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Emanating from the proceedings of CIGRE 2004, a new idea for higher power transmission by recycling and up rating high voltage alternating current transmission lines for high voltage direct current application was presented at the HYDC working group session. To date, there is no known application of the idea. Globally, transmission congestion, power transfer bottlenecks with restricted and limited power transfers and unobtainable servitudes challenge electric power utilities. The literature review shows that since the early sixties, several authors have studied this proposal. However, no applications were done. Admittedly, early HYDC technology was troubled by problems with multi-terminal designs, external insulation breakdown in the presence of DC stress and mercury valve rectifiers struggled with arc backs. To date, power electronic and external insulation technology has grown and matured for confident application both in point to point and multi-terminal application. The economic costs of introducing the DC technology are also more affordable given reducing prices due to higher volume of purchases. With promising developments in insulation and power electronic technology and driven by South Africa's surging growth in the consumption of electrical energy; the subject of upgrading HYAC transmission for HYDC application is revisited. For the research, the emphasis is beyond FACTS and towards a solution that could develop into a new supergrid that could overlay the existing national grid. Thus, the solution is prepared specifically for the case of recycling existing assets for higher power transfers. The working environment is defined by the difficulty in acquiring new powerline servitudes, transmission congestion in complex networks, the need for electrical islands within complex interconnections, and the need for enhanced power system stability and to promote new ancillary services energy management. The focus of this research study was to determine the technical feasibility of upgrading of existing HYAC circuits for HYDC application. It is assumed that the transmission line will remain as is in structure, layout and mechanical design. The changing of external line insulators using live line technology is an accepted modification to the original HYAC line, if required. From the study, we conclude that not all HYAC lines are recommended for upgrade to HYDe. We introduce boundary conditions as a first step towards checking on the suitability of the proposed upgrade from HVAC to HYDC mode. Emanating from this study, the first paper published introduced the initial boundary conditions as being only those lines where the "unused gap" between surge impedance loading and conductor current carrying capability is appreciable and large; generally three to four times surge impedance loading. In the case where the unused gap is the smallest or negligible, then we do nothing. In between, where the unused gap is about two to three times the surge impedance loading, then we can consider active or passive compensation using the HVAC FACTS technology options as proposed by EPRl. Having determined the candidate transmission line configuration for the proposed upgrade to HYDC application, we select the DC operating voltage as based on the voltage withstand capability of external insulation for varying environmental conditions. In addition, the DC voltage will generate allowable electrical fields and corona effects within and outside the transmission servitude. The optimum DC operating voltage would satisfy the conditions of minimum transmission power losses and volt drop for the case of maximum power transfers; within the limits of electrical fields and corona effects. / Thesis (M.Sc.Eng.)-University of KwaZulu-Natal, Durban, 2007.

The main aim of this study was concerned with the design and commissioning of a corona cage, which could be used under Direct Current (DC) conditions. The cage was designed based on empirical formulas and equations as well as electric field simulations. The designed cage was then fabricated. The commissioning of the cage was undertaken in the High Voltage Direct Current (HVDC) laboratory at the University of Durban - Westville (UDW). Tests to determine the effects of a silicone coating as well as wind on the corona performance of conductors were undertaken. The tests were done in order to determine ways of improving the corona performance of conductors under HVDC potential. The tests were carried out using various conductor surface conditions. The wind tests were made possible by using a powerful fan. A silicone coating was also used to determine the effects that it would have in mitigation of corona activity on HVDC conductors. The conductors were tested without the coating, with half of their length coated and then fully coated. Results showed that the effect of wind on corona generation in a corona cage is minimal. The effect of the silicone coating was that it increased the corona currents measured in the corona cage. The conductors with no coating generated the lowest currents, the half coated conductors generated the second highest measured currents and the fully coated conductors generated the most corona. Analysis of the increased currents showed that the increase in corona currents due to the silicone coating could be attributed to three factors. Firstly the coating caused an increase in conductor to cage capacitance. Secondly, partial discharges could have occurred in the silicone due to microscopic air particles and lastly, the increase in corona currents could be ascribed to the effect of the boundary conditions on the boundary between the conductor and the coating. / Thesis (M.Sc.)-University of Durban-Westville, 2003.

