Τα τελευταία χρόνια έχει αρχίσει να διαδίδεται με γρήγορους ρυθμούς η μέθοδος μεταφοράς ισχύος μέσω συνεχούς ρεύματος. Προς αυτή την κατεύθυνση ώθηση έδωσε η ανάπτυξη νέων ημιαγωγικών διακοπτικών στοιχείων οδηγώντας σε περεταίρω ανάπτυξη και χρήση των συστημάτων μεταφοράς με συνεχές ρεύμα. Σε αυτή τη διπλωματική εργασία μελετάται ένα σύστημα μεταφοράς ισχύος με διασύνδεση συνεχούς ρεύματος (HVDC), που συνδέεται ανάμεσα σε δυο εναλλασσόμενα ηλεκτρικά δίκτυα με και χωρίς φορτίο. Την διασύνδεση συνεχούς ρέματος πραγματοποιούν δυο back-to-back AC/DC μετατροπείς VSC, που αναλαμβάνουν τους ρόλους του ανορθωτή και του αντίστροφα ισχύος. Οι μετατροπείς χρησιμοποιούν την διαμόρφωση πλάτους παλμού PWM. Αρχικά μελετάται θεωρητικά το μοντέλο των μετατροπέων και του back-to-back HVDC συστήματος και στην συνέχεια σχεδιάζεται και αναλύεται η λειτουργία του ανάμεσα σε δυο δίκτυα εναλλασσομένου ρεύματος με ή χωρίς την ταυτόχρονη παρουσία φορτίου. Τέλος προσομοιώνεται το σύστημα μέσω του λογισμικού Matlab και συγκεκριμένα της εφαρμογής Simulink για την εξαγωγή συμπερασμάτων. / In the past few years the method of power transmission by means of direct current has expanded rapidly. To this direction a push forward has been given by the development of new semi-conductive switching valves leading to a further development of transmission systems by direct current. In this thesis what is considered is a power transmission system via direct current HVDC connected between two AC electric networks with or without load. The direct current connection is achieved through back-to-back AC/DC converters VSC which undertake the role of rectifier and that of inverter of power. The converters use the Pulse Width Modulation (PWM). At first the converter model and the back-to-back HVDC system is theoretically approached and in the process what is designed and analyzed is its function between two networks of AC current with or without the simultaneous presence of load. Finally the system is simulated through software Matlab and specifically the application of simulink in order to draw conclusions.

Thesis (MScEng)--Stellenbosch University, 2012. / ENGLISH ABSTRACT: The worldwide increase in the popularity of high voltage direct current (HVDC) power transmission application has led to questions regarding the performance of high voltage alternating current (HVAC) insulators when energized under HVDC excitation. These questions have led researchers conduct numerous research projects worldwide. A particular question NamPower (the power utility in Namibia) has faced is: how do these insulators perform and age when energized under HVDC excitation in heavily polluted environments? This question was only partly answered by some insulator ageing tests conducted under laboratory conditions for HVDC excitation. However, the natural ageing tests on insulators, which are preferred, have to date been confined predominantly to HVAC excitation voltages. Thus, this research was initiated to investigate the natural ageing performance of insulators under both HVAC and HVDC excitations, when subjected to harsh marine pollution environments. This research project involved performance and ageing tests on three identical sets of line insulators made from different insulator materials, energised under HVAC and both polarities HVDC excitations respectively. The tests were conducted at Koeberg insulator pollution test station (KIPTS), which is a natural marine pollution insulator test station located near Cape Town, along the west coast of South Africa, approximately 50 m from the sea. The set of insulators consisted of EPDM silicone alloy rubber, HTV silicone rubber, RTV silicone rubber coated porcelain, Porcelain and Glass insulators. The HVAC excitation voltage was chosen as 12.7 kV r.m.s. phase-to-ground and it was decided to use a HVDC excitation voltage equal to this value. The research results showed that the insulators made from HTV silicone rubber performed better than the insulators made from EPDM silicone alloy rubber under all excitation voltages. It is also showed that RTV silicone rubber coated porcelain insulators performed better than Glass and Porcelain insulators under all excitation voltages. / AFRIKAANSE OPSOMMING: Die wêreld wye toename in gewildheid van hoë spanning gelyk stroom (HSGS) krag transmisie aplikasie het gelei na vrae oor die effektieweteit van hoë spanning wissel stroom (HSWS) insolators in