Rozprawy doktorskie na temat „GRID POWER”

Thesis: S.M., Massachusetts Institute of Technology, School of Engineering, Center for Computational Engineering, Computation for Design and Optimization Program, 2015.
Cataloged from PDF version of thesis.
Includes bibliographical references (pages 87-91).
This thesis examines two problems concerning the secure and reliable operation of the electric power grid. The first part studies the distributed operation of the electric power grid using the power flow problem, which is vital to the operation of the grid. The power flow problem is a feasibility problem for finding an assignment of complex bus voltages that satisfies the power flow equations and is within operational and safety limits. For reliability and privacy reasons, it is desirable to solve the power flow problem in a distributed manner. Two novel distributed algorithms are presented for solving convex feasibility problems for networks based on the Method of Alternating Projections (MAP) and the Projected Consensus algorithm. These algorithms distribute computation among the nodes of the network and do not require any form of central coordination. The original problem is equivalently split into small local sub-problems, which are coordinated locally via a thin communication protocol. Although the power flow problem is non-convex, the new algorithms are demonstrated to be powerful heuristics using IEEE test beds. Quadratically Constrained Quadratic Programs (QCQP), which occur in the projection sub-problems, are studied and methods for solving them efficiently are developed. The second part addresses the robustness and resiliency of state estimation algorithms for cyber-physical systems. The operation of the electric power grid is modeled as a dynamical system that is supported by numerous feedback control mechanisms, which depend heavily on state estimation algorithms. The electric power grid is constantly under attack and, if left unchecked, these attacks may corrupt state estimates and lead to severe consequences. This thesis proposes a novel dynamic state estimator that is resilient against data injection attacks and robust to modeling errors and additive noise signals. By leveraging principles of robust optimization, the estimator can be formulated as a convex optimization problem and its effectiveness is demonstrated in simulations of an IEEE 14-bus system.
by Ming Qing Foo.
S.M.

The installation of power generation systems based on renewable energy sources has been increasing exponentially over the last decades. However, in spite of the well-known merits of such energy sources, the expansion of renewable-based generation (RG) plants, which interface the grid through power converters, can produce also negative impacts on the electrical grid, due to its power processing mechanism, which is different from traditional generation plants. In fact, the regulation capability of the grid can decrease as much as the share of the RG increases. To avoid this, power conversion systems belonging to RG plants are requested to be more grid-friendly, and responsive to the electrical network conditions. In this way, they can contribute to the electrical network stability as other generation does, instead of behaving as simply grid-feeding systems focused on injecting as much power as possible.This PhD dissertation is focused on the control of grid-connected power converters with grid support functionalities based on the Synchronous Power Controller (SPC) concept. The SPC is an established solution for controlling grid connected power converters and equipping them with emulated and improved synchronous machine characteristics. In addition to the general goal of improving the grid interaction of the RG plants, grid support functionality stands as a main property among the characteristics given by the SPC. In this dissertation the virtual admittance structure, contained in the electrical block of the SPC, which emulates the stator output impedance of the synchronous machines, is analyzed. Moreover, it is extended to a study case where the admittance value can be different for positive- and negative-sequence components. The designed virtual admittance block contains three branches, which are responsible for positive-sequence current injection, negative-sequence current injection and other harmonic components, respectively. The converter¿s performance under asymmetrical grid fault is especially considered in this case.The analysis and arrangements in the design of the SPC¿s power loop controller is another contribution of this research. Other methods that consider synchronous machine emulation normally construct the controller by reproducing the synchronous generation swing equation. Based on the virtual implementation, which is free from mechanical constraints, one can set a proper damping factor achieving thus better dynamics compared to the traditional synchronous machines. However, the increase of the damping factor changes the inherent power-frequency (P-f) droop characteristics, which may lead to undesired deviations in the active power generation. In the framework of this PhD, a method that modifies the conventional swing equation emulation and lets the inherent P-f droop characteristics be configurable, independently of the inertia and damping characteristics, is proposed.The work presented in this dissertation is supported by mathematical and simulation analysis. Moreover, in order to endorse the conclusions achieved, a complete experimental validation has been conducted. As it will be shown, the performance of the SPC has been validated in tests once the main parts, namely virtual admittance and power loop controller, and other parts are settled. The simulation and experimental test scenarios include events like changes in the power operation point, frequency sweeps, voltage magnitude changes, start-up and parallel converters operation, which are given under different control configurations like the different structures for the power loop controller and different control parameters. This PhD research also compares the transient performance of the SPC-based power converters with the ones achieved with conventional control methods.
Los convertidores de potencia conectados a la red actúan comúnmente como interfaz entre plantas de generación basadas en energía renovable y la red eléctrica, permitiendo así el procesado de energía eólica y fotovoltaica y su inyección a red. El control de estos convertidores conectados a la red ha sido objeto de estudio en las últimas décadas, ya que su comportamiento y prestaciones influye de forma determinante tanto en la calidad de la red eléctrica, así como en el cumplimiento de los requisitos de conexión a la red fijados por los códigos de red. Junto con la expansión de las plantas de generación de energía renovable, su impacto en el sistema eléctrico ha crecido también, lo cual ha hecho que se lleven a cabo muchos trabajos de investigación orientados a armonizar la penetración de renovables con la estabilidad de la red. Con los sistemas de control actuales la capacidad de regulación de la red disminuye tanto como la proporción de la generación renovable aumenta. En las redes eléctricas del futuro, se espera que los convertidores de potencia, que actúan como interfaz, exploten sus posibilidades de cómputo y control permitiendo mejorar la interacción de la generación renovable con la red. En este contexto los controles de tipo “droop control”, los cuales son ampliamente utilizados en sistemas de generación tradicionales, se pueden aplicar a los convertidores conectados a red para habilitar funciones de soporte de red, ya que estos contribuyen al control de tensión y frecuencia primaria ajustando el intercambio de potencia activo y reactiva de forma proporcional a la desviación de la frecuencia y magnitud de la tensión en el punto de conexión. En el caso de regulación de frecuencia, y para que este sea bidireccional, el convertidor puede interactuar con la red con la ayuda de sistemas de almacenamiento de energía. Sin embargo, la inclusión del “droop control” no conlleva una solución global. Incluso si se ajusta de forma óptima y se dispone de reserva de energía, aún hay cuestiones como la respuesta inercial que no se pueden dar con este tipo de control. La generación en los sistemas tradicionales se lleva a cabo principalmente por generadores síncronos. Comparados con estos, los convertidores conectados a la red difieren principalmente en la falta de la característica electromecánica. En consecuencia, la estática y la dinámica de las unidades de generación de energía renovable son diferentes en comparación con los generadores síncronos. La dinámica de estos convertidores es altamente dependiente de los sistemas de sincronización (PLL), cuyo comportamiento se degrada en condiciones de red adversas o distorsionadas. Además, el control de potencia normalmente depende control de potencia instantáneo. Debido a las diferentes dinámicas, la inercia total en la red no aumenta junto con la integración de las energías renovables. Sin embargo, los códigos de red han incluido requerimientos tales como “inercia sintética" en los requisitos. Otras deficiencias del control del convertidor convencional incluyen el rendimiento inferior bajo condiciones de avería de red, en conexión de red débil y conexión de red de relación X / R baja. Esta tesis doctoral estudia y valida el control de los convertidores conectados a la red con funcionalidades de soporte de red. El objetivo general del trabajo es mejorar las características de interacción de la red de las plantas de generación de energía renovable mediante la especificación de los convertidores conectados a la red con características de la máquina síncrona emulada y mejorada. La tesis ha aportado contribuciones o ha mostrado originalidades en los siguientes aspectos: Un enfoque de ajuste de bucle de control de corriente interno generalizado; Diseño detallado y validación de la admisión virtual para convertidores conectados a la red; Diseño detallado y validación del circuito de control de potencia para la emulación de inercia y amortiguación.

