Three-phase voltage source inverters are increasingly employed in power systems and industrial applications. Various pulse width modulation strategies have been applied to control the voltage source inverters. This dissertation presents a hybrid 2D-3D space vector modulation algorithm for three-phase voltage source inverters with both three-wire and four-wire topologies. The voltage magnitude and phase angle of the inverters fundamental output phase voltage are precisely controlled under either balanced or unbalanced load conditions, and hence, the space vector algorithm offers synchronization controllability over generation control in distributed generation systems. The numerical efficiency and simplicity of the proposed algorithm are validated through conducting MATLAB/Simulink simulations and hardware experiments. Mathematical description and harmonic analyses of output phase voltages of three-phase voltage source inverter which employs a hybrid 2D-3D SVM are presented in this dissertation. Explicit time domain representation of the harmonic components in addition to the total harmonic distortion of the output phase voltages are given in terms of system and switching parameters. The dissertation also investigates the harmonic characteristics and low total harmonic distortion performance against the linearity of modulation region which helps in the harmonic performance and design studies of such inverters employing the hybrid 2D-3D SVM. Experimental results are used to validate these analyses. In addition, the performance and the harmonic contents of the inverter output phase voltage when applying the proposed hybrid 2D-3D SVM are compared to that obtained from conventional 2D SVM and 3D SVM. As a result, the proposed new algorithm shows advantages in terms of low total harmonic distortion and reduced harmonic contents in both three-wire and four-wire systems.

As the world's energy use continues to grow, the development of clean distributed generation becomes increasingly important. Fuel cells are an environmentally friendly renewable energy source that can be used in a wide range of applications and are ideal for distributed power applications. In this study, the power conversion element of a dual single-phase, 10 kW stand-alone fuel cell system is analyzed. The modular converter consists of a DC-DC front-end cascaded with a half-bridge inverter. The entire system is accurately modeled, to help determine any interactions that may arise. Control strategies based on simplicity, performance, and cost are evaluated. A simple voltage loop, with careful consideration to avoid transformer saturation, is employed for the phase-shifted DC-DC converter. Several experimental transfer functions were measured to confirm the modeling assumptions and verify the control design of the DC-DC converter. Two control options for the inverter are explored in detail, and experimental results confirm that the modulation index must be controlled to regulate the output voltage during various load conditions. The final system is implemented without the use of current sensors, thus keeping the inverter cost down. Experimental results using a power supply are given for resistive, inductive, and nonlinear loads and the performance is acceptable. Fuel cell test results, including transient response, are also displayed and analyzed. / Master of Science

