Variable speed drives allow for more precise speed control of induction motors, are of high power factor, and offer fast response characteristics, compared to older technologies, such as motor-generator sets and eddy current clutches. However, due to the high switching frequencies as well as the high dV/dt in the output increased dielectric stresses are produced in the insulation system of the motor they supply. Due to the use of these solid state drives there have been concerns of premature failure in large, medium and high voltage, motors. To fully understand and deal with these concerns requires studying the degradation mechanisms, in the insulation system, caused by these drives; which, on an actual motor is both extremely costly as well as impractical. Therefore, coil samples which accurately represent the construction of the actual insulation system, must be aged and studied instead. In addition, to ideally replicate the aging process, the same waveform that the motor is subjected to must be applied to these samples. As a result of this requirement, a low power, two-level, high voltage PWM inverter has been built to replicate the most important characteristics of the output waveform of a variable speed drive. This power modulator allows for testing the insulation systems considering a real PWM waveform in which both the fast pulses and the fundamental low frequency are included. The results of these tests show that the effects of PWM waveforms cannot be entirely replicated by a unipolar pulse generator.

High-power voltage sourced converters(VSC’s) are becoming increasingly prevalent in modern transmission systems. These systems primarily use switching schemes generating kHz range harmonics, which may be magnified by one or more system resonances. Despite the high frequency harmonics, VSC systems widely use insulated equipment designed for operation at power frequencies; this includes critical substation components such as medium voltage polymeric cables and terminations. The stress grading systems of non-geometric (compact) cable terminations are susceptible to insulation degradation and eventual flashover failure, under high frequency harmonic stresses. As such, the present work studies high frequency voltage effects in cross-linked polyethylene cable terminations, and their relationship to stress grading (SG) design and material properties. Finite element modeling (FEM) has been used to analyze electric field and resistive heating in termination designs, in response to parametric variations in SG material properties. Experimental studies investigate thermal behaviour in a variety of commercial termination designs, using a high voltage, high frequency test setup developed to replicate conditions of high frequency harmonic resonance in a VSC system. The study results show that high frequency voltage application increases the electric field, resistive heating, and surface temperature rise, in non-geometric (compact) termination designs using field-dependant stress grading materials. Geometric (stress cone) designs are insensitive to high frequency harmonics; however, they have disadvantages compared to compact designs, making them a less practical long-term solution for high frequency applications. Among non-geometric designs, the field-dependent electrical conductivity σ (E), the permittivity ε, and the temperature dependencies of σ (E) and ε strongly influence the termination electrical and thermal behaviour under high frequency stress. Since thermal hotspots in cable termination SG areas may lead to material degradation and eventual failure, recommendations are made for an optimal non-geometric stress grading design, for terminations operating in environments where high frequency harmonics may be present.

Variable speed drives allow for more precise speed control of induction motors, are of high power factor, and offer fast response characteristics, compared to older technologies, such as motor-generator sets and eddy current clutches. However, due to the high switching frequencies as well as the high dV/dt in the output increased dielectric stresses are produced in the insulation system of the motor they supply. Due to the use of these solid state drives there have been concerns of premature failure in large, medium and high voltage, motors. To fully understand and deal with these concerns requires studying the degradation mechanisms, in the insulation system, caused by these drives; which, on an actual motor is both extremely costly as well as impractical. Therefore, coil samples which accurately represent the construction of the actual insulation system, must be aged and studied instead. In addition, to ideally replicate the aging process, the same waveform that the motor is subjected to must be applied to these samples. As a result of this requirement, a low power, two-level, high voltage PWM inverter has been built to replicate the most important characteristics of the output waveform of a variable speed drive. This power modulator allows for testing the insulation systems considering a real PWM waveform in which both the fast pulses and the fundamental low frequency are included. The results of these tests show that the effects of PWM waveforms cannot be entirely replicated by a unipolar pulse generator.

