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This thesis investigates issues affecting grid synchronisation of VSC-HVDC systems with particular regard to, but not limited to, offshore wind power generation during the complex but potentially serious behaviours following solar storms. An averaged value model (AVM) for the contemporary modular multilevel converter (MMC) based VSC-HVDC system is developed and is used in combination with different phase-locked loop (PLL) models and the unified magnetic equivalent circuit (UMEC) transformer model to assess the impacts of geomagnetically induced current (GIC) on grid synchronisation of an offshore VSC-HVDC system. GIC is DC current flowing in the earth caused by strong geomagnetic disturbance events. GIC enters the electric utility grid via the grounded transformer neutral and can cause severe saturation to transformers. This in turn causes disruptions to grid synchronisation. The main contribution of this thesis is that effects of GIC are studied using the UMEC transformer model, which can model saturation. The assessment leads to the development of enhanced fundamental positive sequence control (EFPSC) which is capable of reducing the stress on the system during GIC events. The methods developed can also be applied to other non-symmetrical AC events occurring in VSC-HVDC such as single-phase faults. Additional contributions of the thesis are:A mathematical model of the MMC is derived and forms the foundation of the AVM. The AVM is verified against a detailed equivalent-circuit-based model and shows good accuracy. The PLL is the essential component for grid synchronisation of VSC-HVDC system. Different PLLs are studied in detail. Their performance is compared both qualitatively and quantitatively. This appears to have been done for the first time systematically in the public literature. The UMEC model is verified using hand calculation. Its saturation characteristic is matched to a predefined B-H curve and is also verified. The verifications show that this model is capable of modelling transformer saturation and thus is suitable for this study. The consolidation of the AVM, PLL, UMEC, GIC and EFPSC provides an insight into the how the MMC based VSC-HVDC system behaves under severe geomagnetic disturbances and the possible methods to mitigate the risks and impacts to the power grid.

Os elevados níveis de potência e desempenho solicitados pelas presentes e futuras aplicações de conversores estáticos de potência podem ser atingidos pela interconexão de unidades básicas menores (multiconversor). Com a utilização de uma estratégia de controle adequada consegue-se não apenas a potência total desejada e sua correta divisão entre as várias unidades básicas como também uma redução no conteúdo harmônico das correntes e tensões resultantes. Neste trabalho, algumas topologias de multiconversores monofásicos do tipo fonte de tensão são modeladas, analisadas e comparadas quanto às solicitações de corrente e tensão nos diversos componentes, e quanto ao espectro da corrente resultante. Desta discussão conclui-se que a \"Conexão em Paralelo de Conversores Sem Transformador\" (PCTL- parallel connection/transformerless) é viável, levando-se em conta as demais topologias e a tecnologia disponível em semicondutores de potência. A análise do modelo matemático e da matriz de controlabilidade do caso PCTL trifásico resulta em métodos de desacoplamento das entradas do multiconversor. Baseando-se nestes métodos, são propostas três estratégias de controle utilizando controladores de corrente monofásicos individuais. Dois deles utilizam PWM com portadora triangular, e o outro, um PWM por banda de tolerância. Para um dos casos, baseados em PWM a portadora triangular, propõe-se um processo de minimização de harmônicos de corrente, injetando-se à referência de cada modulador PWM um sinal de \"seqüência zero instantânea\", obtido a partir da solução de um problema de otimização. Para o caso baseado em banda de tolerância, conseguem-se satisfazer os critérios de desacoplamento pela sincronização e igual defasagem das funções de chaveamento dos diversos conversores via malhas PLL, e também pela injeção de uma corrente fictícia de \"seqüência zero instantânea\" às referências do controlador de corrente. Para os controladores propostos, discute-se a operação do PCTL durante a ocorrência de falhas ou manutenção. Apenas os conversores do tipo fonte de tensão são abordados neste trabalho. / The high power and performance levels required for the present and future power electronics converters can be achieved by connecting a set of smaller power units (multiconverter). By using appropriate control, not only perfect sharing of the power among the converters, but also harmonics reduction is obtained. This thesis presents, analyses and compares some multiconverter topologies, based on the values of the voltages and currents in the main components and in the spectra of the resulting current. As a result, the transformerless parallel connection of converters PCTL is shown to be a feasible solution, taking into account the existing power devices technology. The analysis of the three phase PCTL model and its controllability matrix suggests methods for decoupling the PCTL inputs. This results in two methods using carrier based current controller and one method using a \"tolerance band (TB)\" based controller. For the carrier based one, the injection of an optimized zero sequence reference voltage produces a reduction in the ripple of the individual currents. For the tolerance based one, the inputs coupling is reduced by the use of PLL synchronized PWM and by the injection of a \"fictitious zero sequence current in the TB controller. The operation of the PCTL under faults and maintenance condition is discussed. Only the voltage source converter is studied here.

Os elevados níveis de potência e desempenho solicitados pelas presentes e futuras aplicações de conversores estáticos de potência podem ser atingidos pela interconexão de unidades básicas menores (multiconversor). Com a utilização de uma estratégia de controle adequada consegue-se não apenas a potência total desejada e sua correta divisão entre as várias unidades básicas como também uma redução no conteúdo harmônico das correntes e tensões resultantes. Neste trabalho, algumas topologias de multiconversores monofásicos do tipo fonte de tensão são modeladas, analisadas e comparadas quanto às solicitações de corrente e tensão nos diversos componentes, e quanto ao espectro da corrente resultante. Desta discussão conclui-se que a \"Conexão em Paralelo de Conversores Sem Transformador\" (PCTL- parallel connection/transformerless) é viável, levando-se em conta as demais topologias e a tecnologia disponível em semicondutores de potência. A análise do modelo matemático e da matriz de controlabilidade do caso PCTL trifásico resulta em métodos de desacoplamento das entradas do multiconversor. Baseando-se nestes métodos, são propostas três estratégias de controle utilizando controladores de corrente monofásicos individuais. Dois deles utilizam PWM com portadora triangular, e o outro, um PWM por banda de tolerância. Para um dos casos, baseados em PWM a portadora triangular, propõe-se um processo de minimização de harmônicos de corrente, injetando-se à referência de cada modulador PWM um sinal de \"seqüência zero instantânea\", obtido a partir da solução de um problema de otimização. Para o caso baseado em banda de tolerância, conseguem-se satisfazer os critérios de desacoplamento pela sincronização e igual defasagem das funções de chaveamento dos diversos conversores via malhas PLL, e também pela injeção de uma corrente fictícia de \"seqüência zero instantânea\" às referências do controlador de corrente. Para os controladores propostos, discute-se a operação do PCTL durante a ocorrência de falhas ou manutenção. Apenas os conversores do tipo fonte de tensão são abordados neste trabalho. / The high power and performance levels required for the present and future power electronics converters can be achieved by connecting a set of smaller power units (multiconverter). By using appropriate control, not only perfect sharing of the power among the converters, but also harmonics reduction is obtained. This thesis presents, analyses and compares some multiconverter topologies, based on the values of the voltages and currents in the main components and in the spectra of the resulting current. As a result, the transformerless parallel connection of converters PCTL is shown to be a feasible solution, taking into account the existing power devices technology. The analysis of the three phase PCTL model and its controllability matrix suggests methods for decoupling the PCTL inputs. This results in two methods using carrier based current controller and one method using a \"tolerance band (TB)\" based controller. For the carrier based one, the injection of an optimized zero sequence reference voltage produces a reduction in the ripple of the individual currents. For the tolerance based one, the inputs coupling is reduced by the use of PLL synchronized PWM and by the injection of a \"fictitious zero sequence current in the TB controller. The operation of the PCTL under faults and maintenance condition is discussed. Only the voltage source converter is studied here.

The trend of future wind farms moving further offshore requires much higher reliability for each wind turbine in order to reduce maintenance cost. The drive-train system and power electronic converter system have been identified as critical sub-assemblies that are subject to higher failure rates than the other sub-assemblies in a wind turbine. Modern condition monitoring techniques may help schedule the maintenance and reduce downtime. However, when it comes to offshore wind turbines, it is more crucial to reduce the failure rates (or reduce the stresses) for the wind turbines during operation since the harsh weather and a frequently inaccessible environment will dramatically reduce their availability once a failure happens. This research examines the mechanical, electrical and thermal stresses in the sub-assemblies of a doubly-fed induction generator (DFIG) wind turbine and how to reduce them by improved control strategies. The DFIG control system (the rotor-side and the grid-side converter control) as well as the wind turbine control system are well established. The interactions of these control systems have been investigated. This research examines several further strategies to reduce the mechanical and electrical stresses. The control system's coordination with the protection schemes (crowbar and dc-chopper) during a grid fault is presented as well. An electro-thermal model of the power converter has been developed to integrate with the DFIG wind turbine model, for the evaluation of the thermal stresses under different operating states and control schemes. The main contributions of this thesis are twofold. A first contribution is made by providing all the control loops with well-tuned controllers in a more integrated methodology. The dynamics of these controllers are determined from their mathematical models to minimize the interference between different control-loops and also to reduce the electrical transients. This thesis proposes a coordination strategy for the damping control, pitch control and crowbar protection which significantly reduces the mechanical oscillations. On the other hand, an integrated model of the wind turbine and converter electro-thermal system is established that can illustrate the performance integration with different control strategies.

This diploma thesis is devoted to the design of a laboratory measurement tool with DAC converter for audio and low frequency signals. In the introduction part a theoretical background of the ADC and DAC conversions are outlined. In the next chapters the technical solution of the laboratory converter is presented, including DAC with integrated circuit AD1852 and completed with digital audio receiver CS8416. This circuit secures compatibility with external audio devices with digital interfaces, like SPDIF or AES3. The digital receiver and audio DAC are added with serial communication interface for setup of the most audio and input parameters. This setup is operated by external microcontroller ATmega32 and original firmware. The thesis presents block diagram and schematic layout of the laboratory converter, including complete technical documentation for the construction and design.

No description available.

Thesis (MScEng)--Stellenbosch University, 2011. / ENGLISH ABSTRACT: The Solid-State Transformer (SST) is presented as an alternative to the traditional Line-Frequency Transformer (LFT) used for voltage-level transformation in distribution grids. The LFT technology is highly matured which results in low costs and high efficiency. The SST however, introduces several advantages compared to the LFT, some of which are input unity power-factor, near perfect output-voltage regulation, frequency variation as well as harmonic filtering. The SST consists of three power-electronic converter stages: The input stage, the isolation stage and the output stage. The input and isolation stages are implemented with a multilevelconverter topology incorporating a converter-stack for each phase. Each stack consists of N converter building blocks, called cells. In this thesis the design and implementation of the main controller is presented. The main controller, together with N cell controllers, is responsible for the control of the cells within the respective stack. Three main controllers are thus implemented within the SST. Each cell consists of an Active Rectifier (AR) and a DC-DC Converter (DC-DC). The SST control design thus starts with the AR control which is subsequently expanded to Cascaded Active-Rectifier (CAR) control. Design is completed with the addition of the DC-DC control. Time domain simulations of the AR- and CAR-control are presented and discussed. Test measurements, verifying functionality of each control design-phase, are presented and discussed. / AFRIKAANSE OPSOMMING: Die Drywingselektroniese Transformator (DET) word voorgestel as ’n alternatief vir die Lyn Frekwensie Transformator (LFT) wat gebruik word vir spannings-vlak tranformasie op distribusie vlak. The LFT tegnologie is ver gevorderd wat ly tot hoë effektiwiteit en lae kostes. The DET bied wel voordele soos intree eenheids arbeid faktor, na aan perfecte uittree-spannings regulasie, frequensie variasie sovel as harmoniese filrteering. Die DET bestaan uit drie drywingselektroniese omsetter stadiums: Die intree-stadium, die isolasie-stadium en die uittree-stadium. The intree- en isolasie-stadiums word geïmplimenteer met ’n multivlak-omsetter topologie wat bestaan uit ’n omsetter-stapel vir elke fase. Elke stapel bestaan uit N omsetter boustene wat selle genoem word. In hierdie tesis word the ontwerp en implementasie van die hoofbeheerder voorgestel. Die hoofbeheerder, tesame met N selbeheerders, is verantwoordelik vir die beheer van elke sel in die spesifieke stapel. In die DET word daar dus drie hoofbeheerders gebruik. Elke sel bestaan uit ’n Aktiewe Gelykrigter (AG) en ’n GS-GS omsetter (GS-GS). Die DET beheerontwerp begin dus met die (AG) beheer wat daarna uitgebrei word na Kaskade Aktiewe Gelykrigter (KAG) beheer. Die beheer ontwerp word voltooi deur die byvoeging van die GSGS beheer. Tyd-gebied simulasies van die AG- en KAG-beheer word voorgelê en bespreek. Toetsmetings wat die funksionaliteit van elke beheer ontwepsfase verifeer, word voorgelê.