Electric power systems are undergoing significant changes with the deployment of large-scale wind and solar plants connected to the transmission system and small-scale Distributed Energy Resources (DERs) and microgrids connected to the distribution system, making the latter an active system. A microgrid is a small-scale power system that interconnects renewable and non-renewable generating units such as solar photo-voltaic panels and micro-turbines, storage devices such as batteries and fly wheels, and loads. Typically, it is connected to the distribution feeders via power electronic converters with fast control responses within the micro-seconds. These new developments have prompted growing research activities in stability analysis and control of the transmission and the distribution systems. Unfortunately, these systems are treated as separated entities, limiting the scope of the applicability of the proposed methods to real systems. It is worth stressing that the transmission and distribution systems are interconnected via HV/MV transformers and therefore, are interacting dynamically in a complex way. In this research work, we overcome this problem by investigating the dynamics of the transmission and distribution systems with parallel microgrids as an integrated system . Specifically, we develop a generic model of a microgrid that consists of a DC voltage source connected to an inverter with real and reactive power control and voltage control. We analyze the small-signal stability of the two-area four-machine system with four parallel microgrids connected to the distribution feeders though different impedances. We show that the conventional PQ control of the inverters is insufficient to stabilize the voltage at the point-of-common coupling when the feeder impedances have highly unequal values. To ensure the existence of a stable equilibrium point associated with a sufficient stability margin of the system, we propose a new voltage control implemented as an additional feedback control loop of the conventional inner and outer current control schemes of the inverter. Furthermore, we carry out a modal analysis of the four-machine system with microgrids using Koopman mode analysis. We reveal the existence of local modes of oscillation of a microgrid against the rest of the system and between parallel microgrids at frequencies that range between 0.1 and 3 Hz. When the control of the microgrid becomes unstable, the frequencies of the oscillation are about 20 Hz. Recall that the Koopman mode analysis is a new technique developed in fluid dynamics and recently introduced in power systems by Suzuki and Mezic. It allows us to carry out small signal and transient stability analysis by processing only measurements, without resorting to any model and without assuming any linearization. / Ph. D. / Electric power systems are undergoing significant changes with the deployment of large-scale wind and solar plants connected to the transmission system and small-scale Distributed Energy Resources (DERs) and microgrids connected to the distribution system, making the latter an active system. A microgrid is a small-scale power system that interconnects renewable and non-renewable generating units such as solar photo-voltaic panels and micro-turbines, storage devices such as batteries and fly wheels, and loads. Typically, it is connected to the distribution feeders via power electronic converters with fast control responses within the micro-seconds. These new developments have prompted growing research activities in stability analysis and control of the transmission and the distribution systems. Unfortunately, these systems are treated as separated entities, limiting the scope of the applicability of the proposed methods to real systems. It is worth stressing that the transmission and distribution systems are interconnected via HV/MV transformers and therefore, are interacting dynamically in a complex way. In this research work, we overcome this problem by investigating the dynamics of the transmission and distribution systems with parallel microgrids as an integrated system . Specifically, we develop a generic model of a microgrid that consists of a DC voltage source connected to an inverter with real and reactive power control and voltage control. We show that the conventional PQ control of the inverters is insufficient to stabilize the voltage at the point-of-common coupling when the feeder impedances have highly unequal values. Furthermore, we carry out a modal analysis of the four-machine system with microgrids using Koopman mode analysis. Koopman mode analysis is a new technique developed in fluid dynamics and recently introduced in power systems by Suzuki and Mezic. It allows us to carry out small signal and transient stability analysis by processing only measurements, without resorting to any model and without assuming any linearization.

[pt] Com o aumento do número de dispositivos de controle representados nos casos práticos, pode ser verificado o aparecimento de interações entre suas ações de controle. Quando estas interações não são coordenadas podem ocasionar a diminuição da eficiência do método de Newton-Raphson no problema de fluxo de potência, resultando em convergências lentas e frequentemente soluções oscilatórias ou até mesmo a divergência do método. Uma adequada identificação destas interações pode contribuir para tomar as medidas corretivas necessárias e assim evitar este tipo de problema. Com esse objetivo, identificam-se as interações entre múltiplos dispositivos de controle (mais de dois equipamentos de controle) a partir da análise dos autovalores e fatores de participação da matriz de sensibilidade de controles denominada [MSC]. Esta matriz, elaborada com base num modelo alternativo para a representação do controle de tensão local das barras PV, é obtida da redução da matriz Jacobiana expandida do problema de fluxo de potência. Dentro deste contexto, se verifica a presença de autovalores que apresentam informações similares sobre os dispositivos de controle com fortes interações entre suas ações de controle, desenvolvendo-se assim, um método baseado no conceito de colinearidade capaz de identificar e agrupar estes autovalores. Os resultados da avaliação do método desenvolvido aplicado em sistemas de pequeno e grande porte mostram a relevância e a viabilidade da utilização prática dos desenvolvimentos propostos neste trabalho. / [en] The increasing number of control devices represented in practical cases, we can see the appearance of interactions between their control actions. When these interactions are not coordinated (conflict), the efficiency of Newton- Raphson method decrease to the power flow problem, the convergence is slow and the solutions are oscillatory. A correct identification of these interactions can help to take corrective actions and thus avoid this problem. With this objective, the identification of interactions between control devices (more than 2 control equipment) is established from the modal analysis of the sensitivity matrix [MSC]. This sensitivity matrix [MSC] is developed in based to alternative model to represent the local voltage control of the PV buses. This [MSC] is obtained from the reduction of the Jacobean matrix expanded of power flow problem. Within this context, it also checks for the presence of eigenvalues that have similar information about the significant interactions between control devices, thus developing a method based on the use index of sensitivity matrix [MSC] and concept of collinearity able to identify and group these eigenvalues. The results of the evaluation method applied to systems designed for small and large show the relevance and feasibility of practical use of proposed developments in this work.

