Dissertations / Theses on the topic 'Direct current source'

This research focuses on voltage source multilevel converters in high voltage direct current (HVDC) transmission systems. The first Voltage Source Converter based HVDC (VSC-HVDC) systems with series connected IGBTs in a two-level converter represented a solution to meet industrial and economical requirements but is associated with significant drawbacks such as high dv/dt and di/dt, high switching loss, and poor output voltage and current quality. To overcome these issues, the multilevel converter was proposed for HVDC application. The Modular Multilevel Converter (M2C) was the first multilevel converter to be commercially used in the power industry. In this thesis, the M2C is investigated mainly in terms of operating principle, capacitor size and capacitor voltage ripple, capacitor voltage balancing technique and modulation scheme. The results of this investigation show that the M2C offers the following features: improved efficiency, lower supporting voltage and current in the switching devices and low dv/dt. These features make the M2C suitable for HVDC systems. Two new operational principles and modulation strategies for a Hybrid Cascaded Multilevel Converter (HCMC) are proposed in this thesis. Both modulation schemes extend the modulation index linear range and improve the output waveform quality. This gives the HCMC a higher power density than any known multilevel converter topology for the same dc link voltage and switching device rating. Simulations for both types of multilevel converter (M2C and HCMC) are supported by practical results from scaled hardware laboratory converters. Mathematical analysis and calculation of conversion loss for both types of multilevel converter and for the conventional two-level converter are performed. It is shown that both M2C and HCMC provide lower conversion loss compare to the conventional two-level converter. A control strategy for these two multilevel converters in point-to-point and multi-terminal HVDC systems is also studied. Simulation results show that these two converters are able to operate over the entire specified P-Q capability curve and are capable of riding through ac faults without imposing any over-voltage or over-current on the converter switches.

The UK is facing two major challenges in the development of its electricity network. First, two thirds of the existing power stations are expected to close by 2030. Second, is the requirement to reduce its CO2 emissions by 80% by 2050. Both of these challenges are significant in their own right. The fact that they are occurring at the same time, generates a significant amount of threats to the existing power system, but also provides many new opportunities. In order to meet both these challenges, significant amounts of offshore wind generation has been installed in the UK. For the wind generation with the longest connections to land, Voltage Source Converter (VSC) based High Voltage Direct Current (HVDC) transmission has to be used. Due to the high power rating of the offshore wind farms, compared to the limited transmission capacity of the links, a large number of point-to-point connections are required. This has lead to the concept of HVDC grids being proposed, in order to reduce the amount of installed assets required. HVDC grids are a new transmission environment and the fundamental question of how they will protect themselves must be answered. Several new technologies are under consideration to provide this protection, one of which is the HVDC circuit breaker. As HVDC circuit breakers are a new technology, they must be tested in a laboratory environment to prove their operation and improve their Technology Readiness Level (TRL). This thesis is concerned with how such HVDC circuit breakers are operated, rated, and tested in a laboratory environment. A review of the existing circuit breaker technologies is given, along with descriptions of several novel circuit breakers developed in this thesis. A standardized method of rating DC circuit breaker and their associated test circuit is developed. Mathematical analysis of several circuit breakers is derived from first principles and low power prototypes are developed to validate these design concepts. A high power test circuit is then constructed and a semiconductor circuit breaker is tested. The key learning outcomes from this testing are provided.

Doctor of Philosophy
Department of Electrical and Computer Engineering
Behrooz Mirafzal
In this dissertation, a new topology for Direct-Drive Wind Turbines (DDWTs) with a new power electronics interface and a low-voltage generator design is presented. In the presented power electronics interface, the grid - side converter is replaced by a boost Current Source Inverter (CSI) which eliminates the required dc-bus capacitors resulting in an increase in the lifetime of DDWTs. The inherently required dc-link inductor for this topology is eliminated by utilizing the inductance of the Permanent Magnet Synchronous Generator (PMSG). The proposed three-phase boost CSI is equipped with Reverse-Blocking IGBTs (RB-IGBT) and the Phasor Pulse Width Modulation (PPWM) switching pattern to provide a 98% efficiency and high boost ratios ([superscript V]LL/V[subscript dc]) up to 3.5 in a single stage. In this dissertation, Phasor Pulse Width Modulation (PPWM) pattern for the boost – CSI is also modified and verified through simulation and experimental results. In order to realize potential capabilities of the boost inverter and to assist its penetration into renewable energy systems, the boost inverter dynamic behaviors are studied in this dissertation. Then, the developed models are verified using circuit simulations and experiments on a laboratory-scale boost – CSI equipped with RB-IGBTs. The developed dynamic models are used to study the stability of the boost – CSI through root locus of small signal poles (eigenvalues) as control inputs and load parameters vary within the boost inverter's operating limits. The dynamic models are also used to design the control schemes for the boost – CSI for both stand-alone and grid-tied modes of operation. The developed controllers of the boost – CSI are verified through simulation and experimental results. In this dissertation, the boost – CSI steady-state characterization equations are also developed and verified. The developed boost – CSI is used to replace the grid - side converter in a DDWT. A reliability analysis on the power electronics interface of an existing and developed topology is presented to demonstrate the increase in the mean time between failures. The boost – CSI enables conversion of a low dc voltage to a higher line-to-line voltage enabling the implementation of a low-voltage generator. This further enables a reduction in the number poles required in DDWT generators. The feasibility of the presented low-voltage generator is investigated through finite element computations. In this dissertation, a 1.5MW low-voltage generator designed for the proposed topology is compared with an existing 1.5MW permanent magnet synchronous generator for DDWTs to demonstrate the reduction in the volume, weight, and amount of permanent magnet materials required in the generator. The feasibility of the developed system is supported by a set of MATLAB/Simulink simulations and laboratory experiments on the closed-loop stand-alone and grid-tied systems.

