Large scale grid integration of renewables and cross country border energy exchange may be facilitated by multi-terminal high-voltage direct-current (MTdc) grids. However, as the number of terminals and dc lines increases, power flow management between dc nodes becomes amajor challenge. The current carrying capability of dc lines is limited by their thermal and electric stress limits. Thus, the line current must be maintained within the permissible operational region to protect the lines fromdamages. This thesis addresses this fundamental issue through the introduction of inter-line current flow controllers (CFCs) into MTdc grids. An inter-line CFC is a low power rated controllable voltage source that can enhance system performance by suitably redirecting the current flow at the point of connection. It enables regulation of the dc line current flow by changing the voltage at the dc terminals where it is inserted. The research work presented in this thesis is aimed to realise the most feasible CFC topologies to facilitate flexible power flow between dc nodes. Themain contributions of this research work comprise of four parts, namely, (1) design and development of dc CFC topologies, (2) prototyping of the proposed CFCs (3) implementations of centralised and communication free control schemes for densely meshed dc grids, and (4) protection of inter-line CFCs. In the first phase, the characteristics, control and operation for five configurations of interline CFCs are studied, namely, resistive, RC circuit, capacitive, dual H-bridge, and single H-bridge based CFCs. A multi-port CFC is proposed to facilitate current regulation on multiple lines simultaneously. An experimental platform consisting of a three-terminal dc grid and small scale prototypes of the proposed CFCs have been developed to validate the concepts. It is clearly shown through experiments and time-domain simulations that all devices are capable of improving the system performance. A centralised hierarchical control system is proposed to coordinate the operation between multiple CFCs. A novel voltage sharing control scheme is demonstrated. It is shown that such ii Abstract scheme reduces the workload on a single CFC by sharing the required control voltage between multiple CFCs, and, additionally, can be used to avoid control conflicts among active CFCs during communication failure. New protection methodologies are implemented to protect the CFC during internal failures and dc faults. Small-scale dc circuit breakers have been developed to study the performances of 1B and 2B-CFCs under a pole-to-pole fault.

Currently, transmission network security is treated within a deterministic and preventive control framework where network constraints and balancing costs are not fully optimised. This may be inefficient and lead to increased levels of congestion, potentially increasing cost of integration associated with low-carbon generation. In this context, this thesis assesses the effects of coordinating FACTS and HVDC transmission systems under both deterministic and probabilistic security frameworks and assuming an array of available preventive/corrective control actions. An deterministic/probabilistic cost-benefit model has been developed for assessing the economic and reliability performance of alternative control strategies enabled by coordination of flexible devices. Results demonstrate that probabilistic security for flexible AC/DC devices coordination is significantly more beneficial since it can lead to lower operating costs, increased infrastructure utilisation and improved reserve allocation across GB network. The framework is further developed to explicitly consider the likelihood of post-contingency events and wind uncertainty. A tight MILP representation, two novelty transmission losses penalty items and scenario reduction model are proposed to reduce computational burden in the large-scale system. Results demonstrate the advantages of probabilistic security to improving the overall efficiency of the system when integrate uncertain renewable generation. Adding flexible network technology could even increase system costs under a preventive security approach. Finally, a novel two-stage stochastic optimization model that incorporates effect of dynamic line rating (DLR) on transmission network operation with increased penetration of wind generation is proposed. The stochastic model co-optimizes energy and reserve holding levels for the forecasted/expected condition along with re-dispatch actions under real-time operation. The benefits of using DLR are demonstrated in terms of overall cost minimization, albeit reserve levels may be increased due to DLR uncertainty. Results also demonstrate that reserve services can be shared among multiple uncertainties. The impacts of forecast accuracy and reserve price on the performance of DLR is discussed.