High-power voltage sourced converters(VSC’s) are becoming increasingly prevalent in modern transmission systems. These systems primarily use switching schemes generating kHz range harmonics, which may be magnified by one or more system resonances. Despite the high frequency harmonics, VSC systems widely use insulated equipment designed for operation at power frequencies; this includes critical substation components such as medium voltage polymeric cables and terminations. The stress grading systems of non-geometric (compact) cable terminations are susceptible to insulation degradation and eventual flashover failure, under high frequency harmonic stresses. As such, the present work studies high frequency voltage effects in cross-linked polyethylene cable terminations, and their relationship to stress grading (SG) design and material properties. Finite element modeling (FEM) has been used to analyze electric field and resistive heating in termination designs, in response to parametric variations in SG material properties. Experimental studies investigate thermal behaviour in a variety of commercial termination designs, using a high voltage, high frequency test setup developed to replicate conditions of high frequency harmonic resonance in a VSC system. The study results show that high frequency voltage application increases the electric field, resistive heating, and surface temperature rise, in non-geometric (compact) termination designs using field-dependant stress grading materials. Geometric (stress cone) designs are insensitive to high frequency harmonics; however, they have disadvantages compared to compact designs, making them a less practical long-term solution for high frequency applications. Among non-geometric designs, the field-dependent electrical conductivity σ (E), the permittivity ε, and the temperature dependencies of σ (E) and ε strongly influence the termination electrical and thermal behaviour under high frequency stress. Since thermal hotspots in cable termination SG areas may lead to material degradation and eventual failure, recommendations are made for an optimal non-geometric stress grading design, for terminations operating in environments where high frequency harmonics may be present.

With the continuous growth of non-linear power electronic components and the increasing penetration of the distributed generation (DG), the potential for degradation in the power quality of the existing grid exists. There are concerns that the total harmonic distortion (THD) could reach unacceptable levels of 5% or higher. Moreover, there is additional potential of the presence of amplified harmonic components in the power network grid when the harmonic frequencies align with the resonant frequencies that are being injected by power electronic components of the DG. The above conditions could increase the electrical stresses on the insulation system of the power system components, and in particular, cable terminations are a concern. Standard cable terminations are designed to operate under power frequency in the power system network and their service life is considered accordingly. The research work aims to provide an understanding of the performance of the stress grading (SG) system of a commercial cable termination when the voltage waveform is distorted due to the presence of harmonics and when the high frequency and high dV/dt voltage waveforms are present from a typical power electronic drive. An aging experiment was performed for over a 600 hour time period using the pulse width modulated (PWM) high-voltage generator to quantify the impact of high frequency stress on SG system of cable termination. Furthermore, the cable termination was tested under power frequency, distorted voltage waveforms composed of fundamental and low order harmonics using an experiment setup that generate distorted voltage waveforms. Diagnostic techniques such as surface potential distribution measurements and surface temperature monitoring are used to analyze the termination performance. The surface tangential field is calculated based on the gradient of the termination surface potential as measured with an electrostatic voltmeter. The study shows that distorted voltage waveforms with high frequency and high dV/dt components, increase the electric field, resistive heating, and surface temperature rise in the terminations that use the field-dependent SG materials. The rise of electric field by as high as 27.1% and surface temperature rise of as high as 17C demonstrates the severity on the cable terminations. Such electric field enhancements for a period of time have a potential to initiate partial discharge that could lead to degradation of the termination. Moreover, surface temperature rise of 17 deg C could reduce the allowable ampacity of the cable conductor, reduce the short circuit levels, and reduce the feeder loading limits. The field-dependent electrical conductivity (σ(E,T)), permittivity (ε), and the temperature dependencies of (σ(E,T) and ε) have strong impact to degrade the electrical and thermal properties of the termination due to stress from the non-sinusoidal distorted voltage waveform. In order to minimize the surface temperature rise from the hotspot and electrical stress enhancement that eventually lead to insulation degradation and failure, the following recommendations are made for a suitable SG design for a termination to handle the severe voltage stress: Apply the capacitively graded termination in the grid where the distortion levels are low. Under the increased total harmonic distortion levels and HF components, resistively grading with higher degree of nonlinearity (achieved through the use of ZnO filler) is beneficial. The utilities could take preventive maintenance on medium voltage power cable accessories to prevent the termination failure before it actually occurs. Researchers could focus to resolve and minimize the rising power quality issues when the distribution generations are operated, improve the power electronic converters, and provide cost-effective harmonic filter solutions for harmonic mitigation.

The dissertation comprises the general characteristic of the work, 5 chapters, conclusions and recommendations, references and list of published works on the topic of the dissertation. The dissertation covers 149 pages, 78 illustrations, 17 tables. In introduction chapter the problem topicality is considered, the aims and tasks of the work are formulated, the novelty of the research is described, the author’s reports and publications as well as the dissertation structure are presented. Chapter 1 is intended for the review of literature. The survey of ozone sources in working premises and the environment air is presented as well as methods of the ozone concentration determination and its dispersion simulation are discussed. At the end of the chapter conclusions are formulated and the dissertation tasks are concretized. In chapter 2 the methodology of the ozone concentration determination with the ozone analyzer and passive samplers as well as the measurement methods of aerosol particles and meteorological, microclimatic parameters are described. In chapter 3 the results obtained during experimental investigations are discussed: the ozone concentration changes near technogenic sources of this pollutant, the influence of meteorological and microclimatic parameters on the ozone concentrations changes, the relation of aerosol particles with the ozone concentration. Ozone concentrations measured by different methods are compared and the results of comparative measurements are... [to full text] / Disertaciją sudaro įvadas, 5 pagrindiniai skyriai, darbo išvados ir rekomendacijos, literatūros sąrašas, publikacijų sąrašas. Bendra disertacijos apimtis – 149 puslapiai, 80 iliustracijų, 17 lentelių. Įvadiniame skyriuje nagrinėjamas problemos aktualumas, formuluojamas darbo tikslas bei uždaviniai, aprašomas mokslinis darbo naujumas, pristatomi autoriaus pranešimai ir publikacijos, disertacijos struktūra. Pirmasis skyrius skirtas literatūros apžvalgai. Jame pateikta ozono šaltinių darbo patalpose ir aplinkos ore apžvalga, aptariami ozono koncentracijos nustatymo ir jos sklaidos modeliavimo metodai. Skyriaus pabaigoje formuluojamos išvados ir konkretinami disertacijos uždaviniai. Antrajame skyriuje aprašomi ozono koncentracijos nustatymo ozono analizatoriumi ir pasyviaisiais kaupikliais metodika bei aerozolio dalelių ir meteorologinių, mikroklimato parametrų matavimo metodai. Trečiajame skyriuje aptariami eksperimentinių tyrimų metu gauti rezultatai apie: ozono koncentracijos kitimą prie technogeninių šio teršalo šaltinių, meteorologinių ir mikroklimato parametrų įtaką ozono koncentracijos pokyčiams, aerozolio dalelių sąryšį su ozono koncentracija. Lyginamos ozono koncentracijos matuotos skirtingais metodais ir aptariami lyginamųjų matavimų rezultatai. Ketvirtajame skyriuje pateikti RISK programa sumodeliuoti ozono sklaidos nuo šaltinio rezultatai. Penktajame skyriuje pasiūlytas technologinis sprendimo variantas, leidžiantis sumažinti ozono koncentracijas prie technogeninių... [toliau žr. visą tekstą]

Thin dielectric passivation layer is one of the basic construction elements in semiconductor device technology. There are few materials, from which the layers may be manufactured. They are oxides mainly, with Si02 as the most popular of them, although, the phosphor- and boron-silicon glasses are used as passivation layers, too. In choosing a passivant of power thyristors and diodes, there are two important considerations in addition to the usual requirement for providing uniform high breakdown voltage via substrate. One consideration is the thermal stability of the passivant to subsequent high-temperature processes. The other consideration is the bias-temperature stability of the passivation layers affecting the operation life expectancy of a device. In the technology of thyristors and diodes on silicon substrates the bias-breakdown voltage is not uniform over substrate due to non-homogeneity of passivated surface of the p-n junction. In this work, passivation of moat surface by means of electrochemical etching, formation of hydrogen-rich porous silicon layers and glass in-melting steps has been investigated. Passivation quality was controlled by the measurements of surface recombination characteristics after each technological step using a non-invasive technique, which employed microwave probed photoconductivity transients (MW-PCT). It has been shown that electrochemical etching - glass melting steps involved in passivation technological procedures resulted in a decrease of... [to full text] / Puslaidininkinių prietaisų pramušimo įtampos valdymas formuojant griovelį periferiniame perimetre yra viena iš labiausiai paplitusių technologinių operacijų, gaminant galios diodus bei tiristorius Si pagrindu. Aukštavolčių didelės galios puslaidininkinių prietaisų, kurie dirba kelių tūkstančių amperų diapazone, o uždarymo įtampa iki kelių tūkstančių voltų, didelė problema elektrinio lauko pasiskirstymas ties kristalo briauna, kur p-n sandūra išeina į paviršių ir kur vyksta griūtinis krūvininkų skaičiaus didėjimas. Darbo stabilumui užtikrinti būtina pasyvuoti paviršių kristalo periferijoje, ant profiliuoto krašto. Šiame darbe išanalizuota galingų puslaidininkinių struktūrų konstrukcija, pagrindinės charakteristikos, parametrų tarpusavio ryšis, taip pat technologinis procesas ir jo ypatumai. Išanalizuotos technologinio gamybos maršruto silpniausios pozicijos. Nustatyta izoliacinių griovelių ėsdinimo charakteristikų priklausomybė nuo ėsdiklio sudėties, nuo ėsdinimo įrenginio struktūros ir nuo ėsdiklio temperatūros kitimo. Sukurta stiklo pasyvacijos difuzinės krosnies monitoringo sistema, kuri skirta aukštų temperaturų ir dujų srautų matavimui proceso metu. Rekombinacijų charakteristikų kitimo pagalba, matuojant be kontakte MW-PCT technika, įvertinama izoliacinių griovelių pasyvacijos kokybė. Technologiniame gamybos maršrute, po izoliacinio griovelio ėsdinimo operacijos, prieš stiklo pasyvaciją sudarinėjamas porėtojo silicio sluoksnis, taip pat siūloma įvesti homogeniškumo... [toliau žr. visą tekstą]

