In der vorliegenden Arbeit wird ein neues Verfahren zur Dünnschichtverkappung von oberflächennahen Mikrostrukturen vorgestellt. Ausgehend von den speziellen Anforderungen an die Verkappung oberflächennaher Mikrostrukturen, insbesondere von Strukturen mit hohem Aspektverhältnis, wurden die Verwendung eines Fluor-Kohlenstoff-Polymers als Opferschichtmaterial und die Eignung unterschiedlicher Schichtstapel zur Realisierung der Dünnschichtkappe untersucht. Die resultierende Technologie ermöglicht eine durchgehend trockenchemische Prozessierung. Für die Abschätzung der notwendigen Schichtdicken und den geometrischen Entwurf der Kappenstrukturen, wurden auf Basis der Plattentheorie analytische und numerische Modelle erstellt. Verschiedene Materialkombinationen bestehend aus Siliziumoxid, Siliziumnitrid und Aluminium wurden hinsichtlich ihrer mechanischen und thermomechanischen Eigenschaften untersucht und bewertet. Ein weiterer Schwerpunkt lag auf der Entwicklung und Optimierung der Opferschichtprozesse, sowie deren Integration in die Gesamttechnologie. Die Eignung der plasmagestützten Prozesse zur Abscheidung und Strukturierung des Opferpolymers wurde durch die Fertigung von verkapselten Beschleunigungssensoren nachgewiesen. Ein ausreichender hermetischer Verschluss der Dünnschichtkappe konnte durch die Messung der viskosen Dämpfung an Feder-Masse-Schwingern bestätigt werden.

El actual análisis convencional realizado para la modelación de una estructura se basa en fijar los apoyos en la base de la estructura. Dicha idealización no refleja el comportamiento real del suelo donde se cimienta. Puesto que este, dependiendo de sus parámetros geotécnicos, aporta una determinada rigidez que se relaciona con un comportamiento elástico-plástico. Se analizó los efectos en las respuestas de un sistema estructural, considerando interacción suelo estructura, variando los parámetros geotécnicos, la altura de la edificación y los modelos ISE los cuales fueron el modelo académico propuesto A.E. Sargsyan, el modelo propuesto por D. D. Barkan y O.A. Savinov y la Norma Rusa SNIP 2.02.05-87. Los tipos de suelos empleados para el desarrollo fueron arena limosa (SM-S2) y arena pobremente graduada (SP-S3). Las respuestas obtenidas se compararon con un análisis convencional empotrado en la base, con ello, se determinó las variaciones de 3 respuestas principales: el periodo, lo esfuerzo y las derivas. Se determina que el rango estimado en que aumenta el periodo la estructura es de 21% a 41% y la derivas entre un 23% y 58%, demostrando el cambio de comportamiento de la estructura al considerar la rigidez del suelo. / The current conventional analysis performed for the modeling of a structure is based on fixing the supports at the base of the structure. This idealization does not reflect the real behavior of the soil where it is sown. Since this, depending on its geotechnical parameters, provides a certain stiffness that is related to an elastic-plastic behavior. The effects on the responses of a structural system were analyzed, considering soil structure interaction, varying the geotechnical parameters, the height of the building and the ISE models which were the proposed academic model A.E. Sargsyan, the model proposed by D. D. Barkan and O.A. Savinov and the Russian Standard SNIP 2.02.05-87. The types of soils used for development were silty sand (SM-S2) and poorly graded sand (SP-S3). The answers obtained were compared with a conventional analysis embedded in the base, with this, the variations of 3 main responses were determined: the period, effort and drifts. It is determined that the estimated range in which the period increases the structure is from 21% to 41% and the drifts between 23% and 58%, demonstrating the change in behavior of the structure when considering the stiffness of the soil. / Tesis

En esta investigación se plantea una nueva metodología de diseño con el objetivo de obtener una mejora adicional basada en el concepto de transmisibilidad para la reducción del desplazamiento absoluto en la base aislada de una estructura. El incremento del periodo de diseño en los últimos años potencia el método propuesto basado en el concepto de transmisibilidad. Un edificio de 10 pisos es propuesto para esta investigación. Los resultados demuestran una reducción considerable en el desplazamiento de la base aislada cuando está sometida a sismos de gran magnitud. / In this research, a new design methodology based on the concept of transmissibility is proposed for the reduction of the absolute displacement in the isolated base of a structure. An increase in the design period in the last years favors the proposed method based on the transmissibility concept. A 10-story building is proposed for this research. The results show a considerable reduction in the displacement of the isolated base when it is subjected to earthquakes of great magnitude / Trabajo de investigación

Svenska kraftnät is responsible for the operation and development of the Swedish transmission grid. A large number of substations in the transmission grid are approaching their expected service lifetime and must be renewed in the coming years. When renewing substations, total and partial outages in operation are often unavoidable. These outages may cause downtime for any party connected to the substation, whether it be an electricity generation or consumption. A solution is to build a new substation while the old is still in operation. Upon completion, a transfer of operation can be done from the old to the new substation. This however requires laying claim to new land, which affects the local environment and can be a time-consuming process. In an effort to minimise the need for planned outages during substation renewals, Svenska kraftnät wants to investigate techniques for bypassing substations while also allowing the connected party to remain in operation. This thesis has evaluated two differing techniques that allow for substations and any connecting parties to remain mostly in full operation during station renewals. One technique involves constructing a temporary switchgear compartment using air-insulated apparatuses to bypass the station and connect the third party. The other technique uses a mobile gas-insulated switchgear to achieve essentially the same arrangement.  After studying cases where either technique has been employed in station renewals, it was concluded that a solution utilising air-insulated apparatuses is better in several aspects. While the two technologies achieved similar results, the air-insulated technology did so at a lower cost, offered more flexibility, and was easier to integrate into pre-existing stations. Furthermore, the gas-insulated technology required more specialised competence, possibly limiting the availability of qualified contractors. It was concluded that bypassing stations during renewal projects could provide several benefits in terms of less downtime and economical losses, while also minimising the need to claim new land. In order to optimise the method a supply of air-insulated apparatuses and construction materials necessary for the bypassing of a station should be established by Svenska kraftnät. This, mostly reusable, equipment would then be on stand-by, ready to use in future station renewals.

Das kompakte Design von gasisolierten Systemen und die Unabhängigkeit gegenüber Umgebungsbedingungen führt zu einer idealen Eignung des Betriebsmittels für den Einsatz in modernen Energieversorgungssystemen. Ein Betrieb der Anlagen unter Gleichspannungsbelastung ist dabei aufgrund der zunehmenden räumlichen Distanz zwischen Erzeugungs- und Verbrauchszentren unumgänglich. Der sichere Betrieb über die geplante Lebensdauer von mehreren Jahrzehnten ist nur mit einer genauen Teilentladungsdiagnose, welche eine sensitive Messung und zweifelsfreie Interpretation der Ergebnisse beinhaltet, möglich. Dabei ist zu beachten, dass die von Wechselspannungsanwendungen bekannten physikalischen Zusammenhänge der Entladungsprozesse und des Einflusses von dielektrischen Grenzflächen aufgrund der veränderten Belastung mit einem zeitlich konstanten elektrischen Feld und der damit einhergehenden Raum- und Oberflächenladungsakkumulation nicht direkt übernommen werden können. Ziel dieser Arbeit ist daher die Entladungsprozesse an Defekten mit und ohne dielektrischer Grenzfläche in gasisolierten Gleichspannungssystemen zu analysieren und damit einen Beitrag für die sichere Interpretation von Teilentladungsmessungen zu leisten. Auch werden bekannte elektrische und neuartige optische Messmethoden hinsichtlich ihrer Möglichkeiten und Grenzen beim Einsatz unter Gleichspannungsbelastung untersucht. Für die experimentellen Arbeiten an drei verschiedenen Störstellen wird das schwach inhomogene Isoliersystem der Anlagen in drei Modellanordnungen nachgebildet. Die Untersuchung der ablaufenden Entladungsprozesse wird durch eine direkte Messung des Teilentladungsstroms ermöglicht. Dabei wird zwischen impulsbehafteten und impulslosen Anteilen unterschieden. Infolge von Montagefehlern oder unzureichender Materialqualität können feste, metallische Störstellen im Isoliersystem entstehen. Die experimentell betrachteten Abhängigkeiten der Entladungsprozesse von der Störstellenpolarität, dem Isoliergasdruck und der Spannungsbelastung erlauben eine Klassifizierung von vier verschiedenen Entladungsarten. Zusätzlich zu den Untersuchungen im derzeit am häufigsten verwendeten Isoliergas Schwefelhexafluorid konnte ein Vergleich der Ergebnisse mit