This thesis describes a research investigation into novel designs of high voltage pulse transformers using magnetic insulation, which is the only practicable form of insulation for much of the equipment presently used in ultrahigh voltage pulsed-power work, including transmission lines and plasma opening switches. Although its use in transformers would bring important advantages in both size and weight reductions, a number of seemingly insurmountable problems have however so far prevented this. Two novel arrangements are presented in this thesis: one of these is a 500 kV transformer with self-magnetic insulation, and the other one is a 1 MV 'Tesla' transformer with external magnetic insulation. It is shown that both of these overcome the problems inherent in earlier designs and also offer considerable scope for further development in a number of important areas. It is believed that they represent the first working examples of magnetically-insulated transformers anywhere in the world. Modelling considerations of the transformers developed include both theoretical models and predicted characteristics. The filamentary technique used to describe mathematically the arrangements being investigated involves decomposition of the main conducting components into filamentary elements. The resulting equivalent electrical network includes all the mutual interactions that exist between the different filamentary elements, takes magnetic diffusion fully into account and enables the resistances and self and mutual inductances that are effective under fast transient conditions to be calculated. Theoretical results provided by the resulting mathematical models have been successfully validated by comparison with reliable experimental data. Much of the work detailed in the thesis has already been presented in high quality academic journals and at prestigious international conferences, and a solid theoretical and experimental basis has been laid down for future development and new progress into pulsed power system research.

Η έντονη τάση για συνεχή επικοινωνία και μετάδοση πληροφορίας, οδηγεί στην ανάγκη για αύξηση των τηλεπικοινωνιακών υπηρεσιών. Γι’ αυτό η προσπάθεια εύρεσης λύσης και νέων δρόμων για την εξυπηρέτηση των τηλεπικοινωνιακών αναγκών αποτελεί βασικό στόχο και σκοπό. Η οπτική ίνα είναι ο πιο αξιόπιστος τρόπος για την μετάδοση δεδομένων. Τα πλεονεκτήματα της οπτικής ίνας, καθώς και το γεγονός ότι υπερτερεί του χάλκινου αγωγού, οδηγεί στην εξ’ ολοκλήρου αντικατάσταση του δεύτερου από την ίνα. Οι οπτικές ίνες καλύπτουν τις ανάγκες για αύξηση των ταχυτήτων μετάδοσης και παρουσιάζουν μεγάλο εύρος ζώνης. Το 1854 διατυπώθηκε η αρχή της ολικής ανάκλασης από τον John Taydall, , με βάση την οποία λειτουργεί η οπτική ίνα και το 1950, πήρε την πρώτη της μορφή με την ανάπτυξη της γυάλινης ίνας με ένα είδος επιχρίσματος (glass – coated fiber). Τότε χρησιμοποιήθηκε για πρώτη φορά ο όρος «fiber optics» (οπτική ίνα). Έπειτα, το 1990 εξαπλώθηκε η χρήση της σε τηλεπικοινωνιακά δίκτυα. Τα πλεονεκτήματα των οπτικών ινών και δικτύων πυροδότησαν ισχυρά κίνητρα, ώστε οι Εταιρείες Ηλεκτρικής Ενέργειας να επιθυμούν να εισέλθουν στην τηλεπικοινωνιακή αγορά και να διεκδικήσουν μερίδιο από αυτήν. Σκοπός της διπλωματικής εργασίας είναι η αναφορά στις εξελίξεις της τεχνολογίας των οπτικών ινών που μπορούν να προσφέρουν λύση με την εφαρμογή τους σε γραμμές υψηλής τάσης συνδυάζοντας απαιτήσεις ασφάλειας, κόστους και ποιότητας που είναι απαραίτητες για τις τηλεπικοινωνιακές ή ενεργειακές υπηρεσίες. Η διπλωματική εργασία απευθύνεται σε άτομα τα όποια αναζητούν μια συνολική εικόνα των τεχνικών λύσεων στην τεχνολογία των οπτικών, οι οποίες ενσωματώνονται σε γραμμές υψηλής τάσης. Στόχος της είναι να εξοικειώσει τον αναγνώστη με τις έννοιες της οπτικής ίνας, να του δώσει το απαραίτητο υπόβαθρο για την κατανόησή τους και μέσα από την ανάλυση των τεχνικών λύσεων, που περιγράφονται στην εργασία αυτή, να του παρουσιάσει τα χαρακτηριστικά των οπτικών καλωδίων καθώς και τους νέους ορίζοντες που ανοίγονται από την εφαρμογή των οπτικών καλωδίων σε γραμμές ισχύος. Στο δεύτερο κεφάλαιο δίνονται οι ορισμοί και βασικές έννοιες των οπτικών ινών. Παρουσιάζονται οι τύποι των οπτικών ινών διακρίνοντάς τις σε μονότροπες, πολύτροπες ίνες βηματικού δέκτη, πολύτροπες ίνες βαθμιαίου δέκτη, ανάλογα με την κατανομή του δείκτη διάθλασης. Αναφέρονται τα χαρακτηριστικά και οι επιδόσεις κάθε τύπου οπτικών ινών. Επίσης γίνεται μια εκτενής