At present, there are still a large number of people living in isolated areas, particularly in developing countries, who have no immediate access to the main electricity grid. Most of the energy demands of these remote communities are met by diesel-operated power systems, which are relatively affordable and available. With the ever increasing awareness of climate change, many local authorities have taken initiatives to reduce the carbon footprint of certain energy sectors. In some rural applications, diesel generator power systems are augmented by single or multiple renewable energy supply units to form an off-grid hybrid power system.Generally, the majority of off-grid hybrid power systems include a massive battery bank to store excess renewable energy to supply the user load demand during the period when renewable energy is deficient. In the charging and discharging processes, energy losses may occur due to the inefficiency of the charger and the battery cells. Also, inclusion of an energy storage element into a hybrid power system incurs additional investment costs and involves recycling issues. Therefore, it is necessary to minimise the size of storage, whenever possible, and operate the system under an appropriate power management strategy to ensure efficient system operation.The chosen power management strategy impacts long-term performance of a system as well as components’ longevity. The research presented in this thesis describes the development of an advanced power management concept for the operation of a photovoltaic-variable speed diesel generator hybrid power system.A general introduction regarding the research background to hybrid power system applications and fundamentals of solar energy is presented. A component sizing and control program is developed to facilitate hybrid power system design. Then, various off-grid power system configurations are further discussed with emphasis on the system performances and economic aspects.A prediction technique, namely the Hourly-based Prediction Model for solar irradiance and load demand forecasts is discussed. Forecast algorithms for the hourly solar irradiance and load demand predictions are presented. The proposed prediction models are implemented in the power management strategy for the off-grid photovoltaic-variable speed diesel generator hybrid power system. Assessments of the prediction models through comprehensive analyses of statistical measures are presented.HOMER simulation software has been adopted for time series generation and economic analyses for several off-grid power system configurations. Also, the HOMER simulation results for electrical aspects are used as a benchmark to evaluate the component models developed in this thesis. Due to the fact that HOMER offers limited choices of power management strategy and users do not have access to modify the control algorithms, it is impossible to determine the performance of a system under advanced power management strategy. Therefore, analytical performance models of system components have been developed using the MATLAB/Simulink software to allow the implementation of the proposed power management strategy.The concepts and flow charts of the predictive power management strategy are described. This power management strategy consists of predictive and adaptive dispatch. The time step of the predictive dispatch is fixed to one hour while the time step of the adaptive dispatch is one minute. Operation of the additional generator capacity of the hybrid power system is based on the predicted net load. The adaptive dispatch supports the predictive dispatch to handle fluctuations of net load that occur in between prediction intervals.Simulation results of the performance of hybrid power systems using different types of diesel generator and power management strategies are presented. Particular emphasis is on the comparisons of the system performances using non-predictive and predictive power management strategies. These simulations allow quantitative assessment of the system performances in terms of electrical output, fuel consumption and carbon dioxide emission.Last but not least, the entire research is summarised and concluded with suggestions for future research. In short, the photovoltaic-variable speed diesel generator hybrid power system topology and the proposed power management strategy offer an alternative to the off-grid hybrid power system design, with the aims of overcoming the complex technical issues associated with energy storage and of contributing to market extension of hybrid power systems, particularly in remote locations where financial and technical issues are the major concerns.

Renewable power sources with a relatively uneven or constant DC power production require synchronization methods to work with the current utility power grid. The solution to this synchronization problem has been solved with semiconductor based converters and advanced switching algorithms. To enable switching algorithms that work well with the grids amplitude, phase-shift and frequency, the current waveform has to be measured and estimated.    There are many sources of noise that will add distortion of the current waveform, making its appearance less similar to the grids. The distorted measurement affects the accuracy of the converters negatively. Therefore, using a filter algorithm to attenuate the grid noise is required.  This project uses a Kalman filter with the aim to decrease the noise and estimate the current phase shift for a three phase power-grid.  To achieve reliable and fast calculation, implementing the Kalman filter within a FPGA were done.The project contains results from both simulated MATLAB data and the FPGAs real time data. The method was able to estimate the grid within a few Hz frequency deviation and enable some noise reduction. For larger degree of harmonic distortion during steady state operation, the Kalman filter could remove more of the harmonic distortion. Limits and differences with MATLAB are discussed for the FPGA implemented Kalman filter.

This thesis considers the use of three-phase voltage and current source inverters as interfacing units for renewable power, specifically photovoltaic (PV) into the ac grid. This thesis presented two modulation strategies that offer the possibility of operating PV inverters in grid and islanding modes, with reduced switching losses. The first modulation strategy is for the voltage source inverter (VSI), and exploits 3rd harmonic injection with selective harmonic elimination (SHE) to improve performance at low and high modulation indices, where the traditional SHE implementation experiences difficulties due to pulse dropping. The simulations and experimentation presented show that the proposed SHE allows grid PV inverters to be operated with less than a 1kHz effective switching frequency per device. This is vital in power generation, especially in medium and high power applications. Pulse dropping is avoided as the proposed modified SHE spreads the switching angles over 90°, in add ition increasing the modulation index. The second proposed modulation strategy, called direct regular sampled pulse width modulation (DRSPWM), is for the current source inverter (CSI). It exploits a combination of forced and natural commutation imposed by the co-existence of an insulated gate bipolar transistor in series with a diode in a three phase current source inverter, to determine device dwell times and switching sequence selection. The DRSPWM strategy reduces switching frequency per device in a CSI by suspending each phase for 60°, similar to VSI dead-band, thus low switching losses are expected. Other benefits include simple digital platform implementation and more flexible switching sequence selection and pulse placement than with space vector modulation. The validity of the DRSPWM is confirmed using simulations and experimentation. This thesis also presents a new dc current offset compensation technique used to facilitate islanding or grid operation of inverter based distributed generation, with a reduced number of interfacing transformers. The proposed technique will enable transformerless operation of all inverters within the solar farm, and uses only one power transformer at the point of common coupling. The validity of the presented modulation strategies and dc current offset compensation technique are substantiated using simulations and experimentation.

Thesis: Ph. D., Massachusetts Institute of Technology, Department of Electrical Engineering and Computer Science, 2019
Cataloged from PDF version of thesis.
Includes bibliographical references (pages 271-281).
Many of the most critical challenges of the twenty-first century revolve around energy and its management. Improved performance (efficiency, density) in electrical energy management systems require advancements in a number of areas - semiconductor devices, passive energy storage components, and a variety of circuit- and system-level concerns. The sections of this thesis are somewhat distinct and may find application in a great variety of circumstances. Nevertheless, they can be understood as contributions to a single application system: a grid-interface power converter. These kinds of converters have several unique aspects that make them good targets for research, including a heavy reliance on magnetic components, relatively high voltages for application of emerging GaN transistors, wide range of operating voltages and powers, and a twice-line-frequency energy storage component that is difficult to miniaturize. This thesis will present a high-frequency inductor structure with greatly improved density, an exploration of the limits of magnetic-based current sensing, a method for characterizing GaN losses with large-signal excitations, a control approach for miniaturizing grid-interface energy buffers, and a grid-interface circuit with several advantages over the state of the art.
by Alex J. Hanson.
Ph. D.
Ph.D. Massachusetts Institute of Technology, Department of Electrical Engineering and Computer Science

The development of new concepts and methods for improving the efficiency of power networks needs performance evaluation with realistic grid topology. However, much of the realistic grid data needed by researchers cannot be shared publicly due to the security and privacy challenges. With this in mind, power researchers studied statistical properties of power grids and introduced synthetic power grid topology as appropriate methodology to provide enough realistic power grid case studies. If the synthetic networks are truly representative and if the concepts or methods test well in this environment they would test well on any instance of such a network as the IEEE model systems or other existing grid models. In the past, power researchers proposed a synthetic grid model, called RT-nested-smallworld, based on the findings from a comprehensive study of the topology properties of a number of realistic grids. This model can be used to produce a sufficiently large number of power grid test cases with scalable network size featuring the same kind of small-world topology and electrical characteristics found in realistic grids. However, in the proposed RT-nested-smallworld model the approaches to address some electrical and topological settings such as (1) bus types assignment, (2) generation and load settings, and (3) transmission line capacity assignments, are not sufficient enough to apply to realistic simulations. In fact, such drawbacks may possibly cause deviation in the grid settings therefore give misleading results in the following evaluation and analysis. To address this challenges, the first part of this thesis proposes a statistical methodology to solve the bus type assignment problem. This method includes a novel measure, called the Bus Type Entropy, the derivation of scaling property, and the optimized search algorithm. The second part of this work includes a comprehensive study on generation/Load settings based on both topology metrics and electrical characteristics. In this section a set of approaches has been developed to generate a statistically correct random set of generation capacities and assign them to the generation buses in a grid. Then we determine the generation dispatch of each generation unit according to its capacity and the dispatch ratio statistics, which we collected and derived from a number of realistic grid test cases. The proposed approaches is readily applied to determining the load settings in a synthetic grid model and to studying the statistics of the flow distribution and to estimating the transmission constraint settings. Considering the results from the first two sections, the third part of this thesis will expand earlier works on the RT-nested-smallworld model and develop a new methodology to appropriately characterize the line capacity assignment and improve the synthetic power grid modeling.