[pt] O crescente aumento da complexidade dos sistemas elétricos de potência gera a necessidade de desenvolvimento de ferramentas que melhorem as condições de análise. O objetivo deste trabalho é aprimorar a ferramenta computacional de avaliação das condições de segurança (ou estabilidade) de tensão. No que diz respeito às barras associadas ao controle remoto de tensão por geração de potência reativa, são propostos novos modelos que representam mais adequadamente as condições operativas no momento do cálculo dos índices de segurança de tensão. Em relação à barra associada ao controle local de tensão por geração de potência reativa é proposta nova modelagem, aplicável tanto no problema de fluxo de potência, utilizando o método de Newton- Raphson, quanto no cálculo dos índices de segurança de tensão. Este modelo,mais robusto e flexível, inclui o controle de tensão local da barra no problema geral de fluxo de potência, formando um sistema de equações de ordem (2*número de barras+número de barras controladas localmente). Para o tratamento de múltiplas barras swing, é proposto um novo modelo, de novo para representar mais adequadamente as condições operativas. É aplicável tanto no problema básico de fluxo de potência, como no cálculo dos índices de segurança de tensão. O modelo proposto considera que apenas o ângulo de uma barra swing é especificado, com os ângulos das demais barras swing livres para variar. Testes numéricos com sistemas-teste (5 e 6 barras) comprovam a aplicabilidade e adequação dos modelos propostos comparando-os aos modelos usados atualmente. / [en] The crescent increase of the complexity of the electric power systems generates the need of development of tools to improve the analysis conditions. The objective of this work is to improve the computational tool of voltage security (stability) conditions assessment. As regards to the buses associated to remote voltage control by reactive power generation, new models that represent more appropriately the operatives conditions at the moment of the calculations of the voltage security indexes, are proposed. As regards to the bus associated to local voltage control by reactive power generation, it is proposed a new modeling, applicable as much in the power flow problem, using the Newton-Raphson method, as in the calculation of the voltage security indexes. This model, more robust and flexible, includes the local voltage control of the bus in the general power flow problem, constituting an equations system of order (2*number of system buses + number of buses with local voltage control). As regard to the multiples swing buses, it is proposed a new model, again to represents more appropriately the operatives conditions. It is applicable as much in the basic power flow problem, as in the calculation of the voltage security indexes. The proposed model considers that just one swing bus has your voltage angle specified and the others swing buses of the power system have your voltage angles free to vary.

Voltage is one of the most important parameters for electrical power networks. The Distribution Network Operators (DNOs) have the responsibility to maintain the voltage supplied to consumers within statutory limits. On-Load Tap Changer (OLTC) transformer equipped with Automatic Voltage Control (AVC) relay is the most widely used and effective voltage control device. Due to a variety of advantages of adding Distributed Generation (DG), more and more distributed resources are connected to local distribution networks to solve constraints of networks, reduce the losses from power supply station to consumers. When DG is connected, the direction of power flow can be reversed when the DG output power exceeds the local load. This means that the bidirectional power flow can either be from power grid towards loads, or vice versa. The connection point of DG may suffer overvoltage when the DG is producing a large amount of apparent power. The intermittent nature of renewable energy resources which are most frequently used in DG technology results in uncertainty of distribution network operation. Overall, conventional OLTC voltage control methods need to be changed when DG is connected to distribution networks. The required voltage control needs to address challenges outlined above and new control method need to be formulated to reduce the limitations of DG output restricted by current operational policies by DNOs. The thesis presents an analysis of voltage control using OLTC transformer with DG in distribution networks. The thesis reviews conventional OLTC voltage control schemes and existing policies of DNOs in the UK. An overview of DG technologies is also presented with their operation characteristics based on power output. The impact of DG on OLTC voltage control schemes in distribution networks is simulated and discussed. The effects of different X/R ratio of overhead line and underground cable are also considered. These impacts need to be critically assessed before any new method implementation. The thesis also introduces the new concepts of Smart Grid and Smart Meter in terms of the transition from passive to active distribution networks. The role of Smart Meter and an overview of communication technologies that could be used for voltage control are investigated. The thesis analyses the high latency of an example solution of which cost and availability are considered to demonstrate the real-time voltage control using Smart Metering with existing communication infrastructures cannot be achieved cost-effectively. The thesis provides an advanced compensation-based OLTC voltage control algorithm using Automatic Compensation Voltage Control (ACVC) technique to improve the voltage control performance with DG penetration without communication. The proposed algorithm is simulated under varying load and DG conditions based on Simulink MATLAB to show the robustness of the proposed method. A generic 11kV network in the UK is modelled to evaluate the correct control performance of the advanced voltage control algorithm while increasing the DG capacity.