Since the end of the Current Wars in the 19th Century, alternating current (AC) has dominated the production, transmission, and use of electrical energy. The chief reason for this dominance was (and continues to be) that AC offers a way minimize transmission losses yet transmit large power from generation to load. With the Digital Revolution and the entrance of most of the post-industrialized world into the Information Age, energy usage levels have increased due to the proliferation of electrical and electronic devices in nearly all sectors of life. A stable electrical grid has become synonymous with a stable nation-state and a healthy populace. Large-scale blackouts around the world in the 20th and the early 21st Centuries highlighted the heavy reliance on power systems and because of that, governments and utilities have strived to improve reliability. Simultaneously occurring with the rise in energy usage is the mandate to cut the pollution by generation facilities and to mitigate the impact grid expansion has on environment as a whole. The traditional methods of transmission expansion are beginning to show their limits as utilities move generation facilities farther from load centers, which reduces geographic diversity, and the integration of nondispatchable, renewable energy sources upsets the current operating regime. A challenge faces engineers - how to expand generation, expand transmission capacity, and integrate renewable energy sources while maintaining maximum system efficiency and reliability. A technology that may prove beneficial to the operation of power system is high voltage direct current transmission. The technology brings its own set of advantages and disadvantages, which are in many ways the complement of AC. It is important to update transmission planning processes to account for the new possibilities that HVDC offers. This thesis submits a discussion of high voltage direct current transmission technology itself and an examination of how HVDC can be considered in the planning process.
Master of Science

Cette thèse a été consacrée à l'étude des réseaux multi-terminaux haute tension à courant continu (MTDC). Les principales contributions étaient dans le domaine du contrôle automatique non linéaire, appliquées aux systèmes électriques, électronique de puissance et les sources d'énergie renouvelables. Le travail de recherche a été lancé avec l'intention de combler certaines lacunes entre la théorie et la pratique, en particulier: 1) d'enquêter sur diverses approches de contrôle pour le but d'améliorer la performance des systèmes MTDC; 2) d'établir des connexions entre la conception du contrôle empiriques existantes et analyse théorique; 3) d'améliorer la compréhension du comportement multi-échelle de temps des systèmes MTDC caractérisés par la présence de transitoires lents et rapides en réponse aux perturbations externes. En conséquence, ce travail de thèse peut être mis en trois domaines, à savoir la conception non linéaire de commande de systèmes MTDC, analyse des comportements dynamiques de système MTDC et l'application de systèmes MTDC pour le contrôle de fréquence des systèmes de climatisation
This dissertation was devoted to the study of multi-terminal high voltage direct current (MTDC) networks. The main contributions were in the field of nonlinear automatic control, applied to power systems, power electronics and renewable energy sources. The research work was started with the intention of filling some gaps between the theory and the practice, in particular: 1) to investigate various control approaches for the purpose of improving the performance of MTDC systems; 2) to establish connections between existing empirical control design and theoretical analysis; 3) to improve the understanding of the multi-time-scale behavior of MTDC systems characterized by the presence of slow and fast transients in response to external disturbances. As a consequence, this thesis work can be put into three areas, namely nonlinear control design of MTDC systems, analysis of MTDC system's dynamic behaviors and application of MTDC systems for frequency control of AC systems

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.