HSGS aplikasies. Hierdie vrae het gelei na baie navorsings projekte. Een vraag waarmee NamPower (die krag voorsienings maatskapy in Namibia) gekonfronteer was, was hoe gaan die insolators wat onderworpe is aan ’n HSGS toevoer reageer in ’n baie besoedelde omgewing. Hierdie vraag was slegs gedeeltelik beantwoord deur verouderings toetse op insolators wat gedoen is in ’n labaratorium met ’n HSGS toevoer. Inteendeel, die meeste natuurlike verouderings toetse op insolators, soos verkies, is meestal gedoen met ’n HSWS toevoer. Om hierdie rede was hierdie navorsing begin om natuurlike verouderings toetse te doen op insolators onderworpe beide aan HSWS en HSGS toevoere binne ’n marien besoedelde omgewing. Hierdie navorsings projek gaan oor prestering en verouderings toetse op drie, identiese, transmissie lyn insolator stelle, wat onderskeidelik onderworpe was aan HSWS en beide polariteite HSGS toevoere. Die toetse was gedoen by Koeberg insolator besoedeling toets stasie (KIBTS) wat naby Kaapstad geleë is langs die weskus van Suid Africa omtrent 50 m van die see. ’n Stel toets insolators bestaan uit EPDM silikon allooi rubber, HTV silikon rubber, RTV silikon rubber bedekte porselein, Porselein en Glas insolators. Die HSWS waarde waarmee die insolators getoets was, was 12.7 kV w.g.k., fase-na-grond, en dit was besluit om ’n HSGS waarde gelyk aan hierdie spannings waarde te gebruik. Die navorsings resultate wys dat insolators wat gemaak is met HTV silikon rubber presteer beter as insolators wat met EPDM silikon allooi rubber gemaak is onder al die verskillende toevoere. Dit wys ook dat RTV silikon rubber bedekte porselein presteer beter as Porselein en Glas insolators onder al die verskillende toevoere.

Modern day power systems present an open access environment, inspiring participation from small scale and large power suppliers. With multiple players in the system driven by the market, proper monitoring and control of system becomes a major concern. This transformation is accompanied by dynamic consumption patterns and rising power demands. The expanding network encompassing EHV/AC network, HVDC and FACTS devices, along with increased penetration of renewable sources, viz. solar and wind energy at medium and low voltage levels, adds to the problem. Independent System Operators (ISO) are entrusted with ensuring smooth operation, and employing proper preventive measures to eliminate a possible cascade tripping leading to a partial or large-scale blackout. To aid the operator in the process of ensuring secure operation of the grid, there are many tools that provide required information and guidance. Power flow tracing is one such tool that aids the operator in congestion management, transmission pricing, transaction evaluation, loss allocation and reactive power optimization. In this thesis, a novel active power flow tracing approach is proposed that takes into account, the real-time operating conditions and network topology. It provides the decomposition of active power flow in a line into respective components injected by various generators in the system. It also provides the contribution of the generators to various loads in the system. The approach is simple and computationally fast, making it an ideal tool to aid preventive control decisions. Based on the proposed active power flow tracing, a congestion management approach is developed. The approach indicates the least number of generators that need to be coordinated for generation rescheduling, so as to alleviate overloading in affected transmission lines and transformers. The approach also takes into consideration the operating constraints on the system, while computing the optimal rescheduling amongst selected generators using LP technique. The thesis also presents a real power loss allocation approach based on the proposed power flow tracing. Loss allocation is an important part of tariff design as the cost associated with losses amounts to a sizable fraction of total revenue collected from the loads. The approach provides information as to how losses are distributed among loads and how much each generator is providing for the loss share of each load. The approaches developed in the thesis are illustrated on a sample 10-bus equivalent system, IEEE 30-bus, and IEEE 39-bus systems. Results for typical case studies are presented for practical systems of 72-bus equivalent and 203-bus equivalent of Indian Southern grid.