In this dissertation, several volt-var optimization methods have been proposed to improve the expected performance of the distribution system using distributed renewable energy sources and conventional volt-var control equipment: photovoltaic inverter reactive power control for chance-constrained distribution system performance optimisation, integrated distribution system optimization using a chance-constrained formulation, integrated control of distribution system equipment and distributed generation inverters, and coordination of PV inverters and voltage regulators considering generation correlation and voltage quality constraints for loss minimization. Distributed generation sources (DGs) have important benefits, including the use of renewable resources, increased customer participation, and decreased losses. However, as the penetration level of DGs increases, the technical challenges of integrating these resources into the power system increase as well. One such challenge is the rapid variation of voltages along distribution feeders in response to DG output fluctuations, and the traditional volt-var control equipment and inverter-based DG can be used to address this challenge. These methods aim to achieve an optimal expected performance with respect to the figure of merit of interest to the distribution system operator while maintaining appropriate system voltage magnitudes and considering the uncertainty of DG power injections. The first method is used to optimize only the reactive power output of DGs to improve system performance (e.g., operating profit) and compensate for variations in active power injection while maintaining appropriate system voltage magnitudes and considering the uncertainty of DG power injections over the interval of interest. The second method proposes an integrated volt-var control based on a control action ahead of time to find the optimal voltage regulation tap settings and inverter reactive control parameters to improve the expected system performance (e.g., operating profit) while keeping the voltages across the system within specified ranges and considering the uncertainty of DG power injections over the interval of interest. In the third method, an integrated control strategy is formulated for the coordinated control of both distribution system equipment and inverter-based DG. This control strategy combines the use of inverter reactive power capability with the operation of voltage regulators to improve the expected value of the desired figure of merit (e.g., system losses) while maintaining appropriate system voltage magnitudes. The fourth method proposes a coordinated control strategy of voltage and reactive power control equipment to improve the expected system performance (e.g., system losses and voltage profiles) while considering the spatial correlation among the DGs and keeping voltage magnitudes within permissible limits, by formulating chance constraints on the voltage magnitude and considering the uncertainty of PV power injections over the interval of interest. The proposed methods require infrequent communication with the distribution system operator and base their decisions on short-term forecasts (i.e., the first and second methods) and long-term forecasts (i.e., the third and fourth methods). The proposed methods achieve the best set of control actions for all voltage and reactive power control equipment to improve the expected value of the figure of merit proposed in this dissertation without violating any of the operating constraints. The proposed methods are validated using the IEEE 123-node radial distribution test feeder.

[pt] Após a incidência de inúmeros problemas relacionados a fenômenos de instabilidade de tensão, o controle de potência reativa em sistemas elétricos de potência tornou-se um assunto importante os últimos anos. Um adequado controle do perfil de tensão em uma área pode contribuir para evitar este tipo de problema. Com esse objetivo, determinam-se áreas de controle de tensão a partir da análise dos autovalores e autovetores das matrizes de sensibilidade: VCS Voltage Control Sensitivity Matrix e QV. A matriz de sensibilidade [VCS] é constituída por elementos diagonais que relacionam a grