supplied in the form of dry, agglomerated powder and critical to the manufacturing process of MLCCs is good dispersion of these powders, reducing the number of agglomerates which are potential points of breakdown when MLCCs are placed under increasing voltage loads. The main objective of this study was to employ a process engineering focus to examine and identify critical process parameters within the complete dispersion process by experimentation and statistical consideration. A laboratory-scale replica of the industrial process was developed based around a stirred media mill. Dispersions were characterized primarily by measurement of particle size using laser scattering and suspension stability using multiple light scattering. Statistical consideration of the complete dispersion process by factorial analysis and analysis of variance identified three key stages; wetting, deagglomeration and stabilization. A reduction in the number of agglomerates, identified by improved dispersion stability and a narrowing of particle size distribution, was achieved by an increase in mechanical energy input at the wetting and deagglomeration stages. To avoid damage to primary particles, increased energy input during deagglomeration in the stirred media mill was achieved using grinding media beads with reduced diameter, reducing the stress intensity but increasing the number of collisions (“mild dispersion”). Stability of the deagglomerated particles was achieved by steric stabilization and a minimum concentration of polymeric stabilizer was required to enable monolayer coverage of particle surfaces. Below this monolayer point, dispersion stability was reduced due to reagglomeration. An optimum volume fraction of the feed suspension was identified giving the narrowest particle size distribution, avoiding either over­milling due to excess collisions or under-milling due to media dampening. Recommendations have been made to scale-up the findings of this study into an industrial scale process.

The need to resolve the energy shortage and environmental pollution leads to improving and exploiting thin films for photovoltaic (PV) applications. The current promising PV technologies are CdTe and CuInGaSe2 (CIGS), which have achieved high efficiencies and already reached the commercialisation stage. However, the scarcity of elements like indium and tellurium has limited the deployment of these technologies on a terawatt scale. A search for alternative materials has become crucial to replace and overcome current technology limitations. Copper zinc tin sulfide (Cu2ZnSnS4 or CZTS) has attracted a lot of attention as a potential alternative light-absorbing material that consists of abundant elements, non-toxic and inexpensive. Furthermore, CZTS has a direct band gap of 1.4-1.6 eV and high light absorption coefficient of 104 cm-1, which favourably matches the solar spectrum. CZTS material's have reached efficiencies up to 12.6%, as prepared by a hydrazine-based solution method. The danger of this reaction due to hydrazine auto-ignition temperature of 24°C and flash point of 38 °C makes this method unreliable for large-scale production. However, the efficiency gap between CZTS and CIGS is still large, with a conversion efficiency of around >22% for CIGS solar cells. CZTS shows lower open-circuit voltage, Voc, lower short-circuit current density, Jsc, and smaller minority-carrier lifetimes. These deficiencies could be related to the formation of defects in nanocrystals that cause trapping or recombination of carriers. This thesis aims to study the structure and defects in CZTS nanocrystals using transmission electron microscopy (TEM)-based techniques. The hot-injection method was used to synthesize CZTS due to the ability to produce large-scale and high-quality nanocrystals. In addition, CZTS nanoparticles growth was investigated after deposition on Molybdenum on glass substrate, providing annealing conditions that significantly improved grain growth to be suitable for PV applications. A detailed analysis of the CZTS crystal structure was undertaken, confirming a kesterite (tetragonal) structure of annealed CZTS nanocrystals. Furthermore, a fingerprint map for CZTS was obtained using selected area electron diffraction (SAED) and convergent beam electron diffraction (CBED). These techniques provide an approach enabling to distinguish CZTS from secondary phases such as ZnS that have a negative impact on the solar cell performance. Bright-field and dark-field were used to visualize the extended defects exhibited in nanocrystals. Nanocrystals showed that growth of defects in the form of lamellar twinning and dislocations occurred in the {112} planes, which are the preferential growth direction of annealed CZTS. The presence of these defects results in a local change to hexagonal phases in lamellar twinning boundaries. Moreover, high-angle annular dark field (HAADF) imaging was used to obtain high-resolution images of CZTS nanocrystals at a sub-O.l nm resolution that visualized the CZTS crystal unit cell, showing for the first time all atoms of Cu, Zn, Sn, and S are presented. These images allow to investigate the formation of antisite defects that have a significant impact on CZTS performance. These defects formed antisite domain boundaries that lie in different planes, causing disorder on the Cu, Zn, or Sn sites with some of the boundaries affecting local changes in stoichiometry. These studies can provide key information on the defects occurring at the atomic scale that have important consequences on CZTS devices' performance. The growth of nanocrystals 'on molybdenum substrates was also investigated to improve the grain size and the electronic properties of the material. An annealing condition is established that achieved a significant improvement in nanocrystals grain size from the initial average size of as-grown nanoparticles of ~7-12 nm up to 1 μm grain size. Annealing under hydrogen atmosphere with additional to SnS and S in a powder form was used to improve the nanoparticles growth. In addition to present a comparison of nanoparticles growth under other annealing conditions including nitrogen atmospheres and additional elements and binaries such as Na2S, SnS and S. The presence of hydrogen demonstrated an annealing atmosphere produces a significant improvement in nanocrystals growth compared with other annealing atmospheres. A promising efficiency is achieved for the CZTS solar cell of (0.8 %), Voc (253 mV), Jsc (6.84 mA/cm2), fill factor (FF) (45.9%), Rs (44.9 Ώ cm2), and Rsh (169.7 Ώ cm2) with a cell configuration (glass/Molybdenum/CZTS/CdS/intrinsic-ZnO)/Aluminum doped ZnO (AZO)/NiAl).

