Wind power is growing rapidly around the world as a means of dealing with the world energy shortage and associated environmental problems. Ambitious plans concerning renewable energy applications around European countries require a reliable yet economic system to generate, collect and transmit electrical power from renewable resources. In populous Europe, collective offshore large-scale wind farms are efficient and have the potential to reach this sustainable goal. This means that an even more reliable collection and transmission system is sought. However, this relatively new area of offshore wind power generation lacks systematic fault transient analysis and operational experience to enhance further development. At the same time, appropriate fault protection schemes are required. This thesis focuses on the analysis of fault conditions and investigates effective fault ride-through and protection schemes in the electrical systems of wind farms, for both small-scale land and large-scale offshore systems. Two variable-speed generation systems are considered: doubly-fed induction generators (DFIGs) and permanent magnet synchronous generators (PMSGs) because of their popularity nowadays for wind turbines scaling to several-MW systems. The main content of the thesis is as follows. The protection issues of DFIGs are discussed, with a novel protection scheme proposed. Then the analysis of protection scheme options for the fully rated converter, direct-driven PMSGs are examined and performed with simulation comparisons. Further, the protection schemes for wind farm collection and transmission systems are studied in terms of voltage level, collection level  wind farm collection grids and high-voltage transmission systems for multi-terminal DC connected transmission systems, the so-called “Supergrid”. Throughout the thesis, theoretical analyses of fault transient performances are detailed with PSCAD/EMTDC simulation results for verification. Finally, the economic aspect for possible redundant design of wind farm electrical systems is investigated based on operational and economic statistics from an example wind farm project.

A spectrum of phenomena related to the reliability of AlGaN/GaN HEMTs are investigated in this thesis using numerical simulations. The focus is on trap related phenomena that lead to decrease in the power output and failure of devices, i.e. the current collapse and the device degradation. The current collapse phenomenon has been largely suppressed using SiN passivation, but there are gaps in the understanding of the process leading to this effect. Device degradation, on the other side, is a pending problem of current devices and an obstacle to wide penetration of the market. Calibration of I-V measurements of two devices is performed with high accuracy to provide a trustworthy starting point for modelling the phenomena of interest. Traditionally, in simulations of nitride based HEMTs, only direct piezoelectric effect is taken into account and the resulting interface charge is thence independent of the electric field. In this work, the impact of the electric field via the converse piezoelectric effect is taken into account and its impact on the bound charge and the drain current is studied, as a refinement of the simulation methodology. It is widely believed that the current collapse is caused by a virtual gate, i.e. electrons leaked to the surface of the device. We have found a charge distribution that reproduced the I-V measurement that shows current collapse, hence validating the concept of the virtual gate. While it was previously shown that the virtual gate has a similar impact on the I-V curve as is observed during the current collapse, we believe that this is for the first time that a wide range of gate and drain voltages was calibrated. High gate/drain voltage leading to permanent degradation was also investigated. The hypothesis that stress induced defects and dislocations might be responsible for the degradation was tested but not fully confirmed. Finally, the leakage of electrons thought to be responsible for formation of the virtual gate and the current collapse due to the Poole-Frenkel emission, is simulated in order to explain the surface charge distribution responsible for the current collapse and deduced in Chapter 5.

The aim of this thesis is to dielectrophoretically characterise sub-micron particles on the basis of their surface properties and to devise a DEP technique suitable for the fractionation and manipulation of particles on this scale. Polystyrene particles are modified by the attachment of biological ligands using various established localisation techniques and their DEP response observed using micro-electrodes with well defined high and low field regions, corresponding to a previously utilised design and modified in the course of this project for multiple sample handling. The results of these observations are modelled for the first time using a charge relaxation mechanism pertaining to a structured interfacial charge distribution and, through fitting the data to this model, fundamental parameters of the system - the surface conductance and electrokinetic charge - are predicted. The model viability is assessed with reference to both comparisons with alternative measurements and the technical limitations of the data fitting procedure, and corresponding surface charge transport mechanisms are discussed in the light of the DEP response following surface modification. Investigations are made into the possibility of a DEP based device suitable for the transport/fractionation of sub-micron particles. Given the essentially dissipative nature of sub-micro particle ensembles, a Brownian ratchet principle is chosen. A Brownian ratchet is a generic system wherein a net directional drive is effected by biasing Brownian diffusion on a periodically activated anisotropic structure. Without need of thermal gradients or net macroscopic forces Brownian ratchet pumps could be an interesting alternative in many microfluidic applications. Simulated fields and corresponding particle transport rates are compared for two basic electrode structures in order to assess their viability for use as DEP Brownian ratchets and a new design proposed, based on the simultaneous juxtaposition of positive and negative DEP forces. This device is built on the necessary scale using multi-layer fabrication techniques with a silicon elastomer moulded channel. The existence of stochastic transport on the device is investigated experimentally by means of processed video sequences and resulting possibilities for particle separation on the basis of size and surface properties inferred.