Connection of low carbon technology to the LV distribution network stresses the network voltage limits. In certain cases this is such that further low carbon technologies such as, distributed generation sources, heat pumps, and electric vehicles, cannot be accommodated without costly network reinforcement. One solution is use power electronics to regulate the voltage and so remove these network voltage constraints. This can defer or even remove the need for network reinforcement and so reduce investment costs for distribution network operators. This thesis examines how these power electronics can be installed into distribution network for the purposes of voltage control and capacity release in constrained networks. The efficacy of five power electronics solutions relative to both reinforcement and de-regulation (or relaxing) of voltage limits is compared in technical and economic aspects. As de-regulation requires knowledge of how the network loads function in response to supply voltage fluctuation an analysis of how de-regulation effects the loads connected to the grid was undertaken. This revealed there is more reliable operation of these loads with voltages well below the present UK limits, however their tolerance of over voltages is shown to be very limited. Following on from this, it was shown that, when coupled to changes to the nominal system voltages, reinforcement could be deferred by de-regulation. Testing and simulation of both motor loads and the magnetics in common power supply units was performed, were a method was outlined for consideration of the losses in a flyback transformer and a boost inductor when subject to supply voltages outside of their tolerance bands. Detailed models of distribution networks are needed to accurately quantify the befits of deregulation and PEDs, so three detailed test networks where assessed in terms of their hosting capacity for low carbon technologies. A load model was presented for load flow studies that enables the model to be used in scenarios where the supply voltages are subject to wide voltage fluctuations. Additionally, the effect of thermally controlled goods was also considered by implementing either constant energy or dynamic thermal model into these goods. Finally to aid in assessment of the huge variance in LV network topology, metrics were developed to indicate the relative constraints a given network will likely face. These where seen in the cases test to be in good agreement with the results of the substantially more time consuming load flow methods.

Cable termination is the weakest part in any HV underground cable system, as defects may be left in the main insulation during the installation process. In HVDC systems, the converters produce the intended DC voltage for transmission but there may also be AC harmonics superimposed. The frequency of the AC harmonics could be in kHz range. The superimposed harmonics on the HVDC may have synergistic effects on cable insulation and may lead to further degradation in the cable insulation. In addition, for the sake of environmental protection, XLPE is no longer the suitable choice for underground cable insulation as it cannot be recycled. Thermoplastic material may be a candidate. There is little published information on the behaviour of thermoplastic materials under combined AC and DC voltages. In thermoplastic materials, HDPE film and PP film were selected as the target material to study in this project. As the properties of HDPE are similar to those of XLPE, the data obtained from HDPE was regarded as a bench-mark. It was decided to use the thermo-electrical stress to age the samples. For thermal stress, 90°C was chosen as the aging temperature for HDPE, while 90°C and 110°C were chosen for PP. For the electrical stress, AC & DC combined voltage was used to age the samples. DC voltage was 6 kV. The superimposed frequency of AC voltage was 1 kHz, 1.5 kHz, 2 kHz and 2.5 kHz. The AC/DC voltage ratio is from 10%, 30% and 50%. In this project, the effect of frequency and voltage ratio on HDPE and PP degradation under superimposed stresses was studied using the following approaches: Equivalent Phase Resolved Partial Discharge (PRPD) plots, Fourier Transform Infrared - Attenuated Total Reflection (FTIR-ATR) Spectroscopy and Dielectric Spectroscopy (DS) measurements were carried out. The performance of HDPE and PP were compared.

The process of generation, transmission and distribution of electric energy appears to be very simple. However, the continuous flow of power through electric networks is affected by several factors, normally of two kinds: failures caused by ageing of insulating materials and failures caused by defective equipment or design. The energy supplied to small and big consumers can be interrupted for long periods causing disastrous economic losses. The origin of most failures normally is found in electrical insulation, which is an essential component in all electrical systems especially in power equipment such as transformers and circuit breakers, the cost of which is very high. Before the insulation fails, there are manifestations of this condition, called partial discharges (PD) and because of this, PD have been subject of an extensive study and investigation. PD identification relies on the intrinsic characteristics of the generated discharge. This thesis shows the behaviour of radiated discharge through the analysis of the characteristic frequency spectra and pulse distribution to create pattern classification in the laboratory. This research is based on a novel technique that uses wide band, high resolution equipment, composed of an antenna array, that receives the radiated discharge, and a high resolution oscilloscope that accurately measures and samples the propagated signal. The second part of the laboratory setup comprises the characterization of partial discharges using a high voltage transformer and measurement devices that accurately apply a range of voltages to specimens containing insulation defects. The advantages of having accurate PD sources are evident: availability, free of ambiance noise, accurate AC reference and PD location. Results show that pattern recognition to identify PD'S using a non invasive system and point on wave information is possible and reliable.