Thesis (MScEng)--Stellenbosch University, 2012.
ENGLISH ABSTRACT: The implementation of the Caprivi Link Interconnector (CLI) High Voltage Direct Current (HVDC) scheme in 2010 connecting the weak Namibian and Zambian Alternating Current (AC) transmission networks via overhead line is based on Voltage Source Converter (VSC) technology. This world-first combination of attributes presents a unique opportunity to study harmonic interaction between weak AC systems and VSC-based HVDC schemes. Relatively few publications exist that focus on AC and DC harmonic interaction and very few refer to VSC HVDC schemes. Because weak AC systems are much more prone to harmonic distortion than strong AC systems, there is a clear motivation for more detailed work in this field. In order to understand the context wherein AC and DC harmonic interaction exists, the fields of AC power system harmonic analysis and resonance, VSC switching theory, HVDC scheme configurations, Pulse Width Modulation (PWM) techniques and frequency domain analysis techniques are discussed. This thesis then presents the concept of Harmonic Amplitude Transfer Ratio (HATR) by a theoretical analysis of AC and DC harmonic interaction due to the fundamental component, as well as harmonic interaction due to scheme characteristic harmonics and is compared to the simulation results obtained from different software solutions. Simulation and modelling techniques for AC and DC harmonic interaction are discussed including AC and DC systems modelling. The theoretical results and simulation results are compared to the results obtained from a real life case study on the CLI HVDC scheme where a harmonic resonance condition occurred. The correlation of these three sets of results confirms the validity of the theories presented and possible mitigation of the case study resonance problems is explored. The results and conclusion highlight a variety of interesting points on harmonic sequence components analysis, VSC zero sequence elimination, AC and DC harmonic interaction due to the fundamental component and the HATR for different PWM methods, AC and DC harmonic interaction due to scheme characteristic harmonics, modelling techniques and mitigation for the resonance conditions experienced in the analysed real life case study.
AFRIKAANSE OPSOMMING: Die implementering van die Caprivi Skakel Tussenverbinder (CLI) hoogspannings- gelykstroom (HSGS) skema in 2010 wat die swak Namibiese and Zambiese Wisselstroom (WS) transmissienetwerke verbind via „n oorhoofse lyn is gebasseer op Spanningsgevoerde-omsetter tegnologie. Hierdie wêreld-eerste kombinasie van eienskappe verskaf „n unieke geleentheid om harmoniese interaksie tussen swak WS stelsels en Spanningsgevoerde-omsetter Hoogspannings GS stelsels te bestudeer. Relatief min publikasies wat fokus op WS en GS harmoniese interaksie bestaan, en baie min verwys na Spanningsgevoerde-omsetter Hoogspannings GS skemas. Omdat swak WS stelsels baie meer geneig is tot harmoniese verwringing as sterk WS stelsels, is daar „n duidelike motivering vir meer gedetaileerde werk in hierdie veld. Om die konteks te verstaan waarin WS en GS harmoniese interaksie bestaan, word die velde van WS kragstelsel harmoniese analise en resonansie, Spanningsgevoerde-omsetter skakelteorie, Hoogspannings GS skema opstellings, Pulswydte Modulasie (PWM) tegnieke, en frekwensiegebied analiese tegnieke bespreek. Hierdie tesis stel dan die konsep van Harmoniese Amplitude Oordragsverhouding voor deur „n teoretiese analise van WS en GS harmoniese interaksie na aanleiding van die fundamentele komponent, asook harmoniese interaksie a.g.v. harmonieke wat die stelsel kenmerk en word vergelyk met die simulasieresultate verkry uit verskilllende sagteware oplossings. Simulasie- en modelleringstegnieke vir WS en GS harmoniese interaksie word bespreek insluitend WS- en GS stelselmodellering. Die teoretiese resultate en simulasieresultate word vergelyk met die resultate wat verkry is uit „n werklike gevallestudie op die CLI HSGS skema waar „n harmoniese resonansie toestand voorgekom het. Die ooreenkomste tussen hierdie drie stelle resultate bevestig die geldigheid van die teorieë soos uiteengeset voor, en die moontlike verbetering van die gevallestudie resonansie probleme word verken. Die resultate en samevatting beklemtoon „n verskeidenheid punte aangaande harmoniese volgorde-komponent analiese, Spanningsgevoerde-omsetter zero-volgorde uitskakeling, WS en GS harmoniese interaksie na aanleiding van die fundamentele komponent en die Harmoniese Amplitude Oordragsverhouding vir verskillende PWM metodes, WS en GS harmoniese interaksie na aanleiding van skema-kenmerkende harmonieke, modelleringstegnieke, asook verbetering van die resonansie toestande soos ervaar in die analise van die werklike gevallestudie.

U okviru doktorske disertacije, realizovane su podešljive feritne komponente sa jezgrima koja su proizvedena od komercijalno dostupnih ESL 40011 feritnih traka čije se induktivnosti podešavaju primenom jednosmerne struje. Rad realizovanih podešljivih feritnih komponenti je verifikovan u kolu DC-DC konvertora podizača napona. U disertaciji je predložena merna metoda koja omogućava ispitivanje uticaja jednosmerne struje na karakteristike SMD induktora postavljenih u realnom okruženju. Metoda je verifikovana na komercijalno dostupnim SMD induktorima.
As part of this thesis variable ferite components with cores produced from comercialy available ESL 40011 ferite tapes manufactured in Low Temperature Co-fired Ceramic technology are implemented. Inductivity of the components is varied by applying DC current. Functionality of the implemented ferite components is verified in a circuit of DC-DC boost converter. This thesis proposes a measurement method which enables examining the influence of DC current on the characteristics of SMD inductors in real environment. The method is verified on comercialy available SMD inductors

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.

The objective of the proposed research is to investigate feasible approaches to dynamic control of the power grid. Growth in the demand for electric power, and an increase in the penetration of renewable energy resources are causing congestion on an already aging power grid. Conventional grid control involves the use of static assets that operate on long time scales. These assets provide no dynamic control on the grid, and are typically used for scheduled support. Existing solutions (FACTS devices) to dynamic grid control have seen minimal market penetration because of high cost and low reliability. The proposed research provides a solution for dynamic control of the power grid that augments existing grid assets with a thin AC converter (TACC) to realize enhanced dynamic control. The TACC is a direct AC converter with filter elements and no bulk energy storage that dynamically reflects the asset value on the grid. The converter has a fail-normal mode of operation that returns the asset to its initial operating state, thereby not degrading system reliability. Some applications of TACCs include Inverter-Less STATCOMs and Controllable Network Transformers, which are realized by augmenting shunt VAR capacitors and load tap changers respectively. The principle of virtual quadrature sources is proposed to enable conditioning of AC voltages and currents. The concept is a novel method to realize control of phase angle and, or harmonics in single-phase AC converters, with no bulk energy storage. This concept is used to control the TACC and provides the asset with significantly enhanced control capabilities. Scaling of the TACC to utility voltage and power levels has been addressed by proposing a novel multilevel direct AC converter. The concept proposes the use of commercially available low cost semiconductor devices to realize high power converters. The specific application chosen to validate the concept of TACCs, through a medium voltage design, is the Inverter-less STATCOM.

Thesis (MScIng (Electrical and Electronic Engineering))--University of Stellenbosch, 2009.
A bi-directional, direct conversion switch-mode converter is proposed for use in distributed household renewable energy systems. The converter is intended as the central interface between the household energy system’s low voltage Direct Current bus and the high voltage Alternating Current bus. The low voltage DC bus is connected to renewable generation and storage devices, while the high voltage AC bus is connected to the user’s equipment and the mains grid. The converter overcomes the inherent reverse-duty cycle problem associated with bi-directional converters by using a combination step-up / step-down half-bridge converter on the high voltage side of a high frequency transformer. The low voltage side of the transformer is driven by a full bridge inverter that acts as a rectifier during reverse mode. In order to control the flow of power in both directions the converter implements Average Current Mode Control. A method is developed to determine the transfer functions of common switch-mode converters by inspection alone. This method is applied to the proposed converter, and both current and voltage mode control loops are designed with the frequency response method. The control system is implemented using a Digital Signal Processor. A method of simultaneously simulating both the converter hardware and software is developed using VHDL. This method greatly reduced the development effort of the converter. The operation of the proposed converter is verified through this method of simulation. A prototype converter is constructed and successfully tested, thereby proving the viability of the proposed converter topology and control methodology.

Дисертація на здобуття наукового ступеня кандидата технічних наук за спеціальністю 05.09.12 "Напівпровідникові перетворювачі електроенергії" – Національний технічний університет "Харківський політехнічний інститут", Харків, 2019. Дисертацію присвячено вирішенню важливої наукової задачі – створенню трифазних напівпровідникових перетворювачів з покращеними енергетичними та масовими характеристиками. Проведено аналіз характеристик існуючих типів трифазних випрямлячів з електронним зсувом фаз та показано, що трифазний випрямляч з електронним зсувом фаз пульсністю більш ніж 12 має містити узгоджувальний пристрій для балансування рівня вихідних напруг. Запропоновано використання низькочастотної широтно-імпульсної модуляції для балансування вихідної напруги модулів трифазного випрямляча, що дає можливість відмовитися від застосування автотрансформаторів на вході перетворювача. Проаналізовано електромагнітні процеси в перетворювачі на основі багатопульсних випрямлячів з електронним зсувом фаз та показано, що для розглянутого класу перетворювачів доцільно використовувати випрямлячі з пульсністю від 18 до 24 для мереж малої потужності. Запропоновані схемотехнічні рішення багатопульсних випрямлячів з електронним зсувом фаз, можуть бути використані в розробці та проектуванні перспективних джерел постійного струму, що вимагають покращених масових показників, відповідності вимогам по електромагнітній сумісності. Наприклад:– при розробці та проектуванні перспективних джерел постійного струму електрифікованих залізниць. – в наземному електрообладнанні аеропортів, як складова частина комплексу наземного живлення 3х115 В 400 Гц 30-90 кВт та ін.
thesis is devoted to the solution of an important scientific problem - the creation of three-phase semiconductor converters of alternating current to constant current with improved energy and mass characteristics. The characteristics of existing types of three-phase rectifiers with electronic phase shift are analyzed and it is shown that a three-phase rectifier with electronic phase shift with a pulse frequency of more than 12 should contain a matching device for balancing the output voltage level. The use of low-frequency pulse-width modulation for balancing the output voltage of three-phase rectifier modules is proposed, which eliminates the use of autotransformers at the input of the converter. Electromagnetic processes in the converter based on multipulse rectifiers with electronic phase shift are analyzed and it is shown that for the considered class of converters it is advisable to use rectifiers with a pulse range from 18 to 24. The proposed circuit solutions of multi-pulse rectifiers with electronic phase shift can be used in the development and design of promising sources of direct current, requiring improved mass performance and compliance with the requirements for electromagnetic compatibility. For example: - In the development and design of promising sources of direct current electrified railways. - In the ground electrical equipment of airports, as an integral part of the ground supply complex 3x115 V 400 Hz 30-90 kW, etc.

Дисертація на здобуття наукового ступеня кандидата технічних наук за спеціальністю 05.09.12 "Напівпровідникові перетворювачі електроенергії" – Національний технічний університет "Харківський політехнічний інститут", Харків, 2019. Дисертацію присвячено вирішенню важливої наукової задачі – створенню трифазних напівпровідникових перетворювачів з покращеними енергетичними та масовими характеристиками. У багатьох електричних мережах і системах напівпровідникові перетворювачі є одним з основних видів навантаження. Напівпровідникові перетворювачі є для мережі живлення нелінійним навантаженням, і його робота впливає на режими роботи мережі, особливо якщо потужність перетворювача і мережі співрозмірні. Тому при проектуванні як електричних мереж, так і напівпровідникових перетворювачів необхідно враховувати вплив роботи перетворювачів на мережу живлення. В силовій електроніці одним з найпоширеніших перетворювачів є трифазний випрямляч. Схеми випрямлячів трифазного живлення застосовуються в основному для живлення споживачів середньої й великої потужності. Покращення електромагнітної сумісності в цих системах можливе з використанням схем трифазних випрямлячів зі збільшеною пульсністю. Для роботи таких випрямлячів необхідним є використання фазозсувних трансформаторів або автотрансформаторів. Використання фазозсувних магнітних елементів погіршує масові характеристики перетворювача, а при збільшенні пульсності випрямляча ускладняється їх конструкція. Покращення масових характеристик та електромагнітної сумісності можливе з використанням електронного зсуву фаз в трифазних випрямлячах зі збільшеною пульсністю. В дисертаційній роботі запропоновано структуру багатопульсних випрямлячів з електронним зсувом фаз пульсністю більше 12-ти, що дозволяє поліпшити гармонійний склад споживаного струму за рахунок підвищення пульсності випрямляча, отримання високого коефіцієнту потужності за рахунок використання «відстаючих» і «випереджаючих» кутів керування. Обґрунтовано доцільність використання повністю керованих напівпровідникових ключів з зворотною блокуючою здібністю в трифазних випрямлячах з електронним зсувом фаз, що дозволяє отримати високий коефіцієнт зсуву між вживаним струмом і напругою джерела живлення за рахунок уніфікації використовуваних напівпровідникових ключів. Запропоновано використання низькочастотної широтно-імпульсної модуляції для балансування вихідних напруг модулів трифазного випрямляча, що дозволяє реалізувати схему перетворювача без використання вхідних узгоджувальних електромагнітних елементів. Досліджені електромагнітні процеси в трифазних багатопульсних випрямлячах з електронним зсувом фаз при використанні повністю керованих напівпровідникових ключів зі зворотною блокуючою здатністю, що дозволило отримати основні вирази для оцінки комутаційних втрат. Запропоновані схемотехнічні рішення багатопульсних випрямлячів з електронним зсувом фаз, можуть бути використані в розробці та проектуванні перспективних джерел постійного струму, що вимагають покращених масових показників, відповідності вимогам по електромагнітній сумісності. Наприклад: – в наземному електрообладнанні аеропортів, як складова частина комплексу наземного живлення літаків 3х115 В 400 Гц 30-90 кВт; – в перетворювачах частоти з ланкою постійного струму при підключенні AC/AC перетворювача безпосередньо до мережі живлення, тобто без використання масштабуючих трансформаторів.
Thesis for the degree of candidate of technical sciences in the specialty 05.09.12 "Semiconductor power converters" – National Technical University "Kharkiv Polytechnic Institute", Kharkov, 2019. The thesis is devoted to the solution of an important scientific problem - the creation of three-phase semiconductor converters of alternating current to constant current with improved energy and mass characteristics. In many electrical networks and systems, semiconductor transducers are one of the main types of load. Semiconductor transducers are nonlinear for the power supply, and its operation affects the network operating modes, especially if the power of the converter and the network are dimensional. Therefore, when designing both electrical networks and semiconductor converters, it is necessary to take into account the effect of converters on the power supply. In power electronics, one of the most common converters is a three-phase rectifier. Schemes of three-phase power rectifiers mainly used for power supply to consumers of medium and high power. Improvement of electromagnetic compatibility in these systems is possible using schemes of three-phase rectifiers with increased pulsativeness. The use of phase-shift transformers or autotransformers is necessary for such rectifiers. The use of phase-shift magnetic elements, worsens the mass characteristics of the converter, and when the rectifier pulsity increases, their design becomes more complicated. Improvement of mass characteristics and electromagnetic compatibility is possible with the use of electronic phase shift in three-phase rectifiers with increased pulsatility. In the dissertation work the structure of multipulse rectifiers with electron phase shift of pulses more than 12 is proposed, which allows to improve the harmonic composition of consumed current by increasing the rectifier pulsity, obtaining a high power factor by using the "lagging" and "outgoing" angles of control. The expediency of using fully controlled semiconductor keys with reverse blocking capability in three phase rectifiers with electron phase shift is substantiated. The use of low-frequency pulse-width modulation for balancing the output voltages of three-phase rectifier modules is proposed. The proposed circuitry solutions of multi-pulse rectifiers with electron phase shift can be used in the design of advanced DC sources requiring improved mass performance, compliance with electromagnetic compatibility requirements.

Darbe nagrinėjami ir tiriami nuolatinės srovės keitikliai atsinaujinančios energijos šaltiniams. Pateikiami duomenys apie bendrą elektros energijos gamybą ir gamybą iš atsinaujinančių energijos šaltinių Lietuvoje ir pasaulyje. Pirmajame skyriuje nagrinėjamos pagrindinės nuolatinės srovės keitiklių struktūros, pateikiamos jų topologijos, analizuojamos išraiškos parametrams skaičiuoti, pateikiami valdančiųjų signalų formavimo algoritmai ir signalų epiūros. Antrajame skyriuje nagrinėjamos nuolatinės srovės keitiklių (inverterių), keičiančių nuolatinės srovės šaltinio įtampą į kintamąją įtampą, struktūros. Analizuojamos šių keitiklių topologijos, valdymo principai, signalų epiūros, signalų formavimo algoritmai, inverterių savybės, nagrinėjamos išraiškos parametrams apskaičiuoti. Trečiajame skyriuje analizuojamas inverterio išėjimo filtras. Pateikiamos pagrindinės šio filtro topologijos, aptariamos jų savybės, pateikiamos signalų epiūros, analizuojamos išraiškos filtro parametrams apskaičiuoti. Ketvirtajame skyriuje pateikiami inverterio su išėjimo filtru modeliavimo ir eksperimentinio tyrimo rezultatai. Tyrimams suprojektuotas ir surinktas 1 kW galios inverteris su filtru, jo darbo valdymui ir valdančiųjų signalų inverterio raktams formavimui panaudotas mikrokompiuteris su mikrovaldikliu. Gauti tyrimų rezultatai pritaikyti kuriant keitiklį saulės elementų baterijoms. Kiekvieno skyriaus pabaigoje pateikiamos išvados. Baigiamojo darbo pabaigoje pateikiamos apibendrinančios išvados.
The work focuses on direct current (DC) converters for alternative energy sources. The data about the general production of electricity and the production of electricity from alternative energy sources in Lithuania and in the world is presented. In the first section the structures and topologies of DC-DC converters are analyzed, the signal formation algorithms and plots of the signals are given and discussed. In the second section the structures and topologies of DC-AC (Alternating Current) converters are analyzed, the signal formation algorithms and plots of the signals are presented. In the third section the output filter is analyzed. The topologies, characteristics, signal plots and equations are presented. In the fourth section the simulation and experimental investigation results are presented and discussed. The 1 kW DC-AC converter with output filter was designed for experimental investigation and a microcontroller-based microcomputer was used for generation of signals for control of converter output transistors. The obtained investigation results were employed for development of the converter for the solar panel. Conclusions in the each section of the thesis are given. General conclusions are formulated at the end of the work. Thesis consist of: 88 p. text without appendixes, 77 pictures, 9 tables, 28 bibliographical entries. Appendixes included.