Le système de transport d’électricité doit évoluer pour satisfaire les besoins du marché de l’électricité et de l’insertion de la production renouvelable. Les systèmes de transport dits HVDC se développent. Les interfaces d’électroniques de puissance vont jouer un rôle majeur et doivent faire preuve d’une extrême fiabilité, d’une grande efficacité et rester économiquement abordables.La technologie MMC (Convertisseur Modulaire Multi-niveaux) connaît un essor par rapport à des technologies classiques, comme le convertisseur trois-niveaux. Sa topologie étant complexe, deux niveaux de contrôle peuvent être définis. Le premier niveau porte sur le contrôle des interrupteurs pour équilibrer les tensions des sous-modules. Le second niveau contrôle les courants, la puissance et l’énergie dans le système.Cette thèse est axée sur ce deuxième niveau de contrôle. Une approche hiérarchisée et formelle, basée sur l’inversion du modèle pour le contrôle de l’énergie du MMC est présentée. Pour ce contrôle, différentes méthodes ont été proposées et comparées. Cela implique de développer une modélisation, mettre en place un contrôle. Différents modèles et contrôles ont été développés.Le MMC est généralement intégré dans une liaison HVDC où deux stations AC/DC ont un contrôle différent. Un soin particulier doit être apporté à la station dédiée au contrôle de la tension. En effet, la gestion de l’énergie dans le MCC est un point critique pour la stabilité de la tension.Enfin, les différents types de contrôle évoqués ont été étudiés dans le cas d’une liaison HVDC. Il a été montré que les échanges entre le bus DC et les MMC jouent un rôle important pour la régulation de la tension du bus DC / In future, the capability of the electric power transmission continues to grow due to renewable energy production and the needs of electrical market. Consequently, many HVDC transmission systems are developed. Definitely the power electronic interfaces will play a key role to provide high reliability, good efficiency and cost effectiveness for this AC/DC conversion.Recently, the Modular Multilevel Converter (MMC) has taken the advantage over the more classical converter as three-level VSC. Since MMC topology is complex, two different control levels may be distinguished: the control of the switches mainly orientated on the balance of hundreds of voltage on the elementary submodules, the higher level control whose aim is to control the currents, power and energy in the system.This thesis is oriented mainly on the latter. It discusses a hierarchical and formal approach for the MMC to control the energy in all the storage elements. At first it is shown that an energy control is required mandatory. Secondly, it supposes to develop an energetic model which is inverted to design the energy control. Then different solutions of control have been developed and discussed.In the majority of applications, MMC is integrated in an HVDC point to point link where the two AC/DC substations have different roles. A specific attention has to be paid on the station which controls the voltage since the way to manage the energy in the MMC has a critical role in the DC voltage stability.Finally, all these types of control have been tested and discussed on an HVDC. It is shown that the exchange between the DC bus and the MMC placed on both sides play a key role in the DC bus voltage regulation.

Satisfaire les besoins en énergie de manière responsable est possible grâce aux énergies renouvelables, notamment éoliennes et solaires. Cependant ces centres de captation d’énergie sont éloignés dans zones de consommation. Le transport de l’énergie via des réseaux HVDC (haute tension courant continu) permet un rendement et une flexibilité avantageuse face au transport HVAC (haute tension courant alternatif). Ceci est rendu possible grâce aux convertisseurs utilisant l’électronique de puissance. Les récents développements sur les semi-conducteurs à large bande interdite, plus particulièrement le carbure de silicium (SiC) offrent la possibilité de concevoir ces convertisseurs plus simples, utilisant des briques technologiques de plus fort calibre (≤ 10 kV). Cependant le packaging, essentiel à leur bon fonctionnement, ne suit pas ces évolutions. Dans cette thèse, nous explorons les technologies actuelles ainsi que les limites physique et normatives liées au packaging haute tension. Des solutions innovantes sont proposées pour concevoir un module de puissance haute tension, impactant que faiblement les paramètres connexes (résistance thermique, isolation électrique et paramètres environnementaux). Les éléments identifiés comme problématiques sont traités individuellement. La problématique des décharges partielles sur les substrats céramiques métallisés est développée et une solution se basant sur les paramètres géométriques a été testée. Le boitier standard type XHP-3 a été étudié et une solution permettant de le faire fonctionner à 10 kV à fort degré de pollution a été développée. / The supply of carbon-free energy is possible with renewable energy. However, windfarms and solar power plants are geographically away from the distribution points. Transporting the energy using the HVDC (High Voltage Direct Current) technology allow for a better yield along the distance and result in a cost effective approach compared to HVAC (High Voltage Alternative Current) lines. Thus, there is a need of high voltage power converters using power electronics. Recent development on wide bandgap semiconductors, especially silicon carbide (SiC) allow a higher blocking voltage (around 10 kV) that would simplify the design of such power electronic converters. On the other hand, the development on packaging technologies needs to follow this trend. In this thesis, an exploration of technological and normative limitation has been done for a high voltage power module design. The main hot spot are clearly identified and innovative solutions are studied to provide a proper response with a low impact on parasitic parameters. Partial Discharges (PD) on ceramic substrates is analyzed and a solution of a high Partial Discharge Inception Voltage (PDIV) is given based on geometrical parameters. The XHP-3 like power modules are studied and a solution allowing a use under 10 kV at a high pollution degree (PD3) is given.