Under the title of Voltage Stability Analysis and Control, three major subjects have been examined in this PhD project: the fundamental study of voltage stability, the on-line prediction method for voltage collapse, and secondary voltage control systems. The fundamental study was aimed to lay a theoretical foundation for the rest of research in this project. The on-line prediction method and secondary voltage control systems are particularly targeting on the "early prediction and prevention" strategy to tackle the rapid and "uncontrollable" nature of the voltage collapse phenomenon. The work on the fundamental study was presented in chapter 3 of this thesis. The basic characteristics of voltage collapse were examined, the theory concerning the voltage stability determining factors (VSDFs) was established. Based on this fundamental study, a knowledge based system for the on-line prediction of voltage collapse was proposed in chapter 4. The pattern recognition technique was used in this prediction system, and the design and development of such a system were intensively discussed in this chapter. As a well recognised prevention measure to voltage collapse, secondary voltage control systems were systematically investigated in chapter 5,6, and 7 of this thesis. Chapter 5 deals with the principle of secondary voltage control, and the design and analysis of such a system by classic and optimal control were presented in chapter 6 and 7 respectively. As a further study, improving voltage stability through generation dispatch was also briefly discussed at the end of this thesis. An algorithm aimed at this purpose was proposed to determine the generation participation pattern upon system load increase. All the studies in this project were simulated on the standard IEEE test power systems, some of the study results have been published in the different international conferences or academic forums.

Pri.or to 1990, and the advent of deregulation in the UK, the Electricity Supply Industry (ESI) was a state-owned, vertically integrated monopoly. For the first time the way was opened for the introduction of competitiveness into the many and varied aspects ofthe market. This brought with it diversity and additional requirements for customer and engineering services. This new business-driven, customer-orientated environment led to the re~assessment of the utility's customer communication systems. In order to take full advantage of this new environment, an integrated communications and information systems infrastructure is required. This thesis addresses the design and development of an efficient and reliable communications system for the collection and distribution of data within the ESI. Although the communications architecture proposed focuses on meeting the requirements of a specific Regional Electricity Company, SWEB, it is generic and hence may be used in any electric utility. This architecture, based on an open systems approach, provides bearer services to existing and emerging engineering and business applications. The open system aspect permits the deferred design of system components, thereby providing maximum system flexibility and simplicity. An overview ofthe operational systems in the ESI as they existed in 1997 is given. This review considers the power engineering and distribution networks, metering and tariffs, together with SWEB's specific IT and data communication systems. There then follows a review ofthe business and technical environments in the industry, in terms ofthe potential benefits of value-added services and the emerging applications, together with a brief analysis of the post-privatisation growth ofthe telecommunications industry in the UK. A review was carried out on the media used for different communication services involving Power Line Carrier technology (PLC), Telephony, Fibre Optic Cable (FOC), Twisted Pair and Coaxial cable. These media are examined as to their suitability for data communications for the electric utility industry. Various modulation techniques including AM, FM and Spread Spectrum systems were investigated with respect to their transmission characteristics, and a comparison made ofthe modulation techniques used in PLC systems. The 1990s saw considerable interest in utility communications resulting in many trials being carried out throughout the world. A number ofthese are reviewed in this thesis in support of the proposed architecture. The cardinal factor in the design and development of any modern communication system is the compliance with internationally agreed standards and recommendations. The Open Systems Interconnection model is introduced. together with the standards, protocols and profiles particularly relevant to utility communications. The Utility Communications Architecture (UCA) profile, developed by the Electric Power Research Institute (EPRI) is also introduced. The programme ofwork outlined in this thesis was driven by two objectives. The first of these was the aim of utilising the considerable copper infrastructure in the ESI distribution systems for communication purposes. The second was to investigate the feasibility of the provision ofvalue-added services. The generic architecture presented here, based on powerline carrier and other media technology, clearly satisfies the first ofthe above objectives. However PLC inherently lacks the bandwidth for some ofthe value-added services considered, and this may lead slow data transfer rates for certain applications such as downloading large text, image and audio files from internet websites.The second objective was pragmatically modified to address the problems of Remote Metre Reading. To this end a protocol is proposed which requires a minimum data rate transfer thereby conserving network bandwidth.

