Power flows within distribution networks are expected to become increasingly congested with the proliferation of distributed generation (DG) from renewable energy resources. Consequently, the size, energy penetration and ultimately the revenue stream of DG schemes may be limited in the future. This research seeks to facilitate increased renewable energy penetrations by utilising power system component thermal properties together with DG power output control techniques. The real-time thermal rating of existing power system components has the potential to unlock latent power transfer capacities. When integrated with a DG power output control system, greater installed capacities of DG may be accommodated within the distribution network. Moreover, the secure operation of the network is maintained through the constraint of DG power outputs to manage network power flows. The research presented in this thesis forms part of a UK government funded project which aims to develop and deploy an on-line power output control system for wind-based DG schemes. This is based on the concept that high power flows resulting from wind generation at high wind speeds could be accommodated since the same wind speed has a positive effect on component cooling mechanisms. The control system compares component real-time thermal ratings with network power flows and produces set points that are fed back to the DG for implementation. The control algorithm comprises: (i) An inference engine (using rule-based artificial intelligence) that decides when DG control actions are required; (ii) a DG set point calculator (utilising predetermined power flow sensitivity factors) that computes updated DG power outputs to manage distribution network power flows; and (iii) an on-line simulation tool that validates the control actions before dispatch. A section of the UK power system has been selected by ScottishPower EnergyNetworks to form the basis of field trials. Electrical and thermal datasets from the field are used in open loop to validate the algorithms developed. The loop is then closed through simulation to automate DG output control for increased renewable energy penetrations.

This Thesis describes the development and testing of a real-time rating estimation algorithm developed at Durham University within the framework of the partially Government-funded research and development project “Active network management based on component thermal properties”, involving Durham University, ScottishPower EnergyNetworks, AREVA-T&D, PB Power and Imass. The concept of real time ratings is based on the observation that power system component current carrying capacity is strongly influenced by variable environmental parameters such as air temperature or wind speed. On the contrary, the current operating practice consists of using static component ratings based on conservative assumptions. Therefore, the adoption of real-time ratings would allow latent network capacity to be unlocked with positive outcomes in a number of aspects of distribution network operation. This research is mainly focused on facilitating renewable energy connection to the distribution level, since thermal overloads are the main cause of constraints for connections at the medium and high voltage levels. Additionally its application is expected to facilitate network operation in case of thermal problems created by load growth, delaying and optimizing network reinforcements. The work aims at providing a solution to part of the problems inherent in the development of a real-time rating system, such as reducing measurements points, data uncertainty and communication failure. An extensive validation allowed a quantification of the performance of the algorithm developed, building the necessary confidence for a practical application of the system developed.

As part of extensive activities on the reduction of CO2 emissions, a rapid expansion of power generation using new more fuel efficient technologies (large, medium and embedded scale with combined heat and power (CHP) projects) and renewable energy (wind, biomass, solar PV) is currently taking place in numerous European countries, including the UK. The research presented in this thesis is a part of a UK government funded project, which aims to find answers to how to accommodate increased renewable energy into the distribution network. Current ratings, which are limited by the temperature of the conductors used in the distribution network, are based on worst case scenario conditions and are conservative. The temperature limits can be lifted if one takes into consideration the dynamic changes in the surrounding environmental conditions of the conductors. Implementation of real-time thermal rating of existing power systems could result in greater installed capacities of distributed generation (DG). This research aims to provide new insights into the thermal state of overhead line conductors (OHL) and underground cables (UGC) by using Computational Fluid Dynamic methods. An algorithm consists of building the geometry of the calculation domain, meshing, choosing a model, inputting initial conditions, initiation of the calculation, and analysing results. A part of the UK power system was chosen by Scottish Power Energy Networks for monitoring essential data of OHL conductors in order to validate results of the temperatures of the conductors.

Small-scale renewable energy systems are becoming increasingly popular due to soaring fuel prices and due to technological advancements which reduce the cost of manufacturing. Solar and wind energies, among other renewable energy sources, are the most available ones globally. The hybrid photovoltaic (PV) and wind power system has a higher capability to deliver continuous power with reduced energy storage requirements and therefore results in better utilization of power conversion and control equipment than either of the individual sources. Power conditioning units (p.c.u.) for such small-scale hybrid PV-wind generation systems have been proposed in this study. The system was connected to the grid, but it could also operate in standalone mode if the grid was unavailable. The system contains a local controller for every energy source and the grid inverter. Besides, it contains the supervisory controller. For the wind generator side, small-scale vertical axis wind turbines (VAWTs) are attractive due to their ability to capture wind from different directions without using a yaw. One difficulty with VAWTs is to prevent over-speeding and component over-loading at excessive wind velocities. The proposed local controller for the wind generator is based on the current and voltage measured on the dc side of the rectifier connected to the permanent magnet synchronous generator (PMSG). Maximum power point tracking (MPPT) control is provided in normal operation under the rated speed using a dc/dc boost converter. For high wind velocities, the suggested local controller controls the electric power in order to operate the turbine in the stall region. This high wind velocity control strategy attenuates the stress in the system while it smoothes the power generated. It is shown that the controller is able to stabilize the nonlinear system using an adaptive current feedback loop. Simulation and experimental results are presented. The PV generator side controller is designed to work in systems with multiple energy sources, such as those studied in this thesis. One of the most widely used methods to maximize the output PV power is the hill climbing technique. This study gives guidelines for designing both the perturbation magnitude and the time interval between consecutive perturbations for such a technique. These guidelines would improve the maximum power point tracking efficiency. According to these guidelines, a variable step MPPT algorithm with reduced power mode is designed and applied to the system. The algorithm is validated by simulation and experimental results. A single phase H-bridge inverter is proposed to supply the load and to connect the grid. Generally, a current controller injects active power with a controlled power factor and constant dc link voltage in the grid connected mode. However, in the standalone mode, it injects active power with constant ac output voltage and a power factor which depends on the load. The current controller for both modes is based on a newly developed peak current control (p.c.c.) with selective harmonic elimination. A design procedure has been proposed for the controller. Then, the method was demonstrated by simulation. The problem of the dc current injection to the grid has been investigated for such inverters. The causes of dc current injection are analyzed, and a measurement circuit is then proposed to control the inverter for dc current injection elimination. Characteristics of the proposed method are demonstrated, using simulation and experimental results. At the final stage of the study, a supervisory controller is demonstrated, which manages the different operating states of the system during starting, grid-connected and standalone modes. The operating states, designed for every mode, have been defined in such a hybrid model to allow stability and smooth transition between these states. The supervisory controller switches the system between the different modes and states according to the availability of the utility grid, renewable energy generators, the state of charge (SOC) of energy storage batteries, and the load. The p.c.u. including the supervisory controller has been verified in the different modes and states by simulation.

Axial flux permanent magnet generators are of particular interest for power generation in harsh and confined conditions. Due to their compactness and high power density, the ventilation and cooling inside axial flux permanent magnet generators have becoming increasingly important for further performance improvement. This thesis describes the developments of a lumped parameter, thermal modelling technique for axial flux permanent magnet generators. The main aim of this research is to develop a fast and accurate thermal modelling tool which can be used for rapid machine design and ultimately, to replace complex and time consuming CFD analyses in the machine design process. The thesis illustrates the construction of a generic thermal equivalent circuit, which comprises of conductive and convective sub-circuits, to model the conduction and convection heat transfers and temperature distributions in the radial and axial directions, within these machines. The conduction heat transfer between the solid components of these electrical machines is modelled by an annulus conductive thermal circuit derived from previous researchers; whereas, for convection heat transfer between the working fluid (air) and solids, the author has developed two convective thermal circuits, which are demonstrated as the Temperature Passing Method (TPM) and Heat Pick-up method in (HPM) in the thesis. Several case studies were designed to investigate the validity and accuracy of these thermal sub-circuits with both steady and transient boundary conditions. Since all the thermal impedances and capacitances used in the thermal circuits are in dimensionless form, the developed generic thermal equivalent circuit is capable of performing thermal simulations for axial flux generators of different sizes and topologies. Furthermore, special correction factors were introduced into the developed generic thermal equivalent circuit, to take into account the heat transfer in the circumferential direction in axial flux machines. The thesis also demonstrates how the heat transfer in the stator windings is modelled in the generic thermal equivalent circuit. Two analytical models, which are the Simple Concentric Model (SCM) and Concentric-annulus Layer Model (CLM) were developed, for the evaluation of the thermal resistances of the stator windings. The results evaluated from these analytical models were validated by several numerical models and experimental results of two-phase materials published by previous researchers. Lastly, experimental validation of the lumped parameter thermal equivalent circuit model and CFD simulations was conducted. Heat transfer coefficient measurements were carried out on two separate test rigs, which were a simplified single-sided axial flux machine test rig and a large-scale low speed axial flux machine. The experimental results were compared with the numerical results obtained from both the lumped parameter and CFD models. Good agreement between the experimental, lumped parameter model and CFD results were found. These indicate that the developed generic thermal circuit is potentially capable of replacing CFD analyses in the axial flux machines design process.

This thesis examines the contribution of computational modelling to the development of the tidal current energy industry, against the background of increasing commercial, government, academic and public interest. It does this through the practical application of a number of computational techniques in the areas of: 1. Tidal current analysis and prediction 2. Hydrodynamic flow modelling 3. Tidal resource analysis 4. Optimised economic modelling Appropriate survey set-up is essential in gathering data. Given this, processing the data using velocity profiles; statistical techniques; and harmonic analysis can produce valuable data for site development, device design and grid management. This work developed the application of a directional and time-dependent power coefficient and demonstrates its importance in resource evaluation from tidal flow data. It further concludes that hydrodynamic flow modelling of sites prior to development is important in determining suitable sites, given the scarcity of tidal information in the areas suitable for tidal developments. The same scarcity of data, in terms of boundary conditions, interior validation points and depth does limit the accuracy of such models. The work demonstrates that using differing resource analyses can obtain dramatically different results; and develops a correlation relating energy extraction to developed energy extraction using a one dimensional channel model. In doing so it concludes that energy resource estimates may be reduced from contemporary estimates. Overall, computational modelling of tidal current energy conversion systems can have a significant contribution to their design and site development. The most significant capital costs arise from installation, decommissioning and the turbine itself, however significant reduction in the cost of energy production can result from correct placement, array size and component selection This work contributes to knowledge in a number of areas, namely: 1. It is the first published work on survey data analysis prior to deployment of a large-scale prototype tidal current energy conversion system; 2. At the time that the work was carried out, it was the first published work considering the use of the least-squared harmonic method for prediction of energy output from a tidal current energy device; 3. It is the first work to propose a directional power coefficient in the process of resource analysis for a tidal current energy conversion system; 4. The work on economic modelling was the first to produce an optimised economic model for tidal current energy conversion systems (TCECS); 5. It is the first work to use an optimised economic model for TCECSs to demonstrate the effect of device placement on the cost of energy produced; 6. It is the first work to use an optimised economic model for TCECSs to demonstrate that the cost of energy for TCECSs is minimised by maximising the rated power, given no topographical impedence; 7. It proposes a method to determine the energy resource available including energy extraction.

XLPE cable peelings taken from underground High-Voltage cables that had previously been stressed under different ac electro-thermal conditions are investigated, with the aim of identifying properties indicative of the changes brought about by ageing. The inherent endurance ability of these peelings has been tested for three different endurance conditions of high ac electrical and thermal stresses. Space charge measurements via Pulse-Electro-Acoustic and Thermally-Stimulated-Currents are utilised to evaluate the state of the peelings before and after the endurance test in comparison with unstressed material. It was found that the pre-stressing altered the inherent endurance capability of the peelings especially for materials that had experienced thermal and electro-thermal stressing as a cable. The space charge behaviour also showed changes due to the stressing, related to the accumulation and transport of charge, with ac-electrical stressing reducing the time for heterocharge accumulation at the anode and AC electro-thermal stressing facilitating positive charge injection and propagation to the cathode. A double positive peak near one electrode was identified as indicative of irreversible degradation.

Distributed generation in the form of DC microgrids has recently attracted increasing research interest. For integrating primary sources and energy storage devices to the DC bus of a DC microgrid power electronic converters are necessary, but the associated losses may degrade the microgrid efficiency. Therefore, the aim of this work is to develop high-efficiency converters, particularly for fuel cell generators and ultracapacitors energy buffers suitable for use in a stationary distribution system. Based on the evaluation of the fuel cell dynamic performance, a current–fed DC–DC converter design with a lower voltage rating of the switching devices and a higher DC voltage conversion ratio is proposed. A number of optimisation approaches have been applied to further improve the converter efficiency over its full power range. The periodic steady state operation of the converter is analysed in detail; state-space averaging is then used to determine the small signal equations and derive transfer functions. A closed loop controller has been designed and verified by a novel PSpice/Simulink/actual processor co–simulation approach, where the modelling results are validated by experimental results using a model–based design method. To sustain the charging and discharging states of the ultracapacitor, a bidirectional DC–DC converter is required. Based on a comprehensive overview on different DC–DC converter topologies, the research presented here has shown that, bidirectional voltage–fed topology is better suited for dealing with the fast dynamic response of the ultracapacitor. But for a wide input voltage variation, this topology exhibits a higher circulating power flow and higher conduction losses as a consequence. Therefore, a detailed analysis of the bidirectional converter exploring the impact of the circulating power flow interval is developed in this study. Analytic methods have been applied to establish the optimal operation of the bidirectional voltage–fed converter for an ultracapacitor to improve its performance and efficiency. Based on these methods, a novel modulation scheme is proposed that minimises the circulating power flow in the converter, that has been verified by detailed simulation.

This research study focuses on the dielectric response of XLPE model power cables that have combinations of homo- and co-polymer insulation with furnace and acetylene carbon black semicon shields. Three dielectric spectroscopy techniques, which are frequency response analyzer and transformer ratio bridge in both frequency domain, and charging/discharging current system in time domain, were jointly used to measure the low loss XLPE cables in the frequency range from 10-4Hz to 104Hz at temperatures from 20°C to 80°C. Degassing effects and thermal ageing effects have also been studied with the spectroscopy techniques. Thermal-electric behaviour and maximum voltages for thermal breakdown have been theoretically simulated for the model cables. Three loss origins of the XLPE cables have been found with different loss mechanisms. Conduction loss due to thermally activated electron/hole hopping dominates the lower frequency range from 10-4Hz to 1Hz; Semicon loss due to its in series resistance with the insulation layer in cable equivalent circuit dominates the higher frequency range from 102Hz to 104Hz; intrinsic polarization loss of the XLPE insulation has dominant flat loss spectra in the mid-frequency range from 1Hz to 102Hz. Degassing was found to decrease the conductivity of the model cables, while thermal ageing greatly increased the conductivity. Thermal-electric simulation results with FEMLAB have shown that the position of maximum field changes from inner to outer insulation boundary under higher applied voltages. A loss mechanism model with mathematical expression for dielectric loss spectrum calculation is finally proposed to explain the total dielectric loss of polymer power cables.

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