Sequential Selective Harmonic Elimination and Outphasing Amplitude Control for the Modular Multilevel Converters Operating with the Fundamental Frequency

With the growing use of DC voltage for power transmission (HVDC) and DC links for efficient AC motor drives, the R&D efforts are directed to the increase of DC/AC converter’s efficiency and reliability. Commonly used DC/AC converters, based on the carrier-frequency pulse-width modulation (PWM) to form a sinusoidal output voltage with a low level of higher harmonics, have switching time and switching loss issues. The use of multimodule multilevel converters (MMC), operating with the fundamental switching frequency and phase-shift control to form the ladder-style output voltage, reduces switching losses to minimum while keeping the low level of higher harmonics in the output voltage. The discussed sequential harmonic elimination method for MMC, using identical power modules operating with 50% duty cycle and fundamental frequency, is based on the combination of the multiple fixed phase shifts to form a ladder-style sinusoidal voltage with low total harmonic distortion (THD) and symmetrical variable phase shifts to control the output voltage amplitude. The principles of the sequential selective harmonic elimination for MMC topology and amplitude control are described with two examples. The first example is the industrial-frequency DC/AC converter complying with THD requirements of IEEE 519 2014 standard without the output filter. The second example is a high-frequency converter, used as a transmitter, loaded with the resonant antenna, where the evaluation criteria are decreasing of the transmitter losses and increasing of the reliability or life expectancy at elevated temperature

Development of Nuclear Magnetic Resonance Downhole Logging Tool For Hydrocarbones Search

This thesis presents a new development in the design of Nuclear Magnetic Resonance (NMR) downhole logging tools for the oil and gas industry and operates in a high temperature environment. Two main improvements of this tool is suggested and analyzed in this thesis. They are a new NMR antenna containing a Faraday shield placed between the antenna coil and the analyzed fluid and an improved NMR transmitter operating at 175 °C ambient temperature. This is based on multi-module, multilevel topology, which combines the outputs of several identical power modules, which operate at a 50% duty cycle at its fundamental frequency. The NMR antenna equipped with Faraday shield decreases the resonant frequency shift to less than 1%, which makes it possible to perform measurements without engaging a special tuning system for the compensation of antenna detuning. It was shown experimentally that the implementation of the Faraday shield makes the NMR echo signal less dependant on the type of the well fluid being measured. For the multi-module NMR transmitter, a method of DC/AC conversion based on the multi-vector synthesis of the sinusoidal voltage with sequential elimination of the higher harmonics in the output voltage was used. This was done in order to decrease the power dissipation occurring in the resonant loads. The present study reevaluates the prior investigations of a silicon MOSFET’s drain-to-source resistance (Rdson) in a temperature range of up to 200 °C as a function of the temperature and the drain current (Id). An imp-roved Rdson behavioral model in which Rdson and power index (P) of Rdson dependence on the temperature proposed as a function of Id was tested and validated for the temperature range from 25 °C to 200 °C. The multilayer X7R type capacitors, used as the high power NMR transmitter’s filters, were investigated for excessive losses and ways to mitigate these losses were suggested. It was demonstrated that these capacitors behave as piezoelectric resonators when biased with DC voltage and that their mechanical resonances increase the capacitors’ losses and propability of failure. The multimodule NMR transmitter, which uses a combination of the fixed phase shift method for the cancellation of the higher harmonics an increase of the NMR tool signal to noise ratio and NMR transmitter life expectancy. The higher harmonics cancelation in the multimodule NMR transmitter was validated experimentally by testing transmitters with different topologies

Reference system approach within the white-dwarfs theory

Reference system approach of non-relativistic electron fluid theory was adapted for calculation of characteristics of the electron-nuclear model at the densities typical of degenerate dwarfs. Two- and three correlation functions of degenerate relativistic electron gas have been calculated in the momentum-frequency representation in the local field approximation. Main contributions of the Coulomb interactions to the energy and equation of state of the model at $T=0$ K have been calculated in the adiabatic approximation.Comment: 21 pages, 11 figures, 3 table

Возбуждение резонансной нагрузки синусоидальным напряжением синтезированным многовекторным методом снижает потери проводимости и увеличивает надежность

Показано перевагу застосування багатовекторного методу синтезу синусоідальної напруги з подавленням вищих гармонік при роботі на резонансну нагрузку. Для перетворювачів працюючих при дуже високій температурі (175С) виграш в очікуваному часі життя може бути 3 - 4 рази за правилом Ареніуса при тій самій кількості силових транзисторів.A method of DC/AC conversion based on the multi-vector synthesis of the sinusoidal voltage with the sequential elimination of unwanted harmonics in the output voltage and current resulting in lower power dissipation especially for the resonant loads is discussed. Simulation results of the standard 50% duty cycle operation approach and the proposed multi-vector topology are compared and linked to the temperature related characteristics of the MOSFET switches and resulting life expectancy of the DC/AC inverter.Показано преимущество использование многовекторного метода синтеза синусоидального напряжения с подавлением высших гармоник при работе на резонансную нагрузку. Для преобразователей работающих при очень высоких температурах (175С) выигрыш в ожидаемом времени жизни может достигать 3 - 4 раза по правилу Аррениуса при том же количестве используемых силовых транзисторов

An increase of a down-hole nuclear magnetic resonance tool’s reliability and accuracy by the cancellation of a multi-module DC/AC converter's output’s higher harmonics