This thesis provides an investigation of the dynamics and control of turbogenerators from a multivariable control viewpoint. The multivariate control framework chosen -Individual Channel Analysis and Design- is particularly appropriate since it encapsulates the dynamical characteristics of the uncontrolled system with a view to exposing the potential and limitations for subsequent closed-loop control. The main contribution of the thesis is a complete new insight into why excitation/governor control with Power System Stabilisers (PSS) has been so successful for the control of turbogenerators connected to an infinite bus provided by the small-signal multivariable analysis framework, Individual Channel Analysis and Design. The multivariable analysis justifies treating the turbogenerator system as a pseudo- Single-Input Single-Output, (SISO) system where the governor loop is first closed and the exciter loop is treated as a SISO system for the prime purpose of rejecting voltage disturbances. The function of the PSS is identified as that of overcoming an awkward switch-back frequency-domain characteristic of the excitation channel so as to permit high-performance excitation channel bandwidths up to 10 rad/sec that otherwise could not be obtained. Thus, in addition to the control requirements of set point regulation of the terminal voltage and shaft speed, the PSS provides for a second control requirement of strong voltage disturbance rejection over the important frequency range of 0 to 10 rad/sec. The PSS control option is also assessed against other control options. Several other results concerning stability robustness to system uncertainties in different system configurations follow from the analysis in a transparent and immediate way.

Over the past 5 years there has been an increase in the number of applications that require devices that operate in the millimetre range (30-300GHz). This demand has driven research into " devices that will operate at frequencies above 100GHz. This performance has been achieved using two main technologies, the Heterojunction Bipolar Transistor (HBT) and the High Electron Mobility Transistor (HEMT). At present it is a HEMT device that holds the record for the highest operating frequency of any transistor. It is this technology that this project concentrates on. In order to fabricate devices that operate at these frequencies two methods are commonly employed. The first is to vary the material of the device, in particular, increasing the indium content of the channel. The second method is to reduce the physical dimensions of the transistors, including reducing the gate length of the device therefore reducing transit time and gate capacitance. Reducing the separation of the source-drain ohmic contacts or employing a self-aligned ohmic strategy reduces the associated parasitic resistances. This project will concentrate on the scaling of the gate length in addition to the reduction of parasitic resistances with the use of self-aligned ohmic contacts. This work includes the realisation of the first self-aligned 120nm T -Gate. GaAs pHEMT fabricated at the University of Glasgow. These devices required the development of two key technologies, the non-annealed ohmic contact and the succinic acid based selective wet etch. The self-aligned devices showed good RF performance with a ft of 150 GHz and a fmax of 180 GHz which compares favourable with results o~ 120nm GaAs pHEMTs previously fabricated at Glasgow. The investigation of gate length scaling to device performance included the development of two lithographic process capable of producing HEMT with a gate length of 50nm and 30nm respectively in addition to a method ~f sample preparation that allows these devices to be analysed using TEM techniques. This work has lead to the realisation of SOnm T -gate metamorphic HEMTs using a PMMAIcopolymer resist stack, these devices displayed an excellent yield, with over 95% of devices working. The uniformity of the gate process was also high with a threshold voltage of - 0.44SV with a standard deviation of O.OOSV. The devices demonstrated an .it of 330GHz and a fmax of 260GHz making these devices some of the fastest transistors that have ever been fabricated on a GaAs substrate. The second lithography process was developed to realise T -gates with a gate length of less than SOnm. This processed used a two stage "bi-lithography" process to minimise the effect of forward s7attering through the resist. The gate footprint was transferred into a Si02 gate by a dry etch process. This lithography process was integrated into a full process flow for lattice matched InP HEMTs Using this process, HEMTs were fabricated with a T-gate of 2Snm. This is the smallest T -gate device that has been fabricated at the University of Glasgow and is comparable with the smallest HEMT devices in the world.

No description available.