La multiplication des projets HVDC de par le monde démontre l'engouement toujours croissant pour cette technologie de transport de l'électricité. La grande majorité de ces transmissions HVDC correspondent à des liaisons point-à-point et se basent sur des convertisseurs AC/DC de type LCC ou VSC à 2 ou 3 niveaux. Les travaux de cette thèse se focalisent sur l'étude, le contrôle et la commande de systèmes HVDC de type multi-terminal (MTDC), avec des convertisseurs de type VSC classique ou modulaire multi-niveaux. La première étape consiste à obtenir les modèles moyens du VSC classique et du MMC. La différence fondamentale entre ces deux convertisseurs, à savoir la possibilité pour le MMC de stocker et de contrôler l'énergie des condensateurs des sous-modules, est détaillée et expliquée. Ces modèles et leurs commandes sont ensuite linéarisés et mis sous forme de représentations d'état, puis validés en comparant leur comportement à ceux de modèles de convertisseurs plus détaillés à l'aide de logiciels de type EMT. Une fois validés, les modèles d'état peuvent être utilisés afin de générer le modèle d'état de tout système de transmissions HVDC, qu'il soit point-à-point ou MTDC. La comparaison d'une liaison HVDC à base de VSCs classiques puis de MMCs est alors réalisée. Leurs valeurs propres sont étudiées et comparées, et les modes ayant un impact sur la tension DC sont identifiés et analysés. Cette étude est ensuite étendue à un système MTDC à 5 terminaux, et son analyse modale permet à la fois d'étudier la stabilité du système, mais aussi de comprendre l'origine de ses valeurs propres ainsi que leur impact sur la dynamique du système. La méthode de décomposition en valeurs singulières permet ensuite d'obtenir un intervalle de valeurs possibles pour le paramètre de"voltage droop", permettant ainsi le contrôle du système MTDC tout en s'assurant qu'il soit conforme à des contraintes bien définies, comme l'écart maximal admissible en tension DC. Enfin, une proposition de "frequency droop" (ou "statisme"), permettant aux convertisseurs de participer au réglage de la fréquence des réseaux AC auxquels ils sont connectés, est étudiée. Le frequency droop est utilisé conjointement avec le voltage droop afn de garantir le bon fonctionnement de la partie AC et de la partie DC. Cependant, l'utilisation des deux droop génère un couplage indésirable entre les deux commandes. Ces interactions sont mathématiquement quantifiées et une correction à apporter au paramètre de frequency droop est proposée. Ces résultats sont ensuite validés par des simulations EMT et par des essais sur la plate-forme MTDC du laboratoire L2EP. / HVDC transmission systems are largely used worldwide, mostly in the form of back-to-back and point-to-point HVDC, using either thyristor-based LCC or IGBT-based VSC. With the recent deployment of the INELFE HVDC link between France and Spain, and the commissioning in China of a three-terminal HVDC transmission system using Modular Multilevel Converters (MMCs), a modular design of voltage source converters, the focus of the scientific community has shifted onto the analysis and control of MMC-based HVDC transmission systems. In this thesis, the average value models of both a standard 2-level VSC and an MMC are proposed and the most interesting difference between the two converter technologies -the control of the stored energy in the MMC- is emphasised and explained. These models are then linearised, expressed in state-space form and validated by comparing their behaviour to more detailed models under EMT programs. Afterwards, these state-space representations are used in the modelling of HVDC transmission systems, either point-to-point or Multi-Terminal HVDC (MTDC). A modal analysis is performed on an HVDC link, for both 2-level VSCs and MMCs. The modes of these two systems are specifed and compared and the independent control of the DC voltage and the DC current in the case of an MMC is illustrated. This analysis is extended to the scope of a 5-terminal HVDC system in order to perform a stability analysis, understand the origin of the system dynamics and identify the dominant DC voltage mode that dictates the DC voltage response time. Using the Singular Value Decomposition method on the MTDC system, the proper design of the voltage-droop gains of the controllers is then achieved so that the system operation is ensured within physical constraints, such as the maximum DC voltage deviation and the maximum admissible current in the power electronics. Finally, a supplementary droop "the frequency-droop control" is proposed so that MTDC systems also participate to the onshore grids frequency regulation. However, this controller interacts with the voltage-droop controller. This interaction is mathematically quantified and a corrected frequency-droop gain is proposed. This control is then illustrated with an application to the physical converters of the Twenties project mock-up.