The modern power system is changing at a rate faster than would have been expected20 years ago. More and more conventional power plants will be shut down in favour ofdistribution generation (DG). This is happening now with the trend of introducing renewableenergy sources (RES) to the power system.The grids were designed to transfer power from generating units connected to the highvoltage grids towards the end consumers connected to the low voltage grids. With changedpower mix, power flows in the system will change resulting in possible grid problems. Oneof the main problems is keeping the voltage within operational limits of the system. Whenthe generation exceeds the consumption in a distribution network, the power will flow fromthe low voltage network towards the high voltage network (reverse power flow) which willcause the voltage to rise in the low voltage network. Reactive power support from DG canbe a valuable resource to mitigate the problem. Reactive power is necessary to operatethe power system. The main source of reactive power is synchronous generators. If thissource is shut down, the reactive power must come from another source.This thesis investigates if DG could be used to support reactive power to the highvoltage transmission network to control the voltage. For this purpose, a distributionsystem located close toWorms, Germany will be studied. This distribution system consistsof two MV feeders with high penetration of DG, mostly photovoltaic (PV) but also windturbines (WT). Consumption and generation measurement data was provided by the localdistribution system operator (DSO). A few reactive power control methods are introducedand tested on this system. From the results, it is concluded that it is possible to providereactive power support from distribution networks and a voltage dependent reactive powercontrol can be used to this purpose.
Det moderna kraftsystemet förandras snabbara än vad som hade förväntats för 20 årsedan. Fler och fler konventionella kraftverk kommer att stängas till fördel för distributionsgenering.Detta händer nu med trenden att introducera förnybara energikällor tillkraftsystemet.Nätverket utformades för att överföra kraft från generatorer som är anslutna till högspänningsnätetmot konsumenter anslutna till lågspänningsnätet. Med ändrad kraftblandningkommer strömflödena i systemet att förändras vilket resulterar i eventuella nätproblem.Ett av huvudproblemen är att hålla spänningen inom operativa gränser för systemet.När generationen överstiger förbrukningen i ett distributionsnät, kommer strömmen attströmma från lågspänningsnätet till högspänningsnätet vilket kommer att leda till attspänningen stiger i lågspänningsnätet. Reaktivt kraftstöd från distributionsgenering kanvara en värdefull resurs för att mildra problemet. Reaktiv effekt är nödvändig för att drivaelsystemet. Huvudkällan för reaktiv kraft är synkrona generatorer. Om den här källanstängs av måste den reaktiva effekten komma från en annan källa.Denna avhandling undersöker om distributionsgenering skulle kunna användas för attstödja reaktiv kraft till högspänningsöverföringsnätet för att styra spänningen. För dettaändamål studeras ett distributionssystem som ligger nära Worms, Tyskland. Detta distributionssystembestår av två MV-matare med med mycket distributionsgenerering, främstsolceller men även vindturbiner. Förbruknings- och generationsmätningsdata tillhandahöllsav den lokala distributionssystemoperatören. Några reaktiva effektstyrningsmetoderintroduceras och testas på detta system. Av resultaten dras slutsatsen att detär möjligt att tillhandahålla reaktivt kraftstöd från distributionsnät och en spänningsberoendereaktiv effektstyrning kan användas för detta ändamål.

Grid synchronization is a critical concern for proper control of energy transfer between the Distributed Power Generation Systems (DPGS) and the utility power grid. Nonlinear estimation techniques are proposed to track the voltage magnitude, phase angle, and frequency of the utility grid. Instead of directly analyzing in abc coordinate frame, the symmetrical component is employed to separate the positive, negative, and zero sequences in the transformed AlphaBeta stationary coordinate frame. By using the Fortescue's Transformations and Clarke's Transformation, the number of system state variables is reduced to five. The results show that our proposed nonlinear estimation technique is efficient in smart power system synchronization. The MATLAB simulation studies have been conducted to compare the performance of the Extended Kalman Filter (EKF), the Particle Filter (PF), and the Unscented Kalman Filter (UKF). Computer simulations have shown that the efficacy of our proposed nonlinear estimation methods. It also shows that the Unscented Kalman Filter, and the Particle Filter are better estimators, because voltage synchronization problem is nonlinear, and linearization process which the Extended Kalman Filter is based on is not very accurate. The number of particles in Particle Filter can be increased to improve the accuracy, but there exists a trade off between computational effort and estimation accuracy. In our research, considering the same amount of computational complexity, we calculate the Mean Square Error (MSE) to examine the performances of different nonlinear estimation approaches. By comparing the MSE of different estimators, we prove that the Unscented Kalman Filter shows the most accurate performance in voltage synchronization for three phase unbalanced voltage. Our results have shown the potential applications of the nonlinear estimation techniques in the future smart power grid synchronization.

Thesis: S.M., Massachusetts Institute of Technology, Department of Electrical Engineering and Computer Science, 2018.
Cataloged from PDF version of thesis.
Includes bibliographical references (pages 77-79).
Load-independent, fixed-speed operation of prime-movers, such as gas turbines and diesel engines, leads to degraded efficiency at part-loaded conditions. This thesis looks at a hybrid power-system architecture that can boost fuel economy through coordinated variable-speed operation of both prime-movers and drive loads. The propulsion plant of an electric ship serves as an example of a micro-grid with a focus on efficiency and dynamic performance. The proposed power distribution system employs doubly-fed machines for generation and for variable speed loads, and can be used where variable-speed operation improves prime-mover efficiency while minimizing required power electronics ratings. The hybrid power-system architecture achieves reduced fuel footprint, less weight and volume constraints by minimizing system power-electronics rating and allows for a selection of an optimum prime-mover.
by Shibal Ibrahim.
S.M.

When wind power and other intermittent renewable energy (IRE) sources begin to supply a significant part of the load, concerns are often raised about the inherent intermittency and unpredictability of these sources. In order to study the impact from higher IRE penetration levels on the power system, integration studies are regularly performed. The model package presented and evaluated in Papers I–IV provides a comprehensive methodology for simulating realistic time series of wind generation and forecasts for such studies. The most important conclusion from these papers is that models based on coarse meteorological datasets give very accurate results, especially in combination with statistical post-processing. Advantages with our approach include a physical coupling to the weather and wind farm characteristics, over 30 year long, 5-minute resolution time series, freely and globally available input data and computational times in the order of minutes. In this thesis, I make the argument that our approach is generally preferable to using purely statistical models or linear scaling of historical measurements. In the variability studies in Papers V–VII, several IRE sources were considered. An important conclusion is that these sources and the load have very different variability characteristics in different frequency bands. Depending on the magnitudes and correlations of these fluctuation, different time scales will become more or less challenging to balance. With a suitable mix of renewables, there will be little or no increase in the needs for balancing on the seasonal and diurnal timescales, even for a fully renewable Nordic power system. Fluctuations with periods between a few days and a few months are dominant for wind power and net load fluctuations of this type will increase strongly for high penetrations of IRE, no matter how the sources are combined. According to our studies, higher capacity factors, more offshore wind power and overproduction/curtailment would be beneficial for the power system.

Off-grid solar power systems are being used in rural, developing regions of the world to provide electricity to communities that previously didn’t have access. These systems are unable to communicate data about power generation and consumption to the organizations that install them. This information can be critical for the organization to ensure they are providing quality electricity, maintain the health of the components in their system, and evaluate the impact they are having in the community. I designed and built a device to monitor power at both the generation and AC or DC consumption sites. Once daily it sends the data to a website via text message where is it displayed to the installer. The device also stores the data locally on a microSD card. The final device abides by the key specifications set by the prior art of low cost and low power consumption. The device also addresses key challenges of off-grid monitoring including data transmission via text message, battery monitoring, reliability/ durability, and versatility with load site monitoring.

Thesis (B.S.)--California Polytechnic State University, 2009.
Project advisor: David Braun. Title from PDF title page; viewed on Jan. 20, 2010. Includes bibliographical references. Also available on microfiche.