αναφορά στην κατασκευή και στα υλικά των ινών. Οι διαδικασίες κατασκευής είναι η διεργασία OVPO- Πλευρική εναπόθεση, διεργασία OVPO- Αξονική εναπόθεση (VAD), διεργασία MCVD ή IVPO, διεργασία PCVD και η διεργασία απευθείας τήξης. Περιγράφεται η αρχή λειτουργίας της οπτικής ίνας και αναφέρονται τα μηχανικά χαρακτηριστικά της, εφόσον τα οπτικά καλώδια υποβάλλονται σε μικροκάμψεις, τάσεις εφελκυσμού και παραμορφώσεις. Τέλος αναφέρονται αναλυτικώς, τα πλεονεκτήματα και τα μειονεκτήματα των οπτικών ινών. Το τρίτο κεφάλαιο αποτελεί την πρώτη ενότητα όσον αφορά στην ανάλυση του οπτικού καλωδίου. Παρουσιάζονται τα μέρη από τα οποία αποτελείται, τα υλικά από τα οποία κατασκευάζονται τα μέρη αυτά καθώς και οι ιδιότητες των υλικών, όπως μηχανικές και θερμικές ιδιότητες καθώς και επιδράσεις με αλλά υλικά. Επίσης παρουσιάζονται οπτικά και μηχανικά χαρακτηριστικά που έχουν άμεση σχέση με τα υλικά των καλωδίων. Τέλος, δίνονται και διάφορες τεχνικές λύσεις αναφορικά με τα μέρη του καλωδίου και τα χαρακτηριστικά του. Στο τέταρτο κεφάλαιο αναφέρονται αναλυτικά οι τεχνολογίες οπτικών ινών σε γραμμές υψηλής τάσης. Κατηγοριοποιούνται σε οπτικά καλώδια για εναέριες γραμμές υψηλής τάσης, οπτικές ίνες συγχωνευμένες σε υποβρύχια καλώδια και υπόγεια καλώδια. Αναλύονται ξεχωριστά τα οπτικά καλώδια για εναέριες γραμμές, δηλαδή ο οπτικός αγωγός γείωσης (OPGW), τα προσαρτημένα καλώδια (OPAC) και τα αυτοφερόμενα καλώδια (ADSS). Περιγράφεται κάθε οπτικό καλώδιο ξεχωριστά και αναφέρονται οι διάφοροι τύποι κατασκευών κάθε καλωδίου, τα ηλεκτρικά και μηχανικά τους χαρακτηριστικά καθώς , η εγκατάστασή τους και οι απαραίτητες ενέργειες για την προστασία τους. Δίνεται ο παρελκόμενος εξοπλισμός των εναέριων οπτικών καλωδίων, και τα είδη των εξαρτημάτων που χρησιμοποιούνται. Στις δυο τελευταίες ενότητες του κεφαλαίου αναφέρονται έρευνες της CIGRE και γίνεται μια προσπάθεια τεχνικής και οικονομικής σύγκρισης των οπτικών καλωδίων. Στο κεφάλαιο πέντε διακρίνονται 28 παράμετροι, σε παραμέτρους εγκατάστασης και περιβαλλοντικές παραμέτρους, ώστε να είναι επιτυχής η εγκατάσταση οπτικών καλωδίων. Για κάθε παράμετρο δίνονται τυπικές τιμές που αναφέρονται σε κάθε πιθανή εφαρμογή των οπτικών καλωδίων καθώς και πρακτικά παραδείγματα για κάθε περίπτωση. Επίσης με βάση αυτές τις παραμέτρους περιγράφονται τροποποιήσεις που γίνονται για την προστασία τους. Στο κεφάλαιο έξι γίνεται αναφορά στην σημερινή κατάσταση στην Ελλάδα όσον αφορά στην εφαρμογή των οπτικών καλωδίων στις γραμμές ισχύος καθώς και οι μελλοντικές κατευθύνσεις. Δίνονται τα πλεονεκτήματα της ΔΕΗ μέσω των οποίων υπερτερεί έναντι των άλλων εταιρειών, για την είσοδό της στην τηλεπικοινωνιακή αγορά. Αναλύεται η θέση της ΔΕΗ στην αγορά αυτή και η ανάπτυξη του δικτύου της Τellas. Τέλος, η διπλωματική εργασία ολοκληρώνεται με το Παράρτημα Α, στο οποίο αναφέρονται μερικοί τύποι υπόγειων και εναέριων οπτικών καλωδίων από σελίδες εταιρειών που διαθέτουν αυτά τα προϊόντα στην αγορά. / The continuous tendency for communication and transmission of information leads to the increase of services in telecommunications. Therefore, the effort of finding an effective solution for these services is more than necessary. The technology of fiber optics is the most reliable way of transmitting data. The fact that, optical fibers have many advantages and are better than the conductor by coper, has led to the replacement of wires by coper with optical fibers. Nowadays, fiber optics fulfill the demand of increase of transmission rate and bandwith. In 1854, John Taydall was the first to formulate the law of total reflection and in 1950 was developed a glass – coated fiber and for the first time the term “fiber optics” was used. Inevitably later, the spread of fiber optics in networks was a fact. Fiber optics are so effective that, many electrical companies want to enter in the market of telecommunications. The purpose of this work initially, is to describe and define fiber optics. Then the technologies of fiber optics, that serve telecommunications are to be mentioned and especially the technology of applying optical fibers in AC lines. Furthermore, we will mention the situation of these technologies in Greece and we will discuss its future. The definitions and different types of fiber optics are given in the second chapter. The fiber optics are divided into singlemode, multimode stepindex and multimode gradedindex fibers. Furthermore, the characteristics of the manufacturing of an optical