La stimulation transcrânienne par courant continu (tDCS) est une technique de neuromodulation non invasive qui permet de modifier l'excitabilité co11icale au moyen d'un faible courant électrique délivré à travers le crâne. Elle constitue une piste intéressante pour l'élaboration de nouveaux traitements de divers troubles psychiatriques ou neurologiques, mais les mécanismes qui sous-tendent ses effets sont méconnus. L'objectif de ma thèse a été d'évaluer chez la souris les comp011ements affectés par des sessions répétées de tDCS et d'étudier des pistes concernant ses mécanismes d'action. Nous avons montré un effet bénéfique de la tDCS anodique au niveau du lobe frontal (2x20 minutes/jour à 0.2 mA, 5 jours consécutifs) sur les comportements associés à la dépression et sur la mémoire. Nous avons par ailleurs quantifié la prolifération cellulaire au niveau de l'hippocampe et montré que celle-ci est plus importante chez les souris stimulées, suggérant que la tDCS pourrait augmenter la neurogenèse hippocampique. Enfin, nos études sur des souris exposées à différentes drogues (nicotine, alcool, cocaïne) dévoilent que la tDCS diminue certains de leurs effets comportementaux (ex: valeur hédonique) et/ou moléculaires (ex: induction de Zif268 dans les circuits cortico-striataux). Nos résultats sont en adéquation avec les données cliniques recueillies chez l'Homme et soutiennent l'hypothèse que la tDCS pourrait être un traitement additionnel pour diverses pathologies. Ils ouvrent la voie à différentes études complémentaires, pour lesquelles notre modèle animal constitue un outil intéressant
Transcranial direct current stimulation (tDCS) is a non-invasive neuromodulatory technique in which a low current is delivered via electrodes on the scalp to modulate cortical excitability. lt is a promising tool in the treatment of various neurological and psychiatrie illnesses, yet the mechanisms underlying its beneficial effects remain unknown. The goal of my thesis was to describe the effects of repeated tDCS on behavior in mice, and to offer preliminary insights regarding the neurobiological mechanisms involved. Our data indicate that repeated tDCS over the frontal lobe (0.2 mA, 2x20 min/day, 5 consecutive days) has antidepressant prope11ies and improves memory. Interestingly, tDCS increased precursor cell proliferation in the hippocampus, suggesting that tDCS may increase neurogenesis levels in this brain area. Lastly, tDCS decreased the behavioral and/or the molecular effects of nicotine, alcohol and cocaïne. For example, tDCS attenuated cocaine-induced Zif268 expression in specific corticostriatal circuits. Altogether, our findings provide pre-clinical evidence that tDCS cou Id be a beneficial adjunct treatment for several psychiatrie disorders. Our animal mode! will be useful to further investigate the mechanisms underlying the beneficial effects of tDCS on behavior

碩士
國立成功大學
電機工程學系
105
This thesis presents the stability-analysis results of a system composed of two offshore wind farms (OWFs) connected to two onshore power grids through a multi-infeed high-voltage direct-current (MI-HVDC) system. The OWFs are based on doubly-fed induction generators (DFIGs), and the MI-HVDC system is based on voltage-source converters (VSCs). With the focus laid on the influence of the common tie line in the MI-HVDC system, small-signal stability and transient stability are analyzed by utilizing system eigenvalues and time-domain simulations, respectively. The simulation results show that a VSC-based MI-HVDC system can be more stable than two VSC-based HVDC links which operate independently from each other. In addition, the stability of the studied VSC-based MI-HVDC link can be significantly influenced by the electrical parameters of the common tie line.