The aim of the study is to analyze the possibility of how and where wind farms should be integrated on the electrical grid. The challenges mainly concern grid capacity and transmission losses. Economic factors will be regarded as well. To fulfill the aim, the Skellefteälven river in Sweden is selected as study object. A regional grid along the river is thereupon simulated with regards to five existing hydro power plants, four electrical consumption points, and the national grid. Additionally, four wind farms are placed on probable sites around the grid. Considering the large amount of data to be calculated in this study, a grid model assembled through numerical analysis in MATLAB is henceforth deemed optimal. Through load flow simulation, the voltage variations and power losses are calculated. Hence, the costs of the losses is found. The investment costs for upgrading the grid are also determined. As the results show, an upgrade of the electrical grid certainly requires a relatively large investment sum. Nevertheless, the return of the project will eventually surpass the initial costs. Accordingly, there are economic benefits of investing in upgrading the grid capacity.
Syftet med studien är att analysera möjligheten till hur och var vindkraftsparker borde integreras i elnätet. Utmaningarna rör främst nätkapacitet och ledningsförluster. Ekonomiska faktorer kommer även att undersökas. För att uppnå syftet bedrivs en fallstudie, där Skellefteälven väljs som studieområde. Ett regionalnät är utformat längs älven med hänsyn till fem existerande vattenkraftverk, fyra valda konsumtionsnoder och stamnätet. Utöver dessa placeras även fyra vindkraftsparker ut på lämpliga ställen. Med tanke på hur mycket data som behandlas vid beräkningarna simuleras därför nätet med hjälp av numerisk analys i MATLAB. Genom att köra effektflödesberäkningar räknas spänningsvariationer och effektförluster fram. Därifrån kan kostnader för ledningsförluster tas fram. Vidare framtas även investeringskostnader för uppgradering av nätet. Resultaten visar att en uppgradering kräver en relativt stor investeringssumma. Däremot kommer inkomsten efter en genomförd uppgradering tillslut att överstiga initialkostnaden. Därav finns det ekonomiska fördelar med att investera i en ökad nätkapacitet.

The main aim of this thesis is to develop a solar energy system for domestic utilities, using a bi-directional DC-DC boost converter with a battery storage system in it. Topologies of the converters employed in the existing system are not efficient, especially in high power applications because of complicated structures with many power devices. In order to step up the DC voltage of the solar panel to a value suitable for AC power conversion, the DC-DC converter topologies has used either multiple stages of voltage amplification or complicated structures with many power devices as the voltage gain offered by the standard boost converter is not adequate enough. This has led to the use of complicated switching control methods. As a result of such power converters with low power density, the cost of the system is also high with relatively low efficiency. The above-mentioned shortcomings have led researchers to investigate new topologies of converters and efficient control methods. The thesis investigates the existing topologies of DC-DC boost converters pointing out advantages and disadvantages and presents ten new topologies that are superior to existing ones. Detailed analysis of converters is presented, and the mathematical model is developed to determine the voltage gain as a function of duty cycle. The presented converter topologies are also designed, and prototypes fabricated in the laboratory. The fabricated converters are tested experimentally using Arduino micro-controller. Programs are developed to control the converters in different modes of operation and the performance curves are generated. The experimental results support the theoretical model developed and the obtained results are presented in the thesis. A bi-directional DC-AC converter topology is also developed to interface the solar panel to the AC grid. The developed topology makes use of one of the DC-DC converter topologies presented and offers battery backup facility in it. This system can be controlled to store power either from AC grid or from the solar panel and can power the AC load or inject power to the grid. Appropriate control methods are developed for the bi-directional converter system and extensive simulation studies are conducted using MATLAB/Simulink to demonstrate the operation of the system in different modes of operation and simulation results are also presented. The converter system has also been fabricated and tested with solar panels. The test results of different modes of operation are also presented. Finally, the authors’ viewpoint in the development of power electronics in solar application is presented.

The increasing penetration of converter-interfaced generators (CIGs) has raised concerns about the stability and security of future grids (FGs). These resources affect power systems dynamics in many ways including reducing system inertia, interacting with existing generators, changing power flow paths, etc. In this thesis, we carry out a sensitivity study to explore the structural impacts from CIGs on the damping and frequency stability of power systems. Initially, we study the impact of the intermittent power from wind turbine generators (WTGs) on the damping of the electromechanical oscillations in power systems. It will be shown that the inability of WTGs to provide synchronizing and damping torque to the system jeopardize the small signal stability of power systems. Stable operation regions, in terms of wind penetration and tie-line power, are derived and the impact of load flexibility on these regions are discussed. Next, we have studied the impact of the inertia distribution on the damping of the inter-area modes in power systems. It is shown that tie-line power has a significant role on the damping of the inter-area modes. Moreover, we show that dynamic voltage control and inertia emulation can be utilized to improve the damping of the system. By developing an oscillatory recovery model for power system loads, we have also studied the impact of load oscillations on the damping of the inter-area modes. It is shown that the load dynamics can have a significant influence on the electromechanical oscillations of power systems. Finally, the frequency support capability of WTGs is investigated and the performance of different techniques in utilizing the kinetic energy of the WTGs to assist the frequency stability of power systems is evaluated. A novel time-variable droop characteristic is proposed to enhance the contribution of WTGs in supporting system frequency.

Power converters have an outstanding potential in micro and smart grid applications that require flexible and fast power control as well as rigid voltage regulation at the point of common coupling. Power converters are required to properly operate under several modes of operation such as grid-tie and micro-grid modes of operations. In addition, the control system should be designed to enable proper load sharing between several units.

Several control techniques have been proposed in the literature to address most of the control requirements of the power converters under different operating modes mentioned above. However, references found in the literatures are usually centered on the analysis of the system under only one mode of operation and using a single control strategy. Comprehensive study that combines an in depth analysis of the power converters control under several modes are very scarce in the literature.

In this thesis, a detailed survey and analysis of power converter control techniques in Grid-Tie and AC Micro/Smart Grid applications are introduced. This analysis is based on detailed nonlinear time domain simulations as well as average and small signal models for system stability assessment and performance evaluation.

Master of Science
Department of Electrical and Computer Engineering
Shelli K. Starrett
This thesis introduces a Vehicle to Grid (V2G) system which coordinates the charging, and discharging among the Electric Vehicles (EVs) and two-test systems, to help with peak power shaving and voltage stability of the system. Allowing EVs to charge and discharge without any control may lead to voltage variations and disturbance to the grid, but if the charging and discharging of the EVs is done in a smart manner, they can help the power network. In this thesis, fuzzy logic controllers (FLC) are used to control the flow of power between the grid and the electric vehicles. The presented work in this thesis mainly focuses on the control architecture for a V2G station that allows for using EVs batteries to help the grid’s voltage stability. The designed controllers sustain the node voltage, and thus also achieve peak shaving. The proposed architectures are tested on 16 -generator and 6-generator test systems to examine the effectiveness of the proposed designs. Five fuzzy logic schemes are tested to illustrate the V2G system’s ability to influence system voltage stability. The major contributions of this thesis are as follows: 
 • FLC based control tool for V2G station present at a weak bus in the system. • Investigate the effect of the station location and voltage sensitivity. • Comparison of chargers providing real power versus reactive power. • Simulation of controller and system interactions in a daily load curve cycle. Keywords: State of Charge (SOC), Electric Vehicle (EV), Fuzzy Logic Controller (FLC), Vehicle to grid (V2G), and Power System Voltage Stability.

Hybrid renewable energy systems (HRES) have proven to be a more stable and feasible source of energy than heir single source counterparts. The benefit of HRES is their ability to balance the stochastic behavior of wind and solar production. As result of this, they have been used as stand-alone systems with great success. Optimization studies in the field have shown optimum sizing of the components in the system to be a key element in order to increase feasibility. This paper focuses on the HRES impact on internal grid design and cost. The goal of the thesis is to create a mathematical function and graph on the internal grid design/cost relation for a virtual site with varying wind speed and solar irradiation. A secondary goal is to analyze how much Photovoltaics (PV) in Megawatt (MW) that can be connected to the internal grid post realization of the wind farm and to performed this analyze on the two specific case projects, Site A (17.25 MW) in Sweden and Site B (51.75 MW) in Italy. By utilizing a case study methodology, a mathematical model was created based on two case projects, both with potential to be a combined Wind-PV hybrid plants provided by the wind developer OX2. Identifiers for the two cases studied in this thesis where removed with respect to OX2’s ongoing projects. Hybrid renewable energy systems is a method of increasing the utilization of a regions RES, the system has an increase in overall power output compared to the single RES alternative. However, the internal grid cost was shown to be 3.85 % more expensive Site A and 5.3 % in Site B. This stood in direct correlation to the HRES in Site A using 8.6 % more cable for its internal grid and 29.7 % more in Site B, this is highly depending (depending on the location of the PV array). Furthermore, the case projects showed that the maximum PV to be connected post realization of the farm without major curtailment would be 11.5% of the wind farms rated power in the case of site A and 67.6 % in the case of Site B. Variations in wind speed and solar irradiation were shown to have some impact on grid cost. However, the results pointed out that grid cost in HRES is to a higher degree affected by total cable length in the internal grid than fluctuation in available energy sources. The extent of increase in cable length, the total grid investment cost rises up to 53.4 % for the two case projects.

Smart grids seem to be the solution to use energy from renewable and intermittent energy sources in an efficient manner. There are many research projects around the world and two of them are Jeju Smart Grid Test-bed and Smart Grid Gotland. They have in common that they are both island-based projects and connected to the Powergrid on the mainland by HVDC-link. The purpose of this thesis is to compare the two projects and find out what challenges and strategies they have related to wind power integration. The objective of the two projects were somewhat different. Jeju Smart Grid Test-bedare the starting point for South Korea’s smart grid road map, where the objective ultimately is to construct a smart grid on a national scale in South Korea. For Smart Grid Gotland there are three main focus areas; electricity market, power quality and wind power integration. In this thesis focus is on wind power integration. Wind power integration in smart grids would benefit from energy storage technology connected to the wind power-park to even out the power output. Properties for a potential energy storage connected to Näsudden wind power park situated on the southern tip of Gotland has been investigated and the result is that such an energy storage would likely need to be big and expensive, but able to stabilize the power output.

Variable renewable energy sources are today increasingly integrated in the power system as a step towards the renewable society. The large-scale introduction of converter-based energy sources brings challenges in terms of reduced damping to the power system due to the reduced number of synchronous generators. This can be manifested as high rate-of-change-of-frequency and decreased grid stability. To forestall this reduced performance, it is suggested that the grid-following control of today’s converters are restructured to a grid-forming control, enabling the converter to behave closer to a synchronous machine.   This thesis compares grid-following and grid-forming control and seeks to further describe this grid-forming behavior by applying a grid-forming control method on an energy storage enhanced STATCOM-system. A continuous time model and a linearized model based on state space representations are constructed in order to investigate the grid-forming behavior but also how the converter stability is affected by a restructure from grid-following to grid-forming control.   The results indicate that the investigated grid-forming control method displays a behavior similar to synchronous machines and incorporates the ability to provide frequency response services and so called “synthetic inertia” to the grid.  The results also show that the stability of the converter (the ability to provide a bounded output when the system is perturbed) is ensured when the control method is restructured from grid-following to grid-forming and that the investigated grid-forming method is stable also in weak grid situations.

The term Vehicle-to-Grid (V2G) refers to the technology that enables a bidirectional power exchange between the electric grid and the batteries of plug-in electric vehicles (PEV). V2G technology can be a key element of the intelligent network, which may use the batteries of the vehicle as a system of local storage. The vehicle battery may contribute to the stability of the grid and to meeting the energy demand, especially in peak hours. A PEV needs a bidirectional charger to implement V2G, and, consequently, the studies regarding their design, functionality and efficiency are of the utmost interest. This thesis describes the state of art of these chargers and discusses some aspects of a bi-directional converter and some case studies related to this topic. The main objective of this work is to develop the design and the control algorithms of a bidirectional battery charger with capability to charge the battery of a PEV and simultaneously to act as an active filter for the supply line. After the first introductory chapter, the second chapter reports the terminology used in this field of research. Several smart strategies for charging, approaches for the implementation of the battery chargers for PEVs and the recharging standards are briefly described. The analysis of different types of charger is detailed in chapter three. The conventional battery chargers (CBC) with front-end formed by a diode rectifier, battery chargers with power factor correction (PFC), bi-directional battery chargers (BBC), and integral battery chargers (IBC) are considered. In chapter four, definitions are given of the electrical power in non-sinusoidal conditions, together with some examples of the inadequacies of the classical power theory in describing non-linear phenomena that occur during the operation of a power system. The fifth chapter presents the basic concepts of the theory of instantaneous active and reactive power (also known as p-q theory) applied to the compensation of non-sinusoidal systems. Definition of real, imaginary and zero sequence power are introduced and it is shown how this theory makes it easier to understand the phenomena caused by non-sinusoidal voltages or currents. The theory is particularly suitable for the design of a battery charger when it is seen as a power conditioner. Chapter six is devoted to the basic concepts of shunt active filters. They can perform different types of functions, such as the compensation of current harmonics generated by nonlinear loads to prevent their propagation in the network. The compensation algorithm based on powers defined in reference  is very flexible and therefore the theory of instantaneous power has been considered as the basis for the development of the control system of active filters. Some examples of compensation described in the previous chapter were simulated and the results have been included. In chapter seven, sizing of the power devices that constitute the battery charger is considered in relation to the various auxiliary services that it can provide. The power electronic switches, the coupling inductors and the other passive components have been sized in voltage and current. In chapter eight it is considered a charger that supplies its load and simultaneously compensates for non-linear loads connected nearby. These additional features in terms of power conditioning were quantified in order to determine the capacity of a battery charger that is formed by given active and passive components to support the network acting as an active filter. In the ninth chapter the filter inductances of a battery charger are sized for a specific case study in which it is required the capability to recharge the battery and to inject active power in the network, both in the case of single-phase and three-phase connection. Evaluation of the ripple current is an important requirement for the design of inductors. Therefore a precise calculation was made of this magnitude both in the case of a battery charger connected to the single phase grid and operating according to the PWM technique, and, in the case of connection to the three-phase grid, operating according to the technique SVM. In chapter ten a case study is considered regarding the design of an LCL filter. Chapter eleven contains a theoretical study of resonant controllers. They solve the problem posed by the conventional PI controllers that, when used for the control of alternate quantities as it occurs for the currents of a DC/AC converter, are not able to cancel the steady state error due to the finite gain at the operating frequency. Instead, a resonant controller has a gain ideally infinite at the operating frequency and thus ensures a zero steady-state error. The effectiveness of the resonant regulators has been verified by means of simulations. Chapter twelve deals with the regulations regarding connectors, charging modes and ways of connecting the PEV chargers to the grid. They are intended to define a charging procedure common to all the PEVs and to all the charging infrastructures, whether public or private.
Il termine Vehicle-to-Grid (V2G) si riferisce alla tecnologia che permette uno scambio di potenza bidirezionale tra la rete elettrica e le batterie dei veicoli elettrici di tipo plug-in (PEV). La tecnologia V2G può essere un elemento chiave della rete intelligente, che può utilizzare le batterie dei veicoli come un sistema di accumulo locale. Le batterie dei veicoli possono contribuire alla stabilità della rete e a soddisfare la domanda di energia soprattutto nelle ore di punta. Un PEV ha bisogno di un caricatore bidirezionale per implementare il V2G, e, di conseguenza, gli studi riguardo il loro progetto, la funzionalità e l'efficienza sono del massimo interesse. Questa tesi descrive lo stato dell’arte di questi caricabatteria e tratta alcuni aspetti di un convertitore bidirezionale e alcuni casi di studio relativi a questo argomento. L'obiettivo principale di questo lavoro è di sviluppare il progetto e gli algoritmi di controllo di un caricabatteria bidirezionale con capacità di caricare la batteria di un veicolo plug-in e contemporaneamente di agire come filtro attivo nei confronti della linea di alimentazione. Dopo il primo capitolo introduttivo, nel secondo capitolo viene riportata la terminologia usata in questo campo di ricerca. Vengono anche brevemente descritte diverse strategie intelligenti di ricarica, gli approcci per la realizzazione dei caricabatteria dei PEV e gli standard di ricarica. L’analisi dei vari tipi di caricabatteria viene approfondita nel terzo capitolo. Sono considerati il caricabatteria tradizionale (CBC) con front-end costituito da un raddrizzatore a diodi, il caricabatteria dotato di correttore del fattore di potenza (PFC), il caricabatteria bidirezionale (BBC), e il caricabatteria integrale (IBC). Nel capitolo quattro vengono date le definizioni della potenza elettrica in condizioni non sinusoidali assieme ad alcuni esempi delle inadeguatezze della teoria classica della potenza nel descrivere fenomeni non lineari che si verificano durante il funzionamento di un sistema di potenza. Nel quinto capitolo sono presentati i concetti di base della teoria potenza istantanea attiva e reattiva (nota anche come teoria pq) applicata alla compensazione di sistemi non sinusoidali. Vengono introdotte le definizioni della potenza reale, immaginaria e di sequenza zero e viene mostrato come questa teoria renda agevole la comprensione dei fenomeni causati da tensioni o correnti non sinusoidali. Essa è particolarmente adatta per il progetto di un caricabatteria quando esso viene visto come un condizionatore di potenza. Il capitolo sei è dedicato ai concetti di base dei filtri attivi di tipo shunt. Essi possono svolgere diversi tipi di funzioni, come la compensazione delle armoniche di corrente generate da carichi non lineari impedendo la loro propagazione nella rete. L’algoritmo di compensazione basato sulle potenze definite nel riferimento αβ è molto flessibile e quindi la teoria della potenza istantanea è stata considerata come la base per lo sviluppo del sistema di controllo dei filtri attivi. Alcuni esempi di compensazione descritti nel capitolo precedente sono stati simulati e sono stati riportati i risultati. Nel capitolo sette è considerato il dimensionamento dei dispositivi di potenza che costituiscono il caricabatteria in relazione ai diversi servizi ausiliari che esso può fornire. Sono stati dimensionati in tensione e corrente gli interruttori elettronici di potenza, gli induttori di accoppiamento con la rete e gli altri componenti passivi. Nel capitolo otto viene considerato un caricabatteria che alimenta il proprio carico e contemporaneamente compensa i carichi non lineari connessi nelle vicinanze, costituiti da raddrizzatori. Queste funzionalità aggiuntive in termini di condizionamento della potenza di rete sono state quantificate al fine di determinare la capacità di un caricabatteria costituito da determinati componenti attivi e passivi di supportare la rete svolgendo la funzione di filtro attivo. Nel nono capitolo sono state dimensionate le induttanze di filtro di un caricabatteria per uno specifico caso di studio in cui era richiesta la capacità sia di ricaricare la batteria che di iniettare potenza attiva in rete, sia nel caso di connessione monofase che trifase. La conoscenza dell’ampiezza dell’ondulazione di corrente è un requisito importante per il dimensionamento delle induttanze. Perciò è stato effettuato un calcolo preciso di questa grandezza sia nel caso di un caricabatteria connesso alla rete monofase e operante secondo la tecnica di PWM, sia nel caso di connessione alla rete trifase e adozione della tecnica SVM. Nel capitolo dieci viene considerato un caso di studio riguardo il dimensionamento di un filtro LCL. IL capitolo undici contiene uno studio teorico dei regolatori risonanti. Essi risolvono il problema posto dai convenzionali regolatori PI, che quando sono impiegati per il controllo di grandezze alternate, come accade nel caso delle correnti in un convertitore dc-ac, non sono in grado di annullare l’errore a regime a causa del guadagno finito alla frequenza di funzionamento. Un regolatore risonante presenta invece un guadagno idealmente infinito alla frequenza di funzionamento e quindi garantisce un errore a regime nullo. L’efficacia dei regolatori risonanti è stata verificata per mezzo di simulazioni. Nel capitolo dodici sono riportate le normative riguardanti i connettori, le modalità di ricarica e la connessione dei caricabatteria dei PEV alla rete elettrica. Esse mirano a definire una procedura di ricarica comune a tutti i PEV e tutte le infrastrutture di ricarica, siano esse pubbliche o private.