cable are presented. In this chapter, are mentioned the OVPO process, the VAD process, the MCVD or IVPO process, the OCVD process and the process of instant fusion. Later, the description of how an optical fiber works is given and its mechanical features are mentioned. Finally, the advantages and disadvantages of fiber optics are presented. The analysis of optical cable comes first in chapter three. The layers of an optical fiber are presented and also the materials and their mechanical, thermic and chemical attributes. Finally, some technical solutions are given related to the layers and features of the cable. The technologies of applying optical fibers in AC lines are being described in the fourth chapter. The optical cables for overhead lines are divided and analyzed separately. That is to say, Optical Ground Wire (OPGW), Optical Attached Cable (OPAC), and All – Dielectric Self – Supporting optical cable (ADSS). Also, undersea and underground cables are mentioned. In this part, the mechanical and electrical features of each cable are described and also their installation and protection. Moreover, the equipment of the overhead optical cable and its elements. Finally, the optical cables are compared, in order to facilitate the choice between them. In chapter five, an effort is being showed to present a schedule for the successful installation of optical cables. This plan includes the division of parameters in installation and environmental parameters. This chapter also includes examples and practical applications for each parameter. In the end modifications of an optical cable are presented in order to secure its protection. The state in Greece, nowadays is being described in sixth chapter. The last chapter presents the application of optical cables in AC lines in this country and the role that plays the national electricity company in the market of telecommunications. Also the future prospects are being analyzed. In the last part of this work many different types of optical cables are presented, that exist in the market of telecommunications.

Gallium Nitride (GaN) is considered a very promising material for use in the field of power devices as its application in power systems would result in a significant increase in the power density, reduced power losses, and the potential to operate at high frequencies. The wide bandgap of the material allows a high critical electric field to be sustained which can lead to the design of devices with a shorter drift region, and therefore with lower on-state resistance, if compared to a silicon-based device with the same breakdown voltage. The use of an AlGaN/GaN heterostructure allows the formation of a two-dimensional electron gas (2DEG) at the heterointerface where carriers can reach very high mobility values. These properties can lead to the production of High Electron Mobility Transistors (HEMTs) and Schottky barrier diodes with superior performance, even when compared to devices based on state-of-the-art technologies such as Silicon Carbide or superjunctions. Furthermore, epitaxial growth of GaN layers on silicon wafers allows a significant reduction in the production cost and makes these devices competitive from a price perspective. This thesis will deal with a variety of topics concerning the characterization, design and optimization of AlGaN/GaN HEMTs and Schottky diodes with a 600 to 650V rating. Discussion will span several topics from device cross-section physics to circuit implementation and will be based on both experimental results and advanced modelling. More specifically, the thesis is concerned with the characterization of AlGaN/GaN Schottky diodes and extraction of their main parameters such as ideality factor, barrier height and series resistance. A thorough investigation of their reverse recovery performance and a comparison to competing technologies is also given. Several topics which concern the operation of AlGaN/GaN HEMTs are then discussed. The underlying physics of p-gate enhancement mode transistors are analysed followed by a discussion of the challenges associated with the implementation of these devices at a circuit level. Finally, a comparison of the performance of a specific area-saving layout (Bonding pad over active area) and a conventional design is given. The thesis aims to significantly enhance the understanding of the behaviour of these devices to enable better or new commercial designs to emerge.

