Στη παρούσα διπλωματική εργασία θα μελετηθεί η συμπεριφορά των εναέριων γραμμών μέσης τάσης του νομού Άρτας, όταν αυτές πλήττονται από διάφορα αίτια, τα οποία είναι καταγεγραμμένα μέσω του Συστήματος Αναγγελιών Βλαβών (ΣΑΒ) της ΔΕΗ και βρίσκονται στη διάθεσή μας για το έτος 2011. Τα κυριότερα αυτών είναι τα κεραυνικά πλήγματα και η κακοκαιρία και σε αυτά θα δωθεί βαρύτητα. Σκοπός είναι να εξαχθούν, μέσω θεωρητικών υπολογισμών, κάποια συμπεράσματα για το πως θα συμπεριφέρονται οι γραμμές αυτές ανάλογα με τη τοποθεσία, το ανάγλυφο της περιοχής, το ύψος της γραμμής και διάφορους άλλους παράγοντες που θα αναφερθούν λεπτομερώς στο 3ο κεφάλαιο. Για την καλύτερη κατανόηση όμως των προαναφερθέντων και για να έχουμε μια πιο πλήρη εικόνα επι του αντικειμένου, θα πρέπει αρχικά να γίνει μια συνοπτική παρουσίαση ενός τυπικού συστήματος ηλεκτρικής ενέργειας, αφού πρώτα αναλυθεί στα επιμέρους τμήματά του. Αναγκαία επίσης είναι και η αναφορά της IEEE οδηγίας για τη συμπεριφορά έναντι κεραυνών των εναέριων γραμμών διανομής ηλεκτρικής ενέργειας, μιας και οι επικρατούντες κλιματολογικές – περιβαλλοντολογικές συνθήκες είναι από τις βασικές αιτίες πρόκλησης σφαλμάτων στα εναέρια δίκτυα. / This thesis studies the behavior of medium voltage (distribution) lines of county of Artas, when they are affected by various causes, which are written down from the System Notice Damage of Hellenic Public Power Corporation and are at our disposal for the year 2011. The most important of them are lightning strikes and bad weather, to which extra focus will be given. Our purpose is to conclude through theoretical calculations to the behavior of these lines according to the location, relief area, the line’s height and other factors that will be mentioned in chapter 3. In order to have a complete perception of the subject, a brief presentation of a typical power system will take place after it is analysed in subsections. It is also important to mention the guidelines of Intstitute of Electrical and Electronic Engineers (IEEE) on the behaviour of air power distribution lines against lightning, as the prevailing environmental and climatological conditions are the most essential causes of faults on air lines.

No description available.

A new algorithm for modeling and simulation of lossy interconnect structures modeled by transmission lines with Frequency Independent Line Parameters (FILP) or Frequency Dependent Line Parameters (FDLP) is developed in this research. Since frequency-dependent RLGC parameters must be employed to correctly model skin effects and dielectric losses for high-performance interconnects, we first study the behaviors of various lossy interconnects that are characterized by FILP and FDLP. Current general macromodeling methods and Model Order Reduction (MOR) algorithms are discussed. Next, some canonical integrals that are associated with transient responses of lossy transmission lines with FILP are presented. By using contour integration techniques, these integrals can be represented as closed-form expressions involving special functions, i.e., Incomplete Lipshitz-Hankel Integrals (ILHIs) and Complementary Incomplete Lipshitz-Hankel Integrals (CILHIs). Various input signals, such as ramp signals and the exponentially decaying sine signals, are used to test the expressions involving ILHIs and CILHIs. Excellent agreements are observed between the closed-form expressions involving ILHIs and CILHIs and simulation results from commercial simulation tools. We then developed a frequency-domain Dispersive Hybrid Phase-Pole Macromodel (DHPPM) for lossy transmission lines with FDLP, which consists of a constant RLGC propagation function multiplied by a residue series. The basic idea is to first extract the dominant physical phenomenology by using a propagation function in the frequency domain that is modeled by FILP. A rational function approximation is then used to account for the remaining effects of FDLP lines. By using a partial fraction expansion and analytically evaluating the required inverse Fourier transform integrals, the time-domain DHPPM can be decomposed as a sum of canonical transient responses for lines with FILP for various excitations (e.g., trapezoidal and unit-step). These canonical transient responses are then expressed analytically as closed-form expressions involving ILHIs, CILHIs, and Bessel functions. The DHPPM simulator can simulate transient results for various input waveforms on both single and coupled interconnect structures. Comparisons between the DHPPM results and the results produced by commercial simulation tools like HSPICE and a numerical Inverse Fast Fourier Transform (IFFT) show that the DHPPM results are very accurate.

Wind loading considered in the design of overhead transmission lines is based on extreme values of synoptic wind, i.e. boundary layer wind originating from largescale meteorological pressure systems. Localized high intensity wind (HIW) storms such as tornadoes and downbursts are a different type of extreme wind frequently causing failures of overhead lines. This thesis covers the design aspects of overhead transmission lines when subject to localized HIW storms. A comprehensive review of the literature is included on the effects of such wind storms on lines and on mitigation measures. Furthermore, several options for the design of self-supporting transmission towers against localized HIW are discussed based on numerical simulations of several simple load cases on four examples of lattice structures.

Wet snow accumulating and shedding from overhead transmission lines can lead to a number of serviceability, safety and mechanical reliability issues. An innovative and inexpensive method to reproduce wet snow accretions on a cable in a controlled environment is explained. Wet snow sleeves were experimentally reproduced by using this technique to study their shedding mechanism. A numerical modeling technique using nonlinear finite element analysis is proposed to evaluate the dynamic response of an overhead cable subjected to any snow-shedding scenario. A time function is associated to the mass and weight of each snow element, which enables its virtual removal from the model at the time prescribed by the user. The response of a single span of overhead ground wire subjected to total and partial snow shedding scenarios is evaluated.

Minestery of Education of Lithuania, besides “Programme”, in 1994 offered for all schools methodical recommendations for educational process, named “General Educational Standards”. The process has influenced on citizens’ educational of Independent Lithuania. The Standards have allowed looking at subject in a new way. The topic of the given MA work “Teaching of mathematics analysis of elements in Lithuania since 1991”. Work goal is to clarify peculiarities of teaching of mathematics analysis of elements: -To consider the following topics: function, derivative, line, integral. Tasks: 1.To consider teacher’s job in our days and to find out this role in Educational process, to discuss connection between pupil and teacher. 2.To answer the question “What is integration and intersubject relations”. 3.To announce mark’s norms at lessons of mathematics. The given work is an analysis of text books, didactic books, small pedagogical experiments and statistic analysis. In general, to find the answer “How to teach and to be tought?

No description available.

No description available.

ESKOM has relied primarily on impedance-based measurement protection relays for the protection of transmission lines. One of the main disadvantages of distance relays is the limited fault resistance measurement capability. High-resistance faults are characterised by low fault currents, therefore the impedance calculated would be much bigger and so the fault will appear to be beyond the protected line. The main aim of the study is to gain clear understanding of the capability of the existing relays used in Eskom Transmission network, to be able to give recommendations on the refinements to the transmission line protection philosophy required to improve future protection performance. Omicron relay tests, showed that the dynamic characteristics of the three selected relays which were tested covers more fault resistance than that of the normal static impedance and also that the effect of DC offset is negligible with regards to fault resistance measurement capability. Normally the relays have built in algorithms which are able to filter nuisance signals. Theoretical case study that compared the most used relays in Eskom Transmission was done and the results are documented. Settings recommendations to improve fault resistance coverage were deduced from the above study. Fault investigation by using digital simulations (Matlab simulations) has proven the lack of capability to operate for some impedance relays in some fault conditions, as the fault resistance sometimes moves fault impedance beyond relay characteristic even when actual . polarization of the relay is considered. Analysis of the protection performance in transmission proved that high resistance faults accounts for at least half of protection equipment performance index (PEPI) incidents. Finally the author made recommendations to improve the protection performance concerning high resistance faults. / Thesis (M.Sc.Eng.)-University of KwaZulu-Natal, Durban, 2006.

This thesis documents a research study of High Voltage transmission line faults induced by fire. Conductor to conductor and conductor to ground flashovers have been experienced by electricity utilities around the world under conditions of veld and sugar cane fires. These types of faults are unpredictable and negatively impact line reliability and quality of supply. This is a crucial problem when the revenue of the industry is sensitive to voltage dips. Electricity utilities have taken a preventative approach, like clearing vegetation from the line servitude in order to decrease the frequency of line faults. There has also been a drive to collaborate with sugarcane farmers in order to have harvesting fires planned with utilities. Some success has been achieved with these initiatives however there still remains a large number of faults. The focus of this study is on the mechanism of fire-induced flashover. Previou s work has displayed the existence of two theories. The first theory suggests that flashover is due to the reduction in air insulation strength caused by a reduced air density that results from the thermal effect of the fire. The second theory suggests that small particles present in the fire cause electric field distortions that induce flashover. This study is focused on a theory , which indicates that flashover is induced due to an enhanced electric field which is a result of the conductive properties of the flames present in the air gap (the flame conductivity theory). The effects of particles and a reduced air density is said to support this mechanism that is the primary reason for flashover. This thesis present s a summary of the literature where firstly an understanding of air insulation behavior is displayed. Thereafter specific interest is given to the effect of fire and flames wherein the physics of flames are discussed. This then leads to the description of the flame conductivity theory. Chapter 4 deals with a simulative investigation into the effect a conducting flame has on the electric field distribution. This is looked at with a varying flame conductivity and gap length in mind. The simulations specifically cover the 275 kV and 400 kV line configurations. The simulative investigation results in a mapping of electric field enhancement against conductivity values and gap sizes. Thus a flashover probability is assessed by using the two factor flashover criteria when analyzing the electric field stresses. The objective of the experimental work in this study is to obtain insight on how the flame geometry and orientation affects flashover and the dependence of flashover on gap size. Tests involving a fire beneath a conductor were carried out for different gap sizes . Experimentation with particles above a flame was also conducted. It was concluded that flame structure does have an impact on flashover since a flame with sharp edges is more likely to cause flashover. Particles have a reducing effect on air insulation strength. This is mainly due to the fact that the particle reduces the effective air-gap size. No significant effect over and above this is noticed . For gaps spanned by clean Liquid Petroleum Gas (LPG) flames flashover voltage increases as gap-length increases with some degree of nonlinearity. Flame resistances and conductivity were approximated from measured currents and voltages. / Thesis (M.Sc.Eng.)-University of KwaZulu-Natal, Durban, 2005.