The research work presented in this thesis is concerned with the modelling of this new generation of power electronics controllers with a view to conduct comprehensive power systems harmonic analyses. An issue of paramount importance in this research is the representation of the self-commutated valves used by the controllers addressed in this work. Such a representation is based on switching functions that enable the realization of flexible and comprehensive harmonic models. Modularity is another key issue of great importance in this research, and the model of the voltage source converter is used as the basic building block with which to assemble harmonic models of actual power systems controllers. In this research the complex Fourier series in the form of operational matrices was used to derive the harmonic models. Also, a novel methodology is presented in this thesis for conducting transient analysis of electric networks containing non-linearities and power electronic components. The methodology is termed the extended harmonic domain. This method is based on the use of time-dependent Fourier series, operational matrices, state-space representation and averaging methods. With this method, state-space equations for linear circuit, non-linear circuits, and power electronics controllers models are obtained. The state variables are the harmonic coefficients of x(t) instead of x(t) itself. The solution of the state-space equations gives the dynamic response of the harmonic coefficients of x(t). Moreover, a new harmonic power flow methodology, based on the instantaneous power flow balance concept, the harmonic domain, and Newton-Raphson method, is developed and explained in the thesis. This method is based on the instantaneous power balance as opposed to the active and reactive power balance, followed by traditional harmonic power flow methods. The power system and the power electronics controllers are modelled entirely in the harmonic domain.

No description available.

The aim of this PhD project was to develop the technology of dielectrophoresis on the sub-micrometre scale and to use DEP to manipulate sub-micrometre particles and measure their dielectric properties. Of particular interest was the application of DEP to viruses, the largest of which is approximately 250 nm in diameter. A system for virus characterisation, identification and separation based on DEP would be a major milestone in this field of research, as well as having beneficial medical and biotechnological uses. Particles with a diameter between 1nm and 1m are referred to as Colloidal particles and the dynamics of their movement are complicated by the effects of thermal energy and Brownian motion. High electric fields are required to dominate these effects but signals with high potentials and high frequencies are difficult to generate. Semiconductor manufacturing techniques can be used to fabricate micro-electrode structures which can produce high electric fields from relatively low potentials. Lithography based manufacturing techniques were developed to produce suitable electrodes for dielectrophoresis on a scale small enough to manipulate sub-micrometre particles. Detailed electric field patterns were numerically calculated for these electrodes, so that the dielectrophoretic force could be simulated, predicted and compared with experimental measurements of particle movement. The dielectric properties of latex spheres with diameters from 93 nm to 557 nm were determined through observation and measurement of the DEP movement; new theories were postulated to account for the results which did not conform to accepted theories. A rod shaped plant virus, Tobacco Mosaic Virus (TMV) was also studied and its dielectric properties determined from the experimental results. TMV is 300 nm long with a cylindrical radius of 9nm, a shape of particle which is very different from a sphere and one which has not been studied by this method previously. An expression for the frequency dependent dielectrophoretic force on such a particle was derived and values of the dielectrophoretic force on the particle were measured and compared with the theoretical model.

The objective of the work of this thesis is to design, fabricate, and characterise a GaAs MESFET based monolithic microwave integrated circuit (MMIC) downconverter which operates at Ka-band frequency (35GHz). In the course of the project active and passive elements required for the MMIC were designed, fabricated, characterised and their equivalent circuit models extracted. Fabrication processes for passives, actives and MMIC were realised using mainly electron beam lithography (EBL) techniques. The main findings of this thesis were: Devices - Influence of gate recess offset on MESFETs The MESFETs were patterned by EBL and gate recessing was accomplished by selective dry etching. The influence of the gate recess offset on the small signal AC equivalent circuit, DC device characteristics, overall high frequency device performance, and low frequency noise behaviour of 0.2 m gate length GaAs MESFETs implemented in the low noise amplifier (LNA) circuit design in this thesis was investigated. Numerical simulations of the AC small signal equivalent circuit dependence were carried out in order to help understand the effects observed. Good qualitative agreement between measured and simulated response was obtained. - Schottky diodes The performance of Schottky-contact diodes used in the MMIC mixer were studied as a function of their geometry and processing conditions. Passives - CPW losses Losses in coplanar interconnect topologies (coplanar waveguide and slotline) using different metallisation processes were investigated. - CPW to slotline transitions A range of coplanar waveguide to slotline transitions required for the MMIC mixer were studied. Broadband performance with insertion loss of < 0.5dB per transition was observed. Transmission line models of the structures have been implemented to enable circuit performance to be predicted and designed to suit the application frequency. The effect of parasitic modes on transitions performance was also investigated.