Οι συνεχώς αυξανόμενες ενεργειακές απαιτήσεις της σύγχρονης κοινωνίας σε συνδυασμό με τις ραγδαίες κλιματικές αλλαγές μας οδηγούν στην ανάγκη παραγωγής ηλεκτρικής ενέργειας με όσο το δυνατό πιο οικονομικό και φιλικό προς το περιβάλλον τρόπο. Έτσι, οι Ανανεώσιμες Πηγές Ενέργειας αποκτούν ολοένα και μεγαλύτερο μερίδιο στην παραγωγή ηλεκτρικής ενέργειας, αλλάζοντας συνεχώς τον παγκόσμιο ενεργειακό χάρτη. Τα αιολικά συστήματα, τα οποία αξιοποιούν την κινητική ενέργεια του ανέμου, είναι ένα είδος ΑΠΕ. Στην παρούσα εργασία μελετάμε ένα αιολικό σύστημα, το οποίο αποτελείται από μία ανεμογεννήτρια μεταβλητών στροφών που χρησιμοποιεί μία σύγχρονη μηχανή μόνιμου μαγνήτη, έναν μετατροπέα πηγής τάσης, ένα R-L φίλτρο στην πλευρά του δικτύου και μία γραμμή μεταφοράς μικρού μήκους. Η αυξημένη αξιοπιστία και απόδοση της ΣΜΜΜ την καθιστούν ιδιαίτερα ελκυστική λύση για τα αιολικά συστήματα. Στην εργασία αυτή προσομοιώνουμε σε περιβάλλον Matlab/Simulink το σύστημα που περιγράψαμε παραπάνω. Εφαρμόζοντας τις κατάλληλες τεχνικές ελέγχου από την πλευρά της μηχανής προσπαθούμε να πετύχουμε μέγιστη απομάστευση ισχύος από τον άνεμο, ενώ στην πλευρά του δικτύου προσπαθούμε μέσω άμεσου ελέγχου ροής ισχύος να πετύχουμε μοναδιαίο συντελεστή ισχύος με ταυτόχρονη ρύθμιση της τάσης της dc διασύνδεσης. Στη συνέχεια, μέσω ενός πρόσθετου ελέγχου προσπαθούμε να κρατήσουμε την τάση στην έξοδο του φίλτρου σταθερή, κατά τη διάρκεια πτώσης της τάσης του δικτύου. Τέλος, παραθέτουμε τα αποτελέσματα της προσομοίωσης μαζί με έναν σύντομο σχολιασμό και τα συμπεράσματα.
The continuously increasing energy requirements of modern society combined with the rapid climate changes lead us to the need to produce electrical energy in a more economic and environmentally friendly way. Thus, the Renewable Energy Sources gain an increasing share of electrical energy production, constantly changing the global energy map. Wind power systems, which utilize the kinetic energy of the wind, are a kind of RES. In this thesis we study a wind power system, which comprises a variable speed wind turbine, which uses a permanent magnet synchronous machine, a voltage source converter, an R-L filter in the grid side and a short transmission line. The increased reliability and performance of PMSG make it particularly attractive solution for wind power systems. In this thesis we simulate in Matlab/Simulink environment the system described above. Applying appropriate control techniques on the machine side we try to achieve maximum power harvesting from the wind, while on the grid side we try through direct power flow control to achieve unit power factor with simultaneous control of the dc link voltage. Then, using an additional control we try to keep constant the voltage at the end of the R-L filter during a grid voltage drop. Finally, we present the simulation results along with a brief commentary and the conclusions.

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.