Recently, due to the increasing adoption of distributed energy resource (DER) technologies including battery energy storage (BES) and electric vehicle (EV) systems, bidirectional power converters are becoming more popular. These converters are broadly utilized as interface devices and provide a bidirectional power flow in applications where the primary power supply can both supply and receive energy. A dc capacitor, called the dc-link, is an important component of such bidirectional converters. For a wide range of applications, the converter is required to control the dc-link voltage. Commonly, a proportional-integrating (PI) controller is used by the dc capacitor voltage controller to generate a set-point for the inner current controller. This approach tightly regulates the dc-link voltage to a given value. The research presented in this dissertation shows that such an approach compromises the stability margins of the converter for reverse power flow and weak grid conditions. It is shown that by allowing a small variation of dc capacitor voltage in proportion to the amount of power flowing through the converter, the stability and robustness margins are improved. This approach also simplifies the design process and can be applied to both dc/dc and dc/ac (single-phase and three-phase) converters. Moreover, it grants an inherent power sharing capability when multiple converters share the same dc-link terminals; removing the need to a communication link between parallel converters. The proposed controller is equipped with a current limiting mechanism to protect the converter during low-voltage/over-current transients. Detailed analyses, simulations, comparisons, and experimental results are included to illustrate the effectiveness of the proposed control approach. To mathematically establish the properties of the proposed method in a single-phase dc/ac application, this dissertation also derives a new and systematic modeling approach for a grid-connected bidirectional single-phase inverter controlled in stationary frame. Implementing the control system in the stationary frame has advantages over rotating frame. However, the combination of dc and ac state variables and nonlinearities make its stability analysis challenging. In the proposed model, an imaginary subsystem is properly generated and augmented to allow a full transformation to a synchronous rotating frame. The proposed modeling strategy is modular and has a closed form which facilitates further extensions. It is successfully used to demonstrate enhanced stability margins of the proposed controller.

Meshed high-voltage direct current grids are becoming an increasingly important technology for integrating renewable energies into the power system. To control the grids in the best possible way, optimal converter and grid control strategies are needed. This project studies how a four-terminal high-voltage direct current grid is operated and controlled by implementing different grid and converter control strategies. The grid control strategies examined are centralized voltage control and distributed voltage control with and without deadband. Simulations are made in the software PSCAD. Different fault types on the grid are studied to investigate how the power flow and voltage level are affected. An optimal value for both the deadband width and droop constant has been identified. Moreover, the results indicate that centralized droop control is not a suitable grid control strategy, whereas distributed voltage control with and without deadband are. The fault study indicates no differences between distributed voltage control with and without deadband. The power flow and voltage levels are identical for all fault types. / Högspända likströmsnät spelar en allt större roll med att integrera förnyelsebar energi i våra elnät. För att styra dessa nät på bästa möjliga sätt krävs optimala omvandlar- och nätkontrollstrategier. I detta projekt studeras hur ett fyrterminalt högspänt likströmsnät kan styras och drivas genom att implementera olika omvandlar- och nätkontrollstrategier. De nätkontrollstrategier som studerats är centraliserad spänningskontroll och distribuerad spänningskontroll med och utan ett spänningsintervall. Alla simuleringar har utförts i programmet PSCAD. Olika fel i nätet har även studerats för att undersöka hur effektflödet och spänningsnivån påverkas. Ett optimalt värde på både spänningsintervallet och droop konstanten har identifierats. Dessutom har resultat som indikerar att centraliserad spänningskontroll inte är en lämplig nätkontrollstrategi erhållits, medan distribuerad spänningskontroll med och utan spänningsintervall är det. Felsimuleringarna påvisar ingen skillnad mellan distribuerad spänningskontroll med och utan spänningsintervall. Effektflödet och spänningsnivån är identiska för alla fel.