Power transmission systems are seeing significant changes with the introduction of large amounts of renewable energy. Integration of these into the network is challenging on two fronts. Firstly, increased penetration stresses conventional generation as those from renewable are intermittent and fluctuate in power output. Secondly, many of these sources are located in areas which are difficult or impossible to economically connect to the network using convention ac technology. This is a major issue for offshore wind where distances to shore are increasing, particularly in the North Sea region. Large scale, multi-terminal high voltage dc networks may offer a solution to these issues, and modern voltage source converters have a small enough footprint to be located on offshore platforms. However, during dc faults these suffer from high currents and the system is unable to transfer power. Prototype dc circuit breakers have been developed by manufacturers to, in theory, allow seamless operation of the healthy areas of the network when a fault occurs. In order to do this they operate extremely fast; opening in less than 5ms. To achieve this the topologies have become complex and expensive, hindering development of multi-terminal systems. In this thesis the requirements of the converter, dc breakers and overall HVDC network are reassessed. The factors influencing stress within the converter and breaker are quantified. Design adjustments which may be used to mitigate these are then investigated. Simplified circuit analysis allows approximations the stresses to calculated and used for initial design iterations. The impact these design alterations has on normal operation is then addressed. A multi-terminal system model is then used to assess how different circuit breaker topologies can effect the fault ride-through of the network. Fault detection and discrimination algorithms are implemented to ensure accurate representation of the time overhead this incurs. It is shown that the variation between slower, cheaper circuit breakers and faster, more costly circuit breakers is not as large as indicated in the current literature. Ride-through for slower circuit breakers is achieved for only a marginal increase in restart time, which is still below the operating speed of conventional ac breakers.

Offshore wind turbines are preferred rather than onshore ones for their numerous advantages, such as land saving, higher wind speeds and higher power generation. However, AC power transmission would fail to deliver the generated power economically over distances longer than 80 kilometres using submarine cables. The more feasible option is to use High Voltage DC (HVDC) power transmission for offshore wind generation. Unlike AC transmission systems that have established power and current flow control methods, DC power transmission systems have only reliable power flow control techniques for point to point systems, which makes it one of the challenges preventing realisation of Multi Terminal HVDC grids (MT-HVDC) as cables may be subjected to higher currents causing overloading and thermal problems. Different HVDC power flow control schemes are suggested by controlling the AC/DC converters such as voltage droop control and voltage margin control. Other methods of power and current flow control based on the connection of new power electronic equipment to the grid have been also proposed. This thesis presents operation and control of an IGBT based Current Flow Controller (CFC) for MT-HVDC grid applications. The CFC is studied in its preliminary two-port configuration and possible modes of operation and dynamic models are produced. An extended topology is proposed to allow the CFC to be connected to more than two cables at a time. Although the proposed extended CFC topology is simple in construction and gave acceptable results in most case studies, it has shown some drawbacks in certain case studies where controlled currents have significant differences in magnitudes. To resolve this problem, a generalized Modular CFC (MCFC) topology is proposed which allows each current to be controlled independently and overcome the extended topology’s drawback. Moreover, a reduced count switch count topology is proposed which reduces the MCFC cost by half in cases of unidirectional current flow control. All proposed control strategies and topologies are validated using both computer simulation through MATLAB/SIMULINK and PSCAD/EMTDC software packages and experimental validation through Rapid Control Prototyping (RCP) with the aid of Opal RT real time simulator. Studies carried throughout this thesis show that the proposed MCFC may play an important role in current flow control applications in MT-HVDC grids due to its low cost, small footprint and accurate performance.

This thesis describes the development of a novel high-voltage, high-speed Pulsed Electro- Acoustic (PEA) instrument for real-time measurements of space charge distribution in solid insulators subjected to DC, AC or transient voltages. Very high-rate PEA measurement is accomplished with a state of the art data acquisition module and a purpose-built high-voltage FET switch pulse generator. This advanced PEA also has excellent temperature monitoring and control. Cross-linked polyethylene (XLPE) peelings taken from un-aged power cable was used in this study. The average thickness of the XLPE sheet was 150 um. A measurement at 50 Hz power frequency was performed using the new PEA system for validation purposes. The study of DC electric field criteria for the formation of space charge packets in XLPE was carried out to determine the DC onset field for charge packet formation. The average DC applied electric field ranged from 80 - 120 kV/mm. The calculation of the internal electric field showed that the packets required a field of 140 kV/mm for their initiation. The generation of charge packets is governed by the field at the anode and the balance of charge injection, transport and extraction process. Researchers in Toulouse (France) conducted electroluminescence experiments the author and researchers from Bologna (Italy) performed space charge measurements using the PEA at various DC electric field. Complementary information was obtained from these two techniques. The correspondence between the pertinent values of the DC electric field obtained from space charge and EL support the existence of degradation thresholds in this particular XLPE material. Charge packet formation, propagation and EL results have been analysed. Charge recombination appears to be the excitation process of EL. Whilst continuous EL excitation can be initiated without the involvement of charge packets, oscillatory EL has been found, which corresponds to packet formation or annihilation.