Abstract: Described in this paper is a method for improving higher harmonic cancellation in Nuclear Magnetic Resonance (NMR) transmitters, which are used in oil and gas well logging tools operating at 175°C. Multi-module multi-level topology which combines the outputs of several identical power modules operating at 50% duty cycle at the fundamental frequency provide the versatility needed for both low harmonic sine voltage synthesis and amplitude control. Cancellation of the output voltage higher harmonics is achieved by creating fixed relative phase shifts between the individual modules of the multi-module converter. The amplitude control employs the Chireix-Doherty outphasing modulation principle with added feed forward correction circuitry. The possibilities of a 20% increase of the tool signal to noise ratio (SNR), as compared to that of a two-module transmitter has also demonstrated significant increase in the tool life expectancy

Determinantes do risco de endividamento das famílias: Estudo de caso em Portugal

Dissertação de mestrado em Economia Monetária, Bancária e FinanceiraOver the years, the level of indebtedness has increased gradually. Mortgage credit, on the one hand, has become one of the most important, financial obligations of the Portuguese households, as it covers about 80% of all loans. However, on the other hand, the mortgage credit has been able to boost some economic sectors in Portugal, such as construction. Therefore, it is important to study the determinants of the risk of indebtedness in the mortgage credit due to its impact on the households, on the banking system and on the real estate market. At moderate levels, indebtedness improves welfare and increase economy growth, but high levels can be damaging. Therefore, this dissertation focuses on the study of the determinants of debt risk, specifically on the debt of Portuguese households. The objective of this work is to analyze the factors that are mostly associated with indebtedness. I shall use a multiple regression model, with macroeconomic variables. So, we are going to be able to analyze if they affect the rate of household debt. Have evaluated the variables, which had indicated previously, negative or positive impact. Thus, verifying that nearly all the expected results were confirmed. All variables in our model, namely, interest rate, saving, consumption, inflation rate and other variables are associated with the level of household debt, either positively or negatively. This study focused mainly on the concept of indebtedness, the indicators of indebtedness in Portugal, the factors that influence indebtedness and over-indebtedness, and also on the consequences of over-indebtedness and the growth of the indebtedness of Portuguese families.Ao longo dos anos, o nível do endividamento aumentou gradualmente. O crédito hipotecário, por um lado, tornou-se uma das obrigações financeiras mais importantes das famílias portuguesas, uma vez que cobre cerca de 80% de todos os empréstimos. No entanto, por outro lado, o crédito hipotecário tem sido capaz de impulsionar alguns setores económicos, como a construção. Por este motivo, é importante estudar os determinantes do risco de endividamento no crédito hipotecário devido ao seu impacto nas famílias, no sistema bancário e no mercado imobiliário. Em níveis moderados, o endividamento melhora o bem-estar e aumenta o crescimento da economia, mas altos níveis podem ser prejudiciais. Assim, esta dissertação centra-se no estudo dos determinantes do risco da dívida, especificamente sobre o endividamento no crédito hipotecário das famílias portuguesas. O objetivo deste trabalho é analisar os fatores associados ao endividamento. No estudo será usado o modelo de regressão múltipla, com variáveis macroeconómicas para analisar se estas variáveis afetam a taxa de endividamento das famílias. Avaliam-se as variáveis, indicadas anteriormente os impactos negativos ou positivos. Assim, verifica-se que quase todos os resultados esperados se confirmaram. Todas as variáveis do modelo apresentada, nomeadamente taxa de juro, poupança, consumo, taxa de inflação e outras variáveis estão associadas ao nível do endividamento das famílias, de modo positivo ou negativo. Este estudo incidirá principalmente sobre o conceito de endividamento, os indicadores de endividamento em Portugal, os fatores que têm influenciado o endividamento e o sobre-endividamento, e ainda sobre as consequências do endividamento e do crescimento do endividamento das famílias portuguesas

Application of Vortex and Pulsation Coolers in Technologies of Rare Gases Extraction

Two types of gas-dynamic coolers were considered: Rank-Hilsh vortex tubes and resonance tubes, known as devices of Hartmann-Sprenger. These devices have a number of similar physical and structural features: no moving parts, low inertia, environment safety, they can be sources of cold and heat simultaneously, generate pressure pulse, and separate gas mixtures. Gas-dynamic devices can be used in the technologies of rare gases obtaining as coolers of reflux condensers. They use pressure drop and don’t need in additional energy consumption. The temperature drop of mixtures phase equilibrium allows to reduce the concentration of by-products. Due to this, costs of gas concentrates transportation are reduced. Schemes, in which the separation function of the gas-dynamic devices are used, were investigated. The results of tests of step-type coolers in the condition of cryogenic temperatures on methane and helium are presented. An influence of the real gas and a scale factor in the interval of the energy separation chamber’s diameter up to 3 mm was studied

Potential regularization as the method accounting for short-range correlations in the electron liquid theory

In our previous paper [1] we proposed a Coulomb potential regularization as one of the methods for short-range correlation accounting in the electron liquid model. The present paper formulates the criterions of optimal choice of regularization as well as calculates the energetic, structural and dielectric characteristics of the model.В попередній роботі авторів [1] запропоновано ідею регуляризації потенціалу Кулона як один із способів врахування короткосяжних кореляцій в моделі електронноїрідини. В даній роботі сформульовано критерії оптимального вибору регуляризаціїі розраховано енергетичні, структурні та діелектричні характеристики моделі