Puslaidininkinių prietaisų pramušimo įtampos valdymas formuojant griovelį periferiniame perimetre yra viena iš labiausiai paplitusių technologinių operacijų, gaminant galios diodus bei tiristorius Si pagrindu. Aukštavolčių didelės galios puslaidininkinių prietaisų, kurie dirba kelių tūkstančių amperų diapazone, o uždarymo įtampa iki kelių tūkstančių voltų, didelė problema elektrinio lauko pasiskirstymas ties kristalo briauna, kur p-n sandūra išeina į paviršių ir kur vyksta griūtinis krūvininkų skaičiaus didėjimas. Darbo stabilumui užtikrinti būtina pasyvuoti paviršių kristalo periferijoje, ant profiliuoto krašto. Šiame darbe išanalizuota galingų puslaidininkinių struktūrų konstrukcija, pagrindinės charakteristikos, parametrų tarpusavio ryšis, taip pat technologinis procesas ir jo ypatumai. Išanalizuotos technologinio gamybos maršruto silpniausios pozicijos. Nustatyta izoliacinių griovelių ėsdinimo charakteristikų priklausomybė nuo ėsdiklio sudėties, nuo ėsdinimo įrenginio struktūros ir nuo ėsdiklio temperatūros kitimo. Sukurta stiklo pasyvacijos difuzinės krosnies monitoringo sistema, kuri skirta aukštų temperaturų ir dujų srautų matavimui proceso metu. Rekombinacijų charakteristikų kitimo pagalba, matuojant be kontakte MW-PCT technika, įvertinama izoliacinių griovelių pasyvacijos kokybė. Technologiniame gamybos maršrute, po izoliacinio griovelio ėsdinimo operacijos, prieš stiklo pasyvaciją sudarinėjamas porėtojo silicio sluoksnis, taip pat siūloma įvesti homogeniškumo... [toliau žr. visą tekstą] / Thin dielectric passivation layer is one of the basic construction elements in semiconductor device technology. There are few materials, from which the layers may be manufactured. They are oxides mainly, with Si02 as the most popular of them, although, the phosphor- and boron-silicon glasses are used as passivation layers, too. In choosing a passivant of power thyristors and diodes, there are two important considerations in addition to the usual requirement for providing uniform high breakdown voltage via substrate. One consideration is the thermal stability of the passivant to subsequent high-temperature processes. The other consideration is the bias-temperature stability of the passivation layers affecting the operation life expectancy of a device. In the technology of thyristors and diodes on silicon substrates the bias-breakdown voltage is not uniform over substrate due to non-homogeneity of passivated surface of the p-n junction. In this work, passivation of moat surface by means of electrochemical etching, formation of hydrogen-rich porous silicon layers and glass in-melting steps has been investigated. Passivation quality was controlled by the measurements of surface recombination characteristics after each technological step using a non-invasive technique, which employed microwave probed photoconductivity transients (MW-PCT). It has been shown that electrochemical etching - glass melting steps involved in passivation technological procedures resulted in a decrease of... [to full text]

No description available.

The proliferation of distributed energy resources (DER)s has prompted interest in the expansion of DC power systems. The technological limitations that hinder the expansion of DC power systems are the absence of DC circuit breakers and high step-up/high step-down DC converters for interconnecting DC systems. This thesis presents a transformerless high step-up DC-DC converter intended for use as an interconnect between DC systems. The converter is required to operate at medium to high voltage (>1kV) and provide high voltage gain (>5). This work details the steady state operation and dynamic model of the proposed converter. The component ratings are identified and converter design limitations are investigated. A 100V:1kV/4kW prototype is produced to verify the analytic steady state model and measure efficiency. An experimental efficiency of 90% was achieved at a step-up ratio of 1:10, however efficiency at low power is limited due to the need to circulate power.