der klimafreundlichen alternative synthetische Luft die Gemeinsamkeiten und Unterschiede bei Entladungen an festen Störstellen aufzeigen. Dabei ist insbesondere die signifikant veränderte Polaritätsabhängigkeit hervorzuheben. Der Kontakt von metallischen Partikeln mit der Feststoffisolierung kann zur Anlagerung des zuvor freibeweglichen, metallischen Defektes an der dielektrischen Grenzfläche führen. Die Ansammlung von Oberflächenladungen auf dem Feststoff beeinflusst dabei insbesondere den Entladungseinsatz. Aufgrund des zur festen metallischen Störstelle ohne Grenzfläche vergleichbaren Entladungsverhaltens im stationären Zustand ist eine Unterscheidung der Defekte anhand von Impulswiederholraten und Amplituden herausfordernd. Eine Besonderheit bei Gleichspannungsbelastung sind Entladungen, welche auf einer Gas-Feststoff-Quergrenzfläche an beschichteten Elektroden einsetzen können. Die Untersuchung der Ursachen für das Auftreten dieser Entladungen, die elektrischen und optischen Charakteristika der ablaufenden Prozesse und Strategien für die Vermeidung werden untersucht. Aus den Ergebnissen werden Prüfempfehlungen für die Teilentladungsdiagnose von gasisolierten Gleichspannungssystemen abgeleitet. Diese sind wesentlicher Bestandteil für einen zukünftigen Einsatz gasisolierter Gleichspannungssysteme in einem leistungsfähigen Elektroenergiesystem mit hoher Versorgungszuverlässigkeit. / The compact design and the independence from environmental conditions of gas-insulated systems leads to an ideal suitability of this high-voltage equipment for the use in a modern power supply system. The operation of the assets under DC voltage stress is unavoidable due to the increasing distance between the areas of power generation and consumption. The reliable operation during the estimated lifetime of several decades is only feasible with a precise partial discharge diagnosis. Hence, a sensitive measurement and a doubtless interpretation of the results is necessary. Nevertheless, it is necessary to take into account, that under alternating voltage stress established physical mechanisms of the discharge processes and the influence of dielectric interfaces cannot be adopted directly, due to the changed voltage stress with a constant electric field and the related surface and volume charge accumulation. Aim of this thesis is the analysis of defects in gas-insulated systems with and without dielectric interfaces under DC voltage stress and thereby to contribute to a reliable interpretation of partial discharge measurements. In addition, known electrical and novel optical measurement methods are investigated with respect to their capabilities and limitations when used under DC voltage stress. The experimental investigations are carried out in model electrode arrangements. The weakly inhomogeneous electrical field of the gas-insulated systems is replicated in three configurations, one for each defect investigated. The detailed analysis of the discharge processes is enabled by a direct measurement of the partial discharge currents. A distinction between impulse currents and pulseless currents is made. Due to assembly faults or insufficient material quality fixed, metallic protrusions can be created within the insulation system. The experimentally observed dependencies of the discharge processes on the polarity of the defect, the insulating gas pressure and the voltage stress permit a classification of four different types of discharge. In addition to the investigations in the most commonly used insulating gas sulphur-hexafluoride a comparison of the results with measurements in the climate-friendly alternative synthetic air are made. Derived from this, commonalities and differences in the discharge behaviour are discussed. Free moving, metallic particles can adhere to the gas-solid interface. The accumulation of surface charges at the solid insulator influences the partial discharge inception significantly. Due to the steady-state discharge behaviour, which is comparable to the fixed, metallic protrusion without contact to a dielectric interface, distinguishing between the two defects based on pulse repetition rates and amplitudes is challenging. A unique aspect under DC voltage stress are discharges at the orthogonal interface between electrode coating and insulating gas. The analysis of the causes of the occurrence of these discharges, their optical and electrical characteristics and strategies for the prevention are investigated. Derived from the results, recommendations for partial discharge diagnosis of gas-insulated DC systems are discussed. These recommendations are an essential component for the future use of this asset in a high-performance electric power system with high reliability of the power supply.

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.

Les systèmes photovoltaïques à concentration, ou CPV, reposent sur le principe de la concentration des rayons du soleil sur une cellule photovoltaïque à très haut rendement. Le CPV reste jeune face au photovoltaïque (PV) classique qui accumule plus de 30 ans de retour d’expérience.La pérennisation de cette technologie CPV ne passera que par la démonstration d’une certaine maturité. Aussi, la question de la fiabilité de ces systèmes est plus que jamais d’actualité. Dans ce contexte, le Commissariat à L’Energie Atomique et aux Energies Alternatives (CEA) a répondu à la sollicitation lancée par des fabricants de modules CPV français sur la thématique de la conception et de la fiabilisation de modules CPV par une collaboration de ses différents laboratoires, dont le Laboratoire des Modules Photovoltaïques (LMPV). C’est au sein de ce laboratoire que s’effectuent les travaux de thèse. La diversité des éléments constituant un module CPV a poussé les travaux de thèse à se concentrer sur le coeur fonctionnel des modules : les assemblages CPV. Une première partie des travaux de thèse a consisté à mettre en place les bons outils de caractérisation, en partant parfois d’une feuille blanche. La mesure de caractéristique IV dans l’obscurité, la mesure de réponse spectrale, la tomographie RX ou encore l’électroluminescence sont autant de moyens de caractérisation de cellules multi-jonctions amenés par les travaux de thèse. Les efforts conduits sur l’électroluminescence auront permis l’invention d’une nouvelle technique de caractérisation des interfaces cellule/ substrat des assemblages CPV, concrétisée par le dépôt d’un brevet. Une collaboration entre le laboratoire d’accueil et l’Institut de l’Energie Solaire (IES) à Madrid a permis l’accès à la mesure de performance des assemblages CPV sous éclairement. Tous ces moyens ont rendu possible une caractérisation fine des assemblages CPV et ont permis de s’intéresser à leur robustesse-fiabilité, deuxième partie des travaux de thèse. Deux types d’assemblages CPV ont été étudiés durant les travaux de thèse. Le premier, basé sur un substrat Direct Bonded Copper (DBC) correspond à l’état de l’art et est le plus utilisé dans l’industrie CPV. Le deuxième, en rupture technologique avec l’état de l’art, repose sur un Substrat Métal Isolé (SMI), et a été intégralement développé par le CEA et ses partenaires industriels. L’étude de la robustesse de ces assemblages CPV a été faite par l’emploi de tests de vieillissement accéléré dont la nature est justifiée par le retour d’expérience et la définition des spécifications environnementales. Aucune défaillance n’a été observé sur chacun des types d’assemblage. Les assemblages SMI se comportent comme les assemblages DBC, considérés comme références. Les travaux de thèse offrent donc un premier retour d’expérience propre au laboratoire d’accueil et la mise en place d’une infrastructure complète de caractérisation d’assemblage CPV permet aujourd’hui au CEA d’être un acteur clé du CPV en France. / Concentrating Photovoltaic (CPV) is based on the concentration of solar rays on very-high efficienciessolar cells. Multi-junction architectures used in CPV systems reach efficiency superior to44% under concentration. This has created great interest for this technology over the past decade.Nevertheless, CPV has still to be proven reliable. This work contributes to this goal.CPV assemblies -or receivers- are defined by the electrical, mechanical and thermal cohesionof a multi-junction solar cell on an appropriate substrate. The complexity of multi-junctionarchitecture does not allow their characterization with the existing PV tools. Therefore, the firstachievement of the work was the development of a complete infrastructure for the characterizationof such devices. The second part developed accelerated ageing tests and analysis methods to studythe degradation process of these assemblies.A new method for the characterization of die-attached CPV cell assembly has been provensuccessful. It is called EEL for Enhanced ElectroLuminescence. This method is cost effective andreally fast and has therefore been patented. Regarding the characterization of performance of CPVcell assembly under illumination (2nd part of the thesis), collaboration has been made with theInstituto de Energìa Solar (IES) in Madrid, Spain. Thanks