Thesis: Ph. D., Massachusetts Institute of Technology, Department of Electrical Engineering and Computer Science, 2016.
This electronic version was submitted by the student author. The certified thesis is available in the Institute Archives and Special Collections.
Cataloged from student-submitted PDF version of thesis.
Includes bibliographical references (pages 215-223).
Power electronics convert and control power and are an essential part of many electronic devices and systems. Increasingly, power converters need to operate over widely varying operating conditions (voltage, current and power level), which can degrade their performance. In many applications, power converters are also being interconnected to form power systems. This is challenging as the converters may have to operate in various configurations. In order to ensure best performance, power converters and systems need to adapt to the operating conditions by adjusting topology, component values or control characteristics. In this thesis, two applications are considered: universal input power supplies and off-grid electrification. More than 3 billion power supplies were sold worldwide in 2014 [1], with efficiencies ranging from 50-90% [2], which results in significant energy loss. A major portion of these are universal input power supplies (designed to operate at AC line voltages found globally). In this thesis, a control technique, Variable Frequency Multiplier (VFX), is developed which compresses the effective operating range required of a resonant converter by switching the inverter and/or rectifier operation between processing energy at a fundamental frequency and one or more harmonic frequencies. This technique was applied to the inverter stage of a stacked-bridge LLC converter for a universal input power supply. An efficiency of 94.9% to 96.6% was achieved for a 50 W converter operating across the entire (4:1) input voltage range. Even though centralized grid infrastructure is widespread, access to electricity is still limited in many parts of the developing world. More than 1.2 billion people globally do not have access to electricity [3]. In this thesis, an ad hoc modular microgrid architecture is developed and field-tested. Contrary to how conventional power systems are designed, these microgrids do not require pre-planning and can operate in any network configuration. Smart power management units have been designed to arbitrarily connect power sources and loads, forming an autonomous microgrid. Accurate power sharing of multiple power sources is demonstrated. A methodology to attain the lowest system cost by designing power converters that reduce lifetime energy loss has also been developed. This enables affordable and reliable electricity to be provided in off-grid areas. Improvements in these applications will have a significant impact on power utilization from the existing grid infrastructure, and will help define the future of power utilization where this infrastructure does not exist. Moreover, the techniques and designs developed, in this thesis, for adaptable and efficient operation of power converters and systems can be easily extended to other applications.
by Wardah Inam.
Ph. D.

Thesis: Ph. D., Massachusetts Institute of Technology, Department of Electrical Engineering and Computer Science, 2016.
Cataloged from PDF version of thesis.
Includes bibliographical references (pages 145-151).
With the present ac-voltage distribution system, ac-dc converters are key components for driving many dc voltage applications from the ac grid voltage. There are a lot of electronic devices that natively operate from the dc voltage including light emitting diodes (LEDs), personal and laptop computers, and smart phones; for all of them there is a drive to increase functionality and to reduce the volume at the same time. The desire for further miniaturization is, however, facing a dominant obstacle strained by the performance requirements on power electronic circuits. In this thesis, a design technique for high-performance ac-dc power converters will be presented. A new grid interface ac-dc conversion architecture and associated circuit implementations are proposed along with novel control methods. This approach simultaneously address design challenges associated with high performance (e.g., high efficiency, high power factor, miniaturization, and high reliability/lifetime) of ac-dc power conversion systems. The proposed architecture is suitable for realizing ac-dc converters that switch in the HF range (3-30 MHz) with relatively low-voltage components and with zero-voltage switching (ZVS) conditions, enabling significant converter size reduction while maintaining high efficiency. Moreover, the proposed approach can achieve reasonably high power factor about 0.9, while dynamically buffering twice-line frequency energy using small capacitors operating with large voltage swings over the ac line voltage cycle. The ac-dc converter design shows that excellent combinations of power density, efficiency, and power factor can be realized with this approach.
by Seungbum Lim.
Ph. D.

To address the challenges of resource allocation in the Smart Electrical Grid a new power market is proposed. A distributed and autonomous contract net based market system in which participants, represented by the agents, engage in two distinct yet interconnected markets in order to determine resource allocation. Key to this proposed design is the 2 market structure which separates negotiations between consumers and reliable generation from negotiations between consumers and intermittent energy resources. The first or primary market operates as a first price sealed bid reverse auction while the second or secondary market utilizes a uniform price auction. In order to evaluate this new market a simulator is developed and the market is modeled and tested within it. The results of these tests indicate that the proposed design is an effective method of allocating electrical grid resources amongst consumers, generators, and intermittent energy resources with some feasibility and scalability limitations.