Thesis (PhD (Electrical and Electronic Engineering))--University of Stellenbosch, 2009. / ENGLISH ABSTRACT: Prospects of up-rating existing high voltage direct current (HVDC) transmission schemes, as well as the conversion of existing alternating current (AC) to direct current (DC) lines and the development of new HVDC schemes in sub-Saharan Africa, have led to renewed interest in DC research. The radio interference (RI), audible noise (AN) and corona loss (CL) performance of HVDC transmission lines are critical factors when assessing the reliability of the line design. The RI performance is especially important when considering the successful transmission of the carrier signal of the power line carrier (PLC) system. The PLC system is the main form of communication between teleprotection devices on the Cahora Bassa HVDC scheme. The aim of the dissertation is to devise modelling as well as metrological techniques to characterise DC conductor corona. A particle-in-cell (PIC) computational code is developed to gain a better understanding of the physical processes that occur during corona events. The numerical code makes use of the charge simulation method (CSM) and nite element method (FEM) to solve for the Laplace and Poisson eld equations. Higher-order basis functions are implemented to obtain a more accurate solution to the Poisson equation. The computational tool yields insight into the mathematical models for the various ionization, attachment and electron avalanche processes that give rise to corona currents. Together with a designed and developed electrometer-type circuit, the numerical code assists the visualisation of the space charge particle dynamics that form in the electrode gap during corona events. The metrological techniques consider the wideband time domain (TD) as well as the frequency domain (FD) information of the measured corona pulses in the presence of noise. These are then compared to the narrowband CISPR standard measurements centred around 500kHz. The importance of impedance matching when attempting to derive a wideband excitation function is investigated. The TD measurements are quite distinct from the well-published FD measurements, and consider the pulse shape, pulse spectrum and pulse repetition rates. The use of three possible conductor corona test methods to study direct current conductor RI performance under both positive and negative polarities is investigated at high altitude in this dissertation. These include a small corona cage, a short test line and the Eskom Megawatt Park large outdoor corona cage. Derived wideband and narrowband monopolar DC RI excitation functions at 500kHz are consolidated with existing radio noise (RN) measurement protocols and prediction methods. The use of a corona cage to derive excitation functions for monopolar RI predictions is explored and it is shown that a small corona cage, due to the build-up of space charge in the small distance between the electrodes, cannot be used to predict the RI levels on HVDC transmission lines accurately. As a consequence of the physics, computational modelling and both frequency and time domain measurements, it is now possible to explain why a small cage system prevents the accurate RI prediction on transmission lines. The large outdoor corona cage and short test line RI performance predictions agree with existing empirical prediction formulas. / AFRIKAANSE OPSOMMING: Vooruitsigte van die opgradering van bestaande hoogspanningsgelykstroom transmissielyn skemas, asook die omkering van bestaande wisselstroom na gelykstroom lyne en die ontwikkeling van nuwe hoogspanningsgelykstroom skemas in sub-Sahara Afrika, het gelei to hernude belangstelling in gelykstroomnavorsing. Die korona-werkverrigting van hoogspanningsgelykstroom oorshoofselyne in terme van radiosteuring, hoorbare-geraas en koronaverliese is kritiese faktore om in aanmerking te neem wanneer die betroubaarheid van die lynontwerp geëvalueer word. Die radiosteuring-werkverrigting is veral van belang tot die suksesvolle oordrag van die kraglyndragolf draersein wat die hoof kommunikasievorm tussen beskermingstoerusting op die Cahora Bassa transmissielyn skema is. Die doel van hierdie proefskrif is om modellering- sowel as meettegnieke te ontwerp om gelykstroomgeleierkorona te karakteriseer. 'n Partikel-in-sel numeriese kode is ontwikkel om 'n beter begrip te verkry van die siese prosesse gedurende koronagebeure. Die numeriese kode maak gebruik van die lading-simulasiemetode, sowel as die eindige element metode om die Laplace en Poisson veldvergelykings op te los. Hoër-orde basisfunksies is geimplimenteer om 'n meer akkurate oplossing vir die Poisson vergelyking te verkry. Die numeriese kode bied insig tot die wiskundige modelle vir die verskeie ionisasie-, aanhegtings- en lawineprosesse wat lei tot koronastrome in die area om die hoogspanningsgeleier. Die numeriese kode, saam met 'n elektro-meter wat ontwerp en ontwikkel is, dra by tot die begrip van die ruimtelading partikeldinamika wat onstaan in die elektrodegaping gedurende koronagebeure. Die meettegnieke neem die wyeband tydgebied- en frekwensiegebiedinformasie van die koronapulse in ag in die teenwoordigheid van geraas. Dit word dan vergelyk met die nouband CISPR meetstandaard vir 'n frekwensie van 500kHz. Die belangrikheid van impedansie-aanpassing vir wyeband metings met die doel om opwekkingsfunksies af te lei, word ondersoek. Die tydgebiedmetings verskil van die algemene frekwensiegebiedmetings, en ondersoek die pulsvorm, -spektrum en -herhalingskoers. Die gebruik van drie moontlike koronageleier-toetsmetodes om gelykstroom radiosteurings-werkverrigting vir positiewe en negatiewe polariteite te bestudeer by hoë vlakke bo seespieël word ondersoek in die proefskrif. Dit sluit in 'n klein koronakou, 'n kort toetslyn en die Eskom Megawatt Park groot buitelug-koronakou. Afgeleide wye- en nouband monopolêre gelykstroom radiosteuring opwekkingsfunksies by 500kHz word gekonsolideer met bestaande radioruis metingsprotokolle en voorspellingsmetodes. Die gebruik van 'n koronakou om opwekkingsfunksies af te lei vir monopolêre radiosteuringvoorspellings is ondersoek en daar is gevind dat 'n klein koronakou nie gebruik kan word om radiosteuringvlakke op hoogspanningsgelykstroom transmissielyne akkuraat te voorspel nie. Dit is as gevolg van die opbou van ruimtelading in die klein elektrodegaping. Met behulp van die sika, numeriese modellering en beide die frekwensie- en tydgebiedmetings, is dit nou moontlik om te verklaar waarom die klein koronakou die akkurate radiosteuringvoorspellings op transmissielyne onmoontlik maak. Die groot buitelug-koronakou en kort toetslyn radiosteuringvoorspellings stem ooreen met bestaande empiriese voorspellings formules.

En permettant la transmission de fortes puissances sur de grandes distances, les réseaux en haute tension à courant continu (HTCC) représentent l'avenir du transport de l'énergie électrique. Des équipements tels que les postes sous enveloppe métallique (PSEM) seront indispensables dans ces réseaux pour répartir le flux d'énergie, interrompre ou isoler certaines branches. Leur conception nécessite la prise en compte, pour les parties isolantes, de phénomènes spécifiques liés à l’application de champs électriques continus, comme la dépendance de la résistivité avec le champ et la température, mais également l’accumulation de la charge d'espace qui contribue à augmenter sensiblement les risques de claquage. Dans le composite étudié (résine époxyde chargée d’alumine), ce phénomène commence à se manifester dès que les valeurs de champ excèdent quelques kV/mm, correspondant à l’ordre de grandeur des contraintes envisagées dans les futurs PSEM HTCC.Bien que des techniques existent pour déterminer la répartition de ces charges dans les isolants solides, aucune n’est à ce jour directement applicable aux structures isolantes installées dans les PSEM HTCC.L’objectif de ce travail est ainsi de concevoir et de mettre en place une technique de mesure des charges d’espace et du champ électrique interne qui soit à résolution spatiale, non destructive et adaptée à une géométrie d’isolateur cylindrique, de type support isolant de jeu de barres.En utilisant le dispositif expérimental mis en place, le comportement du composite vis-à-vis de la charge d’espace est par la suite étudié, notamment en fonction de différentes contraintes électrothermiques représentatives du fonctionnement des PSEM. Le but final est d'aider à l'optimisation de la conception de ce type d’isolateur, en se basant sur l’analyse des résultats issus de mesures de charges d’espace. / The trend today is to develop high voltage direct current (HVDC) technology for the future electric network because it offers some advantages for the transmission on long distances. The development of HVDC networks leads to an increasing need of gas insulated substations (GIS). A problem to be dealt with when an insulator is subjected to a continuous electric field is the variation of the resistivity with the electric field and the temperature and the accumulation of space charges that can lead to dielectric breakdown. In alumina-filled epoxy resin, used as insulating material for GIS spacer, the influence of space charge start to come out when the electric fields exceed several kV/mm, corresponding to values envisaged for the future HVDC GIS.Despite that non-destructive methods exist to determine the space charge distribution in solid insulators, none of them are directly applicable to insulation structures installed in the HVDC GIS.So, the main objective of this work is to design and set up a measurement technique to observe the internal electric field and the accumulated charges. The developed method must be non-destructive and adapted for a cylindrical geometry of an insulator used as a busbar insulation support.By using the experimental bench set up, the behavior of the composite material regarding the space charge will be studied, in particular according to different thermoelectric stresses. The final aim is to contribute to the optimization of the design of this type of insulator, based on the results from space charge measurements.

Thesis submitted in fulfillment of the requirements for the degree Master of Technology: Electrical Engineering in the Faculty of Engineering at the Cape Peninsula University of Technology 2013 / The development of power electronics did remove most of technical limitations that high voltage direct current (HVDC) used to have. HVDC, now, is mostly used for the transmission of bulk power over long distances and for the interconnection of asynchronous grid. Along with the development of the HVDC, the growth of power demand also increased beyond the utilities capacities. Besides the on-going increasing of power demand, the reforms in electricity market have led to the liberalization and the incorporation of Independent power providers in power system operation. Regulations and rules have been established by regulating authority for grid integration of Independent power providers. With the expected increase of penetration level of those new independent power providers, result of economic reason and actual green energy trend, best method of integration of those new power plants are required. In this research HVDC technology, namely VSC-HVDC is used as interface for connecting independent power providers units to the grid. VSC-HVDC has various advantages such as short-circuit contribution and independent control of active and reactive power. VSC-HVDC advantages are used for a safe integration of IPPs and make them participate to grid stabilization. MATLAB/Simulink simulations of different grid connected, through VSC-HVDC system, IPPs technologies models are performed. For each IPP technology model, system model performances are studied and dynamics responses during the disturbance are analyzed in MATLAB/ Simulink program. The simulation results show that the model satisfy the standard imposed by the regulating authority in terms of power quality and grid support. Also the results show the effect of the VSC-HVDC in preventing faults propagation from grid to integrated IPPs units.

Modern interconnected power systems are becoming highly complex and sophisticated, while increasing energy penetrations through congested inter-tie lines causing the operating point approaching stability margins. This as a result, exposes the overall system to potential low frequency power oscillation phenomena following disturbances. This in turn can lead to cascading events and blackouts. Recent approaches to counteract this phenomenon are based on utilization of wide area monitoring systems (WAMS) and power electronics based devices, such as flexible AC transmission systems (FACTS) and HVDC links for advanced power oscillation damping provision. The rise of hybrid AC-DC power systems is therefore sought as a viable solution in overcoming this challenge and securing wide-area stability. If multiple FACTS devices and HVDC links are integrated in a scheme with no supervising control actions considered amongst them, the overall system response might not be optimal. Each device might attempt to individually damp power oscillations ignoring the control status of the rest. This introduces an increasing chance of destabilizing interactions taking place between them, leading to under-utilized performance, increased costs and system wide-area stability deterioration. This research investigates the development of a novel supervisory control scheme that optimally coordinates a parallel operation of multiple FACTS devices and an HVDC link distributed across a power system. The control system is based on Linear Quadratic Gaussian (LQG) modern optimal control theory. The proposed new control scheme provides coordinating control signals to WAMS based FACTS devices and HVDC link, to optimally and coherently counteract inter-area modes of low frequency power oscillations inherent in the system. The thesis makes a thorough review of the existing and well-established improved stability practises a power system benefits from through the implementation of a single FACTS device or HVDC link, and compares the case –and hence raises the issue–when all active components are integrated simultaneously and uncoordinatedly. System identification approaches are also in the core of this research, serving as means of reaching a linear state space model representative of the non-linear power system, which is a pre-requisite for LQG control design methodology.