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Power system harmonics are always an important issue in power networks as they can cause many negative impacts, such as equipment thermal stress, on installations within power networks. Recently, with the increasing connections of power electronic devices based Renewable Energy Source (RES) and High Voltage Direct Current (HVDC) transmission applications, harmonics in power networks, especially high frequency harmonics (>50th order or 2.5 kHz) are on the rise. Currently, the majority of conventional VTs, such as Wound-type Voltage Transformers (WVT) and Capacitor Voltage Transformers (CVT), are widely installed and used in High Voltage (HV) and Extra High Voltage (EHV) power networks for voltage measurement. Since most of them were mainly designed to measure voltage with the required accuracy at the fundamental frequency (i.e. 50Hz in the UK), they are limited to measuring high frequency harmonics due to the coupling of their internal inductive and capacitive elements. To achieve high frequency harmonic measurements, voltage measurement devices with wide frequency bandwidths are required. Recently, non-conventional VTs, such as optical voltage transducers, are commercially available, which could provide accurate voltage measurements over a wide range of frequency. However, before they can be considered by any power utilities, their frequency response performances must be tested at a rated fundamental voltage with required minimum harmonic injections from 100Hz to 5 kHz. This must require a test system which should be capable of providing a rated fundamental voltage up to 400kV with controllable harmonic injections at required levels from 100Hz to 5 kHz. Therefore, the objective of this project is to design and implement such a test system in the National Grid (NG) HV laboratory at the University of Manchester. However, the design and the implementation of such a test system bring many challenges; for instance, a lack of adequate equipment and considerable power to provide the required harmonic injections above 0.5% to the test object.In this thesis, an Instrument Voltage Transformer Frequency Response (VTFR) test system with three different voltage power source designs is presented; The voltage power source designs are: (i) Design 1 is based on a single power source inductive coupling method to provide both a rated fundamental voltage and controllable harmonics; (ii) Design 2 is based on two separate voltage power sources inductive coupling method to provide both a rated fundamental voltage and controllable harmonics; and (iii) Design 3 is based on two separate voltage power sources capacitive coupling method to provide both the rated fundamental voltage and controllable harmonics. A hybrid approach, which combines the VTFR test system with both the voltage power sources Design 2 and 3, is proposed for testing the frequency response of any type of VTs at their rated fundamental voltages with 1% harmonic injections from 100Hz to 5 kHz. The proposed VTFR test system with voltage power source designs were firstly validated at a relatively low voltage of 33kV in the HV laboratory. Then three different VTFR test systems were constructed based on available equipment for testing VTs from 11kV to 400kV. An 11kV, a 33kV WVT and a 400kV WVT and a 275kV CVT were tested. The test results were analyzed, compared and discussed. The models of the test systems were also established and simulated. Simulation results were analysed, compared and discussed.

The fields of nanomaterial and nanostructures are some of the fastest growing fields in material science today. Carbon nanotubes are at the forefront of these fields and their unique mechanical and electrical properties are of great interest to those working in multiple engineering fields. The overall objective of this study was to design and develop a new process and the equipment necessary, to synthesize carbon nanotubes using high voltage and a high-frequency induction field. This was the first time that a high voltage and an induction field have been used simultaneously in high yield production of carbon nanotubes. The source of carbon came from acetylene with cobalt used as the doping agent. The carbon was molded into carbon nanotubes by the high-voltage field. The carbon nanotubes were characterized using electron microscope techniques. Raman spectroscopy was also used to reveal the defects and graphitic structures of the carbon nanotubes.