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No description available.

Data centres have racks that include servers which support digital services. The growing demand on digital services such as streaming content, web services, telecommunications, financial and social media has led to an unprecedented growth in data centres and therefore their electrical power consumption. A significant portion of data centre energy is consumed by the cooling systems within data centres. Energy consumption of data centres can be reduced by improving their cooling systems. Some data centres are cooled by air cooling while others are cooled by liquid. The liquid cooling approach has greater opportunities for efficient cooling, particularly in high density data centres. The liquid cooling methods can be classified as rack heat exchanger, in server and on chip heat exchangers and total liquid immersion. The liquid immersed concept submerges the server in a dielectric liquid. This liquid is in direct contact with all of the microelectronics and it has properties that do not cause any electrical discharge. The heat from server components, such as the CPU, is transferred via natural convection in the dielectric liquid to water channels, which transport the heat. The main interest of this thesis is to study liquid immersed microelectronics for data centres. The thesis starts with validation of numerical and experimental work to develop the correct physics for immersed server simulation model. Investigations for optimal heat transfer within the servers include varying the fin height and cold plate height and the adoption of baffles on the server water jackets (similar to a cold plate). How such servers can be integrated into a data centre is also investigated. The overview results show that the heat transfer performance when immersing servers is improved when heights of the heat sink fins and distances of the cold plate are optimised. Baffles with a rectangular cross sectional area attached to the water jacket demonstrated the best results where the heat transfer can be improved by up to 71% compared to water jackets with no baffles. Extending these results into a data centre scenario showed that increasing the rack occupancy had a positive effect on the Power Usage Effectiveness (PUE), which indicates an improvement in the cooling system.

There is a growing recognition that current energy systems are unsustainable and require fundamental restructuring. While this is a significant engineering challenge it is not a matter of replacing one set of technologies with another. Technologies are embedded in a wider set of political, social and economic institutions which means that the transition from one energy system to another also requires an understanding of the interactions between the technical and non-technical. This thesis contributes to knowledge in this area by paying specific attention to energy distribution networks; the pipes and wires which deliver our energy services. It is argued that existing approaches to the study of transitions in energy systems have largely black-boxed the network components, tending to concentrate on production and demand. However, infrastructure networks have unique technical and institutional characteristics which require a more systematic treatment. Therefore the aim of the thesis is to make more visible the interplay between actors, institutions and technologies in reproducing and transforming energy distribution networks. For this purpose a novel analytical framework is developed which draws from economics and science and technology studies and incorporates insights from the governance literature. The framework is applied to the cases of electricity and heat distribution in the UK. It is found that following the liberalisation of energy systems, the governance of distribution networks has been siloed from the mainstream energy regime which has focused on promoting competition in other segments of the value chain. Therefore efforts to decarbonise energy supply in the UK have tended to be market based with a short term focus, rather than integrated solutions which recognise the role that flexible distribution networks can play in this transition. A number of policy recommendations are made which inform debates surrounding the development of local heat infrastructures and the reconfiguration of electricity distribution systems

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The construction of a European Super Grid, where existing AC systems are connected through High Voltage Direct Current (HVDC) corridors, is one contribution to accommodating a high penetration of renewable energy by diversifying the energy mix and trading surpluses from some regions. This raises questions over the dynamics of mixed AC and DC systems that need to be thoroughly studied. This thesis examines the level of coupling of AC networks interconnected via HVDC and the provision of system services between AC networks via DC links. Since areas such as the North Sea may see interconnected DC links, control of such multi-terminal DC grids (MTDC) in the case of converter outages is also pursued. To form realistic case studies, a number of mixed AC and DC transmission networks were modelled and validated, in particular a dynamic equivalent model of the Great Britain transmission system coupled via voltage source converter (VSC) HVDC to the Scandinavian transmission network. To answer questions over the interaction of the dynamics of modular multi-level converters (MMC) with AC system dynamics, reduced dynamic models of such converters were developed taking an energy balancing approach. The impacts that two-level or modular multi-level VSCs produce on the dynamics of the hosting AC grids were investigated through perturbation studies. The displacement of conventional generation by converter-interfaced sources sees a decrease in frequency response provision and inertia and calls for alternative provision. Several methods for enhancing the frequency recovery (post outage) of AC grids were designed for and applied to VSCs of HVDC interconnectors. Supplementary droop control was found to improve the frequency nadir after a loss of in-feed event at the cost of passing some of that loss to the adjacent AC systems via the HVDC interconnection. Identifying and exploiting the overload capability of MMC enabled the sharing of primary reserves and improvement of the nadir frequency by allowing extra power to be transferred through HVDC links. Proposals to use the internal energy storage capability of MMCs for inertia provisions were investigated but the results show that the scope for this is very limited. It is important that where MTDC grids are envisaged, the power flow control is able to deal with converter station outages and not cause large disturbance to the DC or AC networks. Deficiencies with proposed methods were identified and a new proposal for coordinated control with multiple bus masters was made and control performance improvements demonstrated.