Conduction and space charge theories in dielectrics under direct voltage conditions are reviewed and analysed. Published information on gas discharge processes in insulation, conductivity, discharge detection and measuring methods are also summarised. A combined technique whereby both discharges and current can be studied simultaneously is employed in the investigation of non-ohmic conduction in solid dielectrics at moderate d.c. stresses. Experimental evidence suggests that none of the conventional laws of conduction adequately explains the observed phenomena e.g. the thickness dependence of conductivity; and that the operating stress 'E' calculated as the ratio of the externally applied voltage to the thickness of the dielectric may not be the actual resultant stress. Some anomalies are explained on the basis of a new dielectric equivalent circuit, a modified Maxwellian 'n' strata dielectric and induced interfacial or space charges. The discharge behaviour of gaseous voids in dielectrics is analysed using an equivalent circuit incorporating the resistivity of the void gas. Extinction is explained partly by an extended Maxwellian polarisation phenomena.

Polymers used as electrical insulators in high voltage (RV) plant are subjected to inherent design and fault level stresses during service. As a result of these stresses the materials degrade and deteriorate, reducing equipment life and lowering reliability of power systems. Defects in polymeric insulators will result in partial discharge (PD) activity, potentially leading to plant failure. PD is both a symptom of a fault in insulation and the source of various stress mechanisms, that cause further material degradation. It is important to understand correlations between PD signal parameters, discharge induced degradation and defect type. In this thesis PD activity and subsequent material degradation for void defects in different polymeric insulators are studied. The ability to relate identifiable specific signal characteristics at a fault would aid asset managers with assessment of the insulation degradation. This project measures and identifies differences in characteristics of PD and PD induced material degradation in different types of void defects to quantify the material physical conditions. The results of this research correlate changes in the PD patterns for single void and multiple void arrangements with induced chemical and physical changes on the surface of the void. This work also shows that PD activity in air voids in polymeric insulators is dependent on the void shape, size and sample construction. This thesis investigates the relationship between the PD characteristics and the induced material degradation in four different polymer materials, to aid development of mechanisms to enable prediction of the level of degradation of a known polymeric insulator by measuring its PD characteristics and vice-versa. The results show that there are identified differences in PD activity and subsequent chemical and morphological changes for the four different polymers due to difference in polymer structure, chemistry, morphology and ability to extract initiating electrons from the material surface. Most of the experiments conducted in past research work on PD in voids have been perfol1ned using self-contained internal voids. This work compares PD characteristics and material degradation in unvented and vented voids. This investigation enables analysis of the influence of vents on PD processes in voids. The results show that that there is a sharp increase in the PD activity and subsequent void surface changes over time for the unvented voids, whereas the PD activity for vented voids increase very slowly over time.

Microshocks can be described as the unpleasant discharges experienced by people capacitively coupled to high voltage equipment once a contact is established between their finger-tips and another object at different potential. The situations in which these discharges occur are complicated and very limited research has been conducted so far in this area. The work presented in this thesis aims to enhance the understanding of the microshocks phenomenon, and in particular, focuses on exposure to microshocks by overhead transmission line workers before developing mitigation techniques to prevent the linesman from receiving microshocks or at least minimising the sensation level associated with receiving the discharges. Results of a survey that included 102 linesmen and conducted to determine the implications of different factors on the frequency and intensity of microshocks are presented. Experimental setups and activities that took place inside and outside the High Voltage laboratory to simulate the environment at which linesmen became subjected to microshocks are discussed. Initially, a simplified 2-dimensional metallic test object "Stickman", which simulates a human body, was placed in various positions relative to a lattice tower structure inside the laboratory. Measurements of induced voltages and currents on the object as a result of capacitive coupling have been made. To accurately represent the structure and shape of the human body, a 3- dimensional test object "Cylinderman" has been designed and introduced to the experimental setup replacing Stickman. Experiments have been modelled successfully using the commercial software package CDEGS (Current Distribution and Electromagnetic Interference, Grounding and Soil Structure Analysis). The software package is shown to be capable of accurately modelling simple objects made of thin conductors as well as complicated structure such as the transmission tower. Furthermore, models of realistic and complicated scenarios under which linesmen experience Microshocks have been developed. Discharge voltage waveforms recorded of human subjects experiencing microshocks have been captured successfully. By examining these waveforms it was concluded that skin impedance dictates the level of voltage drop during discharge. In addition to investigating the effectiveness of earthing the body through the footwear to prevent microshocks from occurring, experiments that lead to the identification of a novel mitigation technique are detailed. The technique aims to increase the charging capacitance to ground -represented originally by the shoe capacitance- to reduce the induce voltage on the individual. Details of a filed patent are also included.