to this collaboration, two types of CPVcell assemblies have been studied. One based on the Direct Bonded Copper (DBC) substrate, correspondingto the state-of-the-art and most used type of substrate in CPV industry. The other is acompletely new type of substrate, inspired by the Insulated Metal Substrate (IMS). This new IMSbased CPV cell assembly has been developed by the CEA and its industrial partners. The reliabilitystudy of these CPV cell assemblies (DBC and IMS) has been conducted through acceleratedageing tests. It has been shown that none of the DBC or IMS cell assembly present infant mortalityor failure upon ageing.This work has launched the CPV activity at INES. Results on receivers now need to be confirmedon complete CPV-modules and systems.

The use of insulated-gate bipolar transistor (IGBT)-based power converters is increasing exponentially. This is due to high performance of these devices in terms of efficiency and switching speed. However, due to the switching action, high frequency electromagnetic interference (EMI) noises are generated. Design of a power converter with reduced EMI noise level is one of the primary objectives of this research. The first part of the work focuses on designing common-mode (CM) filters, which can be integrated with differential-mode (DM) filters for three-phase pulse-width modulation (PWM) rectifier-based motor drives. This work explores the filter design based on the CM equivalent circuit of the drive system. Guidelines are provided for selection of the filter components. Different variants of the filter topology are evaluated to establish the effectiveness of the proposed topology. Analytical results based on Bode plot of the transfer functions are presented, which suggest effective EMI reduction. Experimental results based on EMI measurement on the grid side and CM current measurement on the motor side are presented. These results validate the effectiveness of the filter. In the second part of the work, it is shown that inclusion of CM filters into DM filters results in resonance oscillations in the CM circuit. An active damping strategy is proposed to damp the oscillations in both line-to-line and line-to-ground ac voltages and currents. An approach based on pole placement by state feedback is used to actively damp both the DM and CM filter oscillations. Analytical expressions for state-feedback controller gains are derived for both continuous-and discrete-time models of the filter. Trade-off in selection of the active damping gain on the lower-order grid current harmonics is analysed using a weighted admittance function method. In the third part of the work, single-phase grid-connected power converters are considered. An integrated CM filter with DM LCL filter is proposed. The work explores the suitability of PWM methods for single-phase and parallel single-phase grid-connected power converters. It is found that bipolar PWM and unipolar PWM with 180◦interleaving angle are suitable for single-phase and parallel single-phase power converters, respectively. The proposed configuration along with the PWM methods reduces the CM voltage, CM current, and EMI noise level effectively. It is also shown that the suggested circuit is insensitive to nonidealities of the power converter such as dead-time mismatch, mismatch in converter-side inductors, unequal turn on and turn off of the switches, and propagation delays. In the fourth part of the work, the inter-phase inductor in parallel interleaved power converters is integrated with LCL filter boost inductor. Different variant designs are presented and compared with the proposed structure. It is shown that the proposed structure makes use of standard core geometries and consumes lesser core material as well as copper wire. Hence, it reduces the overall size and cost of the power converter. In the present work, a 10kVA three-phase back-to-back connected with input LCL filter and output dv/dt filter, a 5kVA single-phase grid-connected power converter with LCL filter, and a 7.5kVA parallel single-phase grid-connected power converter with LCL filter are fabricated in the laboratory to evaluate and validate the proposed methods. The experimental results validate the proposed methods that result in significant EMI performance improvement of grid-connected power converters.

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.