Le problème inverse en électroencéphalographie (EEG) est la localisation de sources de courant dans le cerveau utilisant les potentiels de surface sur le cuir chevelu générés par ces sources. Une solution inverse implique typiquement de multiples calculs de potentiels de surface sur le cuir chevelu, soit le problème direct en EEG. Pour résoudre le problème direct, des modèles sont requis à la fois pour la configuration de source sous-jacente, soit le modèle de source, et pour les tissues environnants, soit le modèle de la tête. Cette thèse traite deux approches bien distinctes pour la résolution du problème direct et inverse en EEG en utilisant la méthode des éléments de frontières (BEM): l’approche conventionnelle et l’approche réciproque. L’approche conventionnelle pour le problème direct comporte le calcul des potentiels de surface en partant de sources de courant dipolaires. D’un autre côté, l’approche réciproque détermine d’abord le champ électrique aux sites des sources dipolaires quand les électrodes de surfaces sont utilisées pour injecter et retirer un courant unitaire. Le produit scalaire de ce champ électrique avec les sources dipolaires donne ensuite les potentiels de surface. L’approche réciproque promet un nombre d’avantages par rapport à l’approche conventionnelle dont la possibilité d’augmenter la précision des potentiels de surface et de réduire les exigences informatiques pour les solutions inverses. Dans cette thèse, les équations BEM pour les approches conventionnelle et réciproque sont développées en utilisant une formulation courante, la méthode des résidus pondérés. La réalisation numérique des deux approches pour le problème direct est décrite pour un seul modèle de source dipolaire. Un modèle de tête de trois sphères concentriques pour lequel des solutions analytiques sont disponibles est utilisé. Les potentiels de surfaces sont calculés aux centroïdes ou aux sommets des éléments de discrétisation BEM utilisés. La performance des approches conventionnelle et réciproque pour le problème direct est évaluée pour des dipôles radiaux et tangentiels d’excentricité variable et deux valeurs très différentes pour la conductivité du crâne. On détermine ensuite si les avantages potentiels de l’approche réciproquesuggérés par les simulations du problème direct peuvent êtres exploités pour donner des solutions inverses plus précises. Des solutions inverses à un seul dipôle sont obtenues en utilisant la minimisation par méthode du simplexe pour à la fois l’approche conventionnelle et réciproque, chacun avec des versions aux centroïdes et aux sommets. Encore une fois, les simulations numériques sont effectuées sur un modèle à trois sphères concentriques pour des dipôles radiaux et tangentiels d’excentricité variable. La précision des solutions inverses des deux approches est comparée pour les deux conductivités différentes du crâne, et leurs sensibilités relatives aux erreurs de conductivité du crâne et au bruit sont évaluées. Tandis que l’approche conventionnelle aux sommets donne les solutions directes les plus précises pour une conductivité du crâne supposément plus réaliste, les deux approches, conventionnelle et réciproque, produisent de grandes erreurs dans les potentiels du cuir chevelu pour des dipôles très excentriques. Les approches réciproques produisent le moins de variations en précision des solutions directes pour différentes valeurs de conductivité du crâne. En termes de solutions inverses pour un seul dipôle, les approches conventionnelle et réciproque sont de précision semblable. Les erreurs de localisation sont petites, même pour des dipôles très excentriques qui produisent des grandes erreurs dans les potentiels du cuir chevelu, à cause de la nature non linéaire des solutions inverses pour un dipôle. Les deux approches se sont démontrées également robustes aux erreurs de conductivité du crâne quand du bruit est présent. Finalement, un modèle plus réaliste de la tête est obtenu en utilisant des images par resonace magnétique (IRM) à partir desquelles les surfaces du cuir chevelu, du crâne et du cerveau/liquide céphalorachidien (LCR) sont extraites. Les deux approches sont validées sur ce type de modèle en utilisant des véritables potentiels évoqués somatosensoriels enregistrés à la suite de stimulation du nerf médian chez des sujets sains. La précision des solutions inverses pour les approches conventionnelle et réciproque et leurs variantes, en les comparant à des sites anatomiques connus sur IRM, est encore une fois évaluée pour les deux conductivités différentes du crâne. Leurs avantages et inconvénients incluant leurs exigences informatiques sont également évalués. Encore une fois, les approches conventionnelle et réciproque produisent des petites erreurs de position dipolaire. En effet, les erreurs de position pour des solutions inverses à un seul dipôle sont robustes de manière inhérente au manque de précision dans les solutions directes, mais dépendent de l’activité superposée d’autres sources neurales. Contrairement aux attentes, les approches réciproques n’améliorent pas la précision des positions dipolaires comparativement aux approches conventionnelles. Cependant, des exigences informatiques réduites en temps et en espace sont les avantages principaux des approches réciproques. Ce type de localisation est potentiellement utile dans la planification d’interventions neurochirurgicales, par exemple, chez des patients souffrant d’épilepsie focale réfractaire qui ont souvent déjà fait un EEG et IRM.
The inverse problem of electroencephalography (EEG) is the localization of current sources within the brain using surface potentials on the scalp generated by these sources. An inverse solution typically involves multiple calculations of scalp surface potentials, i.e., the EEG forward problem. To solve the forward problem, models are needed for both the underlying source configuration, the source model, and the surrounding tissues, the head model. This thesis treats two distinct approaches for the resolution of the EEG forward and inverse problems using the boundary-element method (BEM): the conventional approach and the reciprocal approach. The conventional approach to the forward problem entails calculating the surface potentials starting from source current dipoles. The reciprocal approach, on the other hand, first solves for the electric field at the source dipole locations when the surface electrodes are reciprocally energized with a unit current. A scalar product of this electric field with the source dipoles then yields the surface potentials. The reciprocal approach promises a number of advantages over the conventional approach, including the possibility of increased surface potential accuracy and decreased computational requirements for inverse solutions. In this thesis, the BEM equations for the conventional and reciprocal approaches are developed using a common weighted-residual formulation. The numerical implementation of both approaches to the forward problem is described for a single-dipole source model. A three-concentric-spheres head model is used for which analytic solutions are available. Scalp potentials are calculated at either the centroids or the vertices of the BEM discretization elements used. The performance of the conventional and reciprocal approaches to the forward problem is evaluated for radial and tangential dipoles of varying eccentricities and two widely different skull conductivities. We then determine whether the potential advantages of the reciprocal approach suggested by forward problem simulations can be exploited to yield more accurate inverse solutions. Single-dipole inverse solutions are obtained using simplex minimization for both the conventional and reciprocal approaches, each with centroid and vertex options. Again, numerical simulations are performed on a three-concentric-spheres model for radial and tangential dipoles of varying eccentricities. The inverse solution accuracy of both approaches is compared for the two different skull conductivities and their relative sensitivity to skull conductivity errors and noise is assessed. While the conventional vertex approach yields the most accurate forward solutions for a presumably more realistic skull conductivity value, both conventional and reciprocal approaches exhibit large errors in scalp potentials for highly eccentric dipoles. The reciprocal approaches produce the least variation in forward solution accuracy for different skull conductivity values. In terms of single-dipole inverse solutions, conventional and reciprocal approaches demonstrate comparable accuracy. Localization errors are low even for highly eccentric dipoles that produce large errors in scalp potentials on account of the nonlinear nature of the single-dipole inverse solution. Both approaches are also found to be equally robust to skull conductivity errors in the presence of noise. Finally, a more realistic head model is obtained using magnetic resonance imaging (MRI) from which the scalp, skull, and brain/cerebrospinal fluid (CSF) surfaces are extracted. The two approaches are validated on this type of model using actual somatosensory evoked potentials (SEPs) recorded following median nerve stimulation in healthy subjects. The inverse solution accuracy of the conventional and reciprocal approaches and their variants, when compared to known anatomical landmarks on MRI, is again evaluated for the two different skull conductivities. Their respective advantages and disadvantages including computational requirements are also assessed. Once again, conventional and reciprocal approaches produce similarly small dipole position errors. Indeed, position errors for single-dipole inverse solutions are inherently robust to inaccuracies in forward solutions, but dependent on the overlapping activity of other neural sources. Against expectations, the reciprocal approaches do not improve dipole position accuracy when compared to the conventional approaches. However, significantly smaller time and storage requirements are the principal advantages of the reciprocal approaches. This type of localization is potentially useful in the planning of neurosurgical interventions, for example, in patients with refractory focal epilepsy in whom EEG and MRI are often already performed.