The global need for a clean, sustainable and affordable source of energy has triggered extensive research especially in renewable energy sources. In this sector, photovoltaic has been identified as a cheapest, clean and reliable source of energy. It would be of interest to obtain photovoltaic material in thin film form by using simple and inexpensive semiconductor growth technique such as electroplating. Using this growth technique, four semiconductor materials were electroplated on glass/fluorine-doped tin oxide (FTO) substrate from aqueous electrolytes. These semiconductors are indium selenide (InxSey), zinc sulphide (ZnS), cadmium sulphide (CdS) and cadmium telluride (CdTe). lnxSey and ZnS were incorporated as buffer layers while CdS and CdTe layers were utilised as window and absorber layers respectively. All materials were grown using two-electrode (2E) system except for CdTe which was grown using 3E and 2E systems for comparison. To fully optimise the growth conditions, the as-deposited and annealed layers from all the materials were characterised for their structural, morphological, optical, electrical and defects structures using X-ray diffraction (XRD), Raman spectroscopy, scanning electron microscopy (SEM), atomic force microscopy (AFM), optical absorption (UV- Vis spectroscopy), photoelectrochemical (PEC) cell measurements, current-voltage (I-V), capacitance-voltage (C-V), DC electrical measurements, ultraviolet photoelectron spectroscopy (UPS) and photoluminescence (PL) techniques. Results show that InxSey and ZnS layers were amorphous in nature and exhibit both n-type and p-type in electrical conduction. CdS layers are n-type in electrical conduction and show hexagonal and cubic phases in both the as-deposited and after annealing process. CdTe layers show cubic phase structure with both n-type and p- type in electrical conduction. CdTe-based solar cell structures with a n-n heterojunction plus large Schottky barrier, as well as multi-layer graded bandgap solar cells were fabricated. This means that the solar cells investigated in this thesis were not the conventional p-n junction type solar cells. The conventional cadmium chloride (CdCl2 or CC) treatment was applied to the structures to produce high performance devices; however, by modifying the treatment to include cadmium chloride and cadmium fluoride (CdCl2+CdF2 or CF) device performance could be improved further. The fabricated devices were characterised using I-V and C-V measurement techniques. The highest cell efficiency achieved in this research was ~10%, with an open circuit voltage of 640 mV, short-circuit current density of 38.1 mAcm-2, fill factor of 0.41 and doping concentration of 2.07xl016 cm'3. These parameters were obtained for the glass/FTO/n-InxSey/n- CdS/n-CdTe/Au solar cell structure.

The thesis is essentially concerned with the modeling and fault analysis of Modular Multilevel Converter (MMC) based High Voltage Direct Current (HVDC) systems with DC circuit breakers connected. A generalized MMC model is proposed and the system behaviour of MMCs subject to both DC line-to-line and line-to-ground faults is analyzed. Various stages of DC voltage / current transient processes are studied and analyzed. Simulation results are presented to illustrate the behaviour of the system under such faults. A hybrid multi-terminal HVDC transmission system model consisted of one MMC, one Full-Bridge Modular Multilevel Converter (FB-MMC) and one conventional two-level Voltage Source Converter (VSC) has been studied in this thesis. In the first part, the steady-state behaviors of these three converters are described, and the simulation results testing the normal performance of MMC, FB-MMC and VSC are given. Considering the necessity of analysis on the hybrid three-terminal DC grid under different DC fault conditions, five fault conditions have been tested, i.e. pole-to-pole DC fault near MMC, pole-to-pole DC fault near FB-MMC, pole-to-ground DC fault at FB-MMC, pole-to-ground DC fault at MMC and pole-to-ground DC fault at VSC. To isolate the DC fault, DC breakers are discussed and the hybrid DC breaker is adopted for the simulation. Because of the DC breaker, after the fault occurs, the three-terminal hybrid DC grid can be transformed a two-terminal hybrid HVDC with these healthy converters, so that the healthy HVDC can continue to operate. The simulation results demonstrate the behaviors of the three different converters under two different DC fault conditions. Finally two simplified models are proposed for studying DC fault behaviour of MMC based HVDC systems, one is an average model and the other is a diode model.

In order for the island of Ireland to achieve the ambitious government target of 40% of electrical energy from renewable energy sources, mainly from wind turbine generation, significant changes in power system network topology and operational procedure are required. The impact of these changes on power system stability needs to be fully investigated to ensure secure and reliable operation of the power system. Therefore, the aim of this thesis is to analyse the impact of wind turbine generation on small signal and frequency stability whilst taking advantage of phasor measurement unit (PMU) technology installed in the All-Island power system for the purpose of stability analysis and parameter estimation. PMU data of active power from the main AC North-South interconnector as well as from fixed-speed and doubly-fed induction generator based wind turbines is employed to obtain the All-Island inter-area and wind turbine oscillation modes. A correlation coefficient analysis between inter-area frequency oscillation and the active power output of thermal power plants and wind turbine generation is conducted to identify the source of the oscillations. A number of case studies with differing operational procedures are simulated using DigSILENT to understand the mechanisms that may influence the small signal stability performance of a power system with high DFIG penetration. The implementation of Prony Analysis to analyse small signal stability is conducted by employing both simulated and actual PMU ringdown data. Both wide-area and single-site PMU based techniques are assessed for purposes of monitoring small signal stability. The frequency response metrics are assessed with respect to the power system’s intertia and system non synchronous penetration to understand the significance of these parameters during frequency events. A method to estimate a synchronous generator’s inertia constant from PMU data during a frequency disturbance is proposed and described.