Electromagnetic effects of lightning currents during a direct hit to tall communication towers, other instrumented towers and chimneys can be hazardous to associated cables, as well as, electrical and electronics systems. The standard practice in telecommunication and other related fields is to bond the cable sheath to the tower and ground connection is made before it enters the base station. However, in some specific cases when power, signal and data logging cables are to be supported on the same tower, isolation of power cables is demanded. In a totally different situation, attempts are also made to have a dedicated isolated down conductor. A critical review of the situation demanded a more quantitative answer to the following questions: (i) whether it is possible to electrically isolate a dedicated down conductor, (ii) is it possible to electrically isolate the cables and their terminal equipment both mounted on towers serving as down conductor and if so, what will be the nature of current induced in the cables and (iii) as per the standard practice, if the cable sheaths are connected to the tower/structure, what will be the nature of the current shared by them. Addressing these important issues formed the scope of the present work. For the tall structures considered in this work, for the critical time periods, wave nature of the current dominates. This called for electromagnetic modeling covering Transverse Magnetic (TM) mode of the wave propagation. Owing to the complex geometrical features involved with the problem, both experiments on electromagnetically scaled laboratory models, as well as, theoretical simulation is attempted. An electromagnetically scaled laboratory model is employed for the time domain experimental investigation. This approach, which has been validated earlier, is further scrutinized to ensure its adequacy. In order to achieve generality and noting the fact that the associated parameters are rather difficult to be varied in the experimentation, theoretical investigation is also employed. For this, both NEC-2, as well as, an in-house thin wire time domain code developed for this work is employed. NEC-2 could handle multi-wire multi-radius junctions, while in-house time domain code could handle proximity and non-cylindrical shapes encountered with tower lattice elements. The investigation of induction to isolated cables on simple down conductors and towers is considered first. The induced current is shown to be bipolar oscillatory with the period of oscillation governed by the length of the cable. It is shown that the level of induction for good earth termination is below 5 – 10 % while that with moderate inductance in the earth termination can enhance the induction to higher levels. The level of induction is shown to be not critically dependent on the length of the cable, gap between cable and down conductor/tower. When multiple cables are mounted, they seem to influence each other and individually carry currents of lower amplitude. Also, the effect of shape and proximity of the tower lattice elements on induction is investigated. If the cable is housed inside a metallic tray, the amplitude of induced current is shown to be quite small. Subsequently, the evaluation of electrical stress between the isolated down conductor on tower and simplified representation of the structure is considered. A suitable definition of the electric stress for the wave regime is evolved and then it is shown that, at present, the voltage difference defined by the path integral of electric field across shortest path between the two entities is the best indicator for the stress. The electrical stress in the case of isolated down conductor on tower, as well as, down conductor with isolated cable is shown to reach very dangerous levels. On the other hand, the stress on the isolated cables on towers also serving as down conductors is shown to be relatively moderate. Interestingly, it is shown that the electrical stress and the voltage difference is dependent on the gap and for the critical time period, can be much lower than that calculated as a product of equivalent tower surge impedance and the stroke current, even before the arrival of ground end reflections. Finally, the current shared by cables connected to the down conductor is investigated. For the case of simple cylindrical down conductor with cable connected to it at the top, it is shown that the amount of current shared by the cable is not dependent on its length and the relative radii (cross section) have only a weak influence. For the case with down conductor formed by L and + angles, it is shown that the placement of cable at their interior corner can reduce the initial current shared by the cable. In order to model best possible situation with towers, experiments are conducted with cable inside an aluminum pipe. Even in this case, cable current builds up with successive reflections to become comparable with the current through the pipe itself. Subsequent investigation with 1:40 and 1:20 tower models lead to several interesting observations. Cables running along leg/face of the tower whether placed inside or outside the tower, always shares good amount of current. Further, frequent bonding of the sheath to the tower increases the current shared by the cable. Cable when housed in a metallic tray shares less than 50% of the current shared without the tray. Even though a complete quantification is not to be achieved in this work, it has made a good beginning with some significant contribution towards lightning protection issues pertaining to tall towers and structures.