Scientists at European Organization for Nuclear Research (CERN) are currently conducting feasibility studies for the Compact linear collider (CLIC); their proposed next experimental setup for gathering information on the fundamental particles of matter. This experiment will involve the simultaneous pulsing of 1300 klystron modulators to produce a 140us, 39GW pulse with a 50 Hz repetition rate. This proposal presents many demands for the connected power system as an effort is made to "hide" this pulse from the local distribution network - instead drawing only the constant average power of approximately 300MW. This challenge is considered in this work. In order to understand the optimal approach both the power system architectures and power electronics interfaces must be considered simultaneously. An approach to the optimisation of the power system architecture is described in this thesis. It is clear from this exercise that the optimum power converter topology for the interface between the electricity distribution network and the klystron modulators is the Modular Multilevel Converter (MMC). This converter is mainly used in modern HVDC transmission circuits as a result of its high efficiency and ability to produce high quality AC waveforms. Pulsing of the klystron modulators does however create further challenges for the inner control loops of an MMC. The placement of the pulse can create imbalances in the DC capacitors of the MMC submodules which may result in tripping of the converter if not corrected. This thesis proposes three arm balancing solutions to be applied together with the decoupled AC and DC side controller designed for the specified application. These proposed solutions to the aforementioned problems are successfully validated using simulation work in PLECS and using data from a laboratory scale prototype of one of the MMC interface power converters.

This paper presents renewable energy, power electronics, and distributed generators. The focus is on wind farm generator, photovoltaic cell, and battery bank system. Power Conditioning system improves the performance of a power system. Apart from the benefits of converting between DC/AC, there is adequate control of real power and additional control of economic reactive power. This is possible because of multiple sources in the system. This project throws light on the basic principle of power system conditioning, its operation and control, and the economic studies.

The Energy Independence and Security Act of 2007 established that the current electric grid was inadequate to serve the United States needs. Congress mandated that the U.S. transition to a more intelligent grid for the future. The Department of Energy was tasked with making this goal a reality. Six years later in 2013, only marginal progress has been made. Outside of smart meter rollouts and pilots programs funded through the American Recovery and Reinvestment Act of 2009 (ARRA), many issues still need to be addressed in order to realize the U.S. Smart Grid vision. Most of the barriers to progress are not technological; the research and business community are rising to the occasion and meeting the challenge through innovation. However, policy issues present a large barrier to overcome. With issues ranging from vague Smart Grids goals issued by the Department of Energy to a general lack of consumer knowledge about the Smart Grid. This paper seeks to identify the gaps in the current electric grid and policy schema are inadequate and suggest recommendations to encourage and expedite the growth of the U.S. Smart Grid.

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.

Creative ways of utilising renewable energy sources in electricity generation especially in remote areas and particularly in countries depending on imported energy, while increasing energy security and reducing cost of such isolated off-grid systems, is becoming an urgently needed necessity for the effective strategic planning of Energy Systems. The aim of this research project was to design and implement a new decision support framework for the optimal design of hybrid micro grids considering different types of different technologies, where the design objective is to minimize the total cost of the hybrid micro grid while at the same time satisfying the required electric demand. Results of a comprehensive literature review, of existing analytical, decision support tools and literature on HPS, has identified the gaps and the necessary conceptual parts of an analytical decision support framework. As a result this research proposes and reports an Iterative Analytical Design Framework (IADF) and its implementation for the optimal design of an Off-grid renewable energy based hybrid smart micro-grid (OGREH-SμG) with intra and inter-grid (μG2μG & μG2G) synchronization capabilities and a novel storage technique. The modelling design and simulations were based on simulations conducted using HOMER Energy and MatLab/SIMULINK, Energy Planning and Design software platforms. The design, experimental proof of concept, verification and simulation of a new storage concept incorporating Hydrogen Peroxide (H2O2) fuel cell is also reported. The implementation of the smart components consisting Raspberry Pi that is devised and programmed for the semi-smart energy management framework (a novel control strategy, including synchronization capabilities) of the OGREH-SμG are also detailed and reported. The hybrid μG was designed and implemented as a case study for the Bayir/Jordan area. This research has provided an alternative decision support tool to solve Renewable Energy Integration for the optimal number, type and size of components to configure the hybrid μG. In addition this research has formulated and reported a linear cost function to mathematically verify computer based simulations and fine tune the solutions in the iterative framework and concluded that such solutions converge to a correct optimal approximation when considering the properties of the problem. As a result of this investigation it has been demonstrated that, the implemented and reported OGREH-SμG design incorporates wind and sun powered generation complemented with batteries, two fuel cell units and a diesel generator is a unique approach to Utilizing indigenous renewable energy with a capability of being able to synchronize with other μ-grids is the most effective and optimal way of electrifying developing countries with fewer resources in a sustainable way, with minimum impact on the environment while also achieving reductions in GHG. The dissertation concludes with suggested extensions to this work in the future.

Thesis (Ph. D.)--Massachusetts Institute of Technology, Engineering Systems Division, 2008.
Includes bibliographical references.
This research used a combination of a grounded theory approach and system dynamics to study the electric power system in Kenya and to model the feedback at work in the development of the system. The ethnographic study revealed the challenges faced by consumers in choosing between grid and off-grid power options. Examination of this challenge leads to the hypothesis that competition between the grid and off-grid markets is contributing to the low growth in power consumption and that there is the potential for off-grid to become the dominant option in the future. This theory guided the construction of a system dynamics model focusing on consumers' decision-making and their interaction with the operation of the system. I then used the model to explore the dynamics of the system through scenario testing. There were two key outcomes from the model. The first showed that given the parameters chosen in most cases there is a clearly dominant option, although it changes over time. This finding points to the second key outcome the model, which is that there are realistic scenarios under which off-grid generation will become the dominant supply source. This shift could be induced by either reduced overhead on photovoltaic panels or high fuel prices. The outcomes from this research have implications for future electricity planning in Kenya and elsewhere in Africa. In particular, there is a need to decouple the system from external prices or account for the extreme uncertainty in fuel prices. Given the potential shift to large-scale off grid power generation, energy planners also need to look at options for managing a decentralized power system architecture and consider how to build in options for future reintegration if a large-scale centralized generation source comes online.
(cont.) This research has both academic and applied contributions. On the academic side, it extends the range of engineering systems modeling to include qualitative factors found in an African environment. These factors include the addition of reliability and availability of the electric power grid and the biases in decision-making, which differ from those in industrialized countries. While the model clearly has direct application in Kenya, it was designed with flexibility to be expanded to include other countries and regions and could be a useful tool for understanding policy trade-offs in African electrification planning.
by Katherine Deaton Steel.
Ph.D.

Outages and power interruptions are a common and unenviable part of power distribution system operations. Growing demands on reliability in distribution systems has opened up for new technological solutions for fault location at MV and LV level in distribution systems, previously reserved for transmission systems. This report compiles and compares available methods for fault location at distribution level and maps the current fault location process at the power distribution company Ellevio, with the aim of reaching a recommendation for a new fault location scheme. The advocated method is an impedance based method motivated by its reliability, applicability and affordability. The performance and implementation procedure is evaluated through a number of case studies where the methods impact on power reliability demonstrated as well as the need for grid analysis before implementation. Fault indicators and fault current, through relay communications, was identified as key factors for a successful implementation of the method.

The objective of the research is to develop a cost-effective, dynamic grid controller called the controllable network transformer (CNT) that can be implemented by augmenting existing load tap changing (LTC) transformers with an AC-AC converter. The concept is based on using a fractionally rated direct AC-AC converter to control the power through an existing passive LTC. By using a modulation strategy based on virtual quadrature sources (VQS), it is possible to control both the magnitude and the phase angle of the output voltage of the CNT without having any inter-phase connections. The CNT architecture has many advantages over existing power flow controllers, like absence of low frequency storage, fractional converter rating, retro-fitting existing assets and independent per-phase operation making it potentially attractive for utility applications. The independent control of the magnitude and the phase angle of the output voltage allow independent real and reactive power flow control through the CNT-controlled line. In a meshed network with asymmetric network stresses this functionality can be used to redirect power from critically loaded assets to other relatively under-utilized parallel paths. The power flow controllability of CNT can thus be used to lower the overall cost of generation of power. The solid state switches in the CNT with fast response capability enable incorporation of various additional critical functionalities like grid fault ride through, bypassing internal faults and dynamic damping. This bouquet of features makes the CNT useful under both steady state and transient conditions without compromising the grid reliability.

Successful integration of the smart grids is crucial for ensuring the efficiency, resiliency, and sustainability of future power systems. With a 46.53% increase in total primary energy supply between 2008 and 2018 (IEA, 2020a), Turkey has the fastest-growing energy market within OECD countries (Erdin and Ozkaya, 2019).Though, Turkey’s current electrical grid faces many challenges; such as high loss rates from the transmission and distribution lines (Damar, 2016; Düzgün, 2018; IBRD/The World Bank, 2016; TEİAŞ, 2019), frequent power outages (Guner and Ozdemir, 2011; Öztürk, 2017; Yanılmaz, 2016) and several incidents of large-scale blackouts (OECD/IEA, 2016; Project Group Turkey, 2015). Smart grid technologies can address Turkey’s power system’s challenges with a holistic approach, as the smart grid does not have a strict definition yet but has distinctive characteristics. This paper provides an overview of Turkey’s current electric power system’s challenges while analyzing Turkey’s progress up to the present day towards the smart grid transition. Also, in order to summarize fundamental smart grid technologies globally, a smart grid framework was designed. Founded upon the framework; Turkey’s previous actions for its power system development were assessed from the smart grid perspective, and further steps were proposed to accomplish a successful smart grid transition. Country-specific remarks regarding the electrical grid were highlighted, such as the risk of terrorism, high seismic activity in the region, and emerging nuclear power in the country. As a result, it is concluded that Turkey has initiated the process towards the smart grid transition not only to achieve a “smart grid ideal” but mainly to meet its growing energy demands. Furthermore, it is also concluded that if the smart grid technologies’ scope could be extended throughout the country, Turkey would overcome the challenges with its power system in short to medium-term.

In recent years, renewable and distributed generation (DG) systems have contributed towards an efficient and an economic way of transporting electricity to end-users as the generation sources are in general located nearer the loads. DG and renewable energy systems are modifying the old concept of distribution network by instigating a bi-directional power flow into the grid, facilitated through the use of power electronic grid-connected converters. A challenge associated with grid-connected converters arises when they are interacted with a grid that is not stiff, like weak micro grids. Small grid parameter variations in these systems can considerably affect the performance of the converter control and lead to higher values in current total harmonic distortion (THD) and loss of control and synchronization. Thus, the control of grid-connected power converters needs to be regularly updated with latest variation in grid parameters. Model Predictive Direct Power Control (MP-DPC) has been chosen as the control strategy for the work presented in this thesis due to its advantages over traditional control techniques such as multivariable control, no need of phase-locked loops (PLLs) for grid synchronization and avoidance of cascaded control loops. Two novel methods for estimating the grid impedance variation, and hence the grid voltage, are presented in this thesis along with a detailed literature review on control of grid-connected converters with special emphasis on impedance estimation techniques. The first proposed estimation method is based on the difference in grid voltage magnitudes at two consecutive sampling instants while the second method is based on a model-fitting algorithm similar to the concept of cost-function optimization in model predictive control. The proposed estimation methods in this thesis are integrated within the MP-DPC, therefore updating the MP-DPC in real-time with the latest variation in grid impedance. The proposed algorithms provide benefits such as: quick response to transient variations, operation under low values of short-circuit-ratio (SCR), robust MP-DPC control, good reference tracking to grid parameter variations and operation under unbalanced grid voltages. The thesis also presents the advantages and drawbacks of the proposed methods and areas where further improvement can be researched. The work presented has been tested on a three phase two-level grid-connected converter prototype, which is connected to a low voltage substation highly dominated by inductive component of grid impedance. It can be adapted and modified to be used for general grid impedance estimation, medium or high voltage applications, in case of multilevel grid-connected converter topologies or photo-voltaic (PV) grid-connected applications.

Företaget detta examensarbete är utfört åt heter YIT och är ett stort företag som erbjuder tjänster så som, tekniska installationer inom fastighetsteknik, service av fastighetstekniska installationer och utveckla energieffektivitet i dess lösningar och tjänster. De finns i Norden, Baltikum, Ryssland och Centraleuropa. YIT erbjöd oss att göra ett arbete på Eson Pac AB där vi skulle göra om enlinjeschemat på kraftnätet samt märka ut vart alla centraler och undercentraler finns på situationsritning. Vår uppgift på Eson Pac AB blev då att uppdatera de befintliga enlinjeschemat, situationsritningar och göra nya skyltar till de centraler/undercentraler som tillkommit i byggnaden. Det resulterade i att dokumentationen över Eson Pac AB:s kraftnät blev slutfört och komplett.

Kyoto University (京都大学)
0048
新制・課程博士
博士(エネルギー科学)
甲第21385号
エネ博第373号
新制||エネ||73(附属図書館)
京都大学大学院エネルギー科学研究科エネルギー社会・環境科学専攻
(主査)教授 手塚 哲央, 教授 下田 宏, 准教授 MCLELLAN,Benjamin
学位規則第4条第1項該当

Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Electrical Engineering and Computer Science, 2011.
Cataloged from PDF version of thesis.
Includes bibliographical references (p. 213-215).
Grid connected power conversion is an absolutely critical component of many established and developing industries, such as information technology, telecommunications, renewable power generation (e.g. photovoltaic and wind), even down to consumer electronics. There is an ever present demand to reduce the volume and cost, while increasing converter efficiency and performance. Reducing the losses associated with energy conversion to and from the grid can be accomplished through the use of new circuit topologies, enhanced control methods, and optimized energy storage. The thesis outlines the development of foundational methods and architectures for improving the efficiency of these converters, and allowing the improvements to be scaled with future advances in semiconductor and passive component technologies. The work is presented in application to module integrated converters (MICs), often called micro-inverters. These converters have been under rapid development for single-phase gridtied photovoltaic applications. The capacitive energy storage implementation for the double-line-frequency power variation represents a differentiating factor among existing designs, and this thesis introduces a new topology that places the energy storage block in a series-connected path with the line interface. This design provides independent control over the capacitor voltage, soft-switching for all semiconductor devices, and full four-quadrant operation with the grid.
by Brandon J. Pierquet.
Ph.D.

Thesis: Ph. D., Massachusetts Institute of Technology, Department of Electrical Engineering and Computer Science, 2018.
This electronic version was submitted by the student author. The certified thesis is available in the Institute Archives and Special Collections.
Cataloged from student-submitted PDF version of thesis.
Includes bibliographical references (pages 185-189).
In this thesis we present a two-stage ac/dc grid-connected converter for computer applications. Also known as off-line power supplies, these converters have to meet various demanding specifications such as a wide input voltage range (typically 0-376 V), large voltage step down (typical output voltages range from 12-48 V), harmonic current limits and galvanic isolation. The focus of this work is in the reduction in volume of ac/dc converters while keeping efficiency constant or improving it, which is challenging to achieve while meeting all the specifications. The thesis breaks down the converter in subsystems and explores architectural and topological trade-offs, modeling, component selection and control methods. The performance of each individual subsystem is experimentally verified. The first stage of the converter is a step-down power factor correction (PFC) converter. This stage interacts with the grid and draws the necessary ac power from the line and rectifies it. Following the PFC is a capacitor bank, which is used to both buffer the ac power from the line and to provide hold-up energy to the output. The capacitor selection process is detailed in the thesis. The second stage of the converter provides isolation and regulation to the output. Two different approaches to the second stage converter are presented: using commercially available, "plug and play" converters and developing a custom converter. The full system is evaluated with both solutions and is compared to other state of the art converters. The final prototype achieves an efficiency of 95.33% at full power (250 W) and 230 Vac input, and a power density of 35 W/in3.
by Juan Antonio Santiago-González.
Ph. D.

Power-combining schemes involving planar grids of solid-state devices quasi-optically coupled in free space are an efficient means of combining power at microwave and millimeter-wave frequencies. The quasi-optical coupling of these grid circuits makes them ideal for millimeter-wave and submillimeter-wave applications by eliminating waveguide sidewall losses and machining difficulties. The planar property of the grids potentially allows thousands of devices to be integrated monolithically. In this thesis, a grid mixer suitable for mixing or detecting quasi-optical signals is presented. The mixer is a planar grid structure periodically loaded with diodes. The grid mixer power handling and dynamic range scales as the number of devices in the grid. The conversion loss and noise figure of the grid are equal to that of a conventional mixer. A variation of the grid mixer, the sideband generator grid, is shown to be an efficient package for increasing the theoretical operating frequency and output power of monolithic planar diodes at terahertz frequencies. Techniques for designing power grid oscillators to produce Watt-level output powers are described. Designs and experimental results for MESFET grid oscillators operating in X-band with output powers of 0.9 W to 10.3 W are presented in detail. Methods that make use of finite-element electromagnetic solvers for analyzing grid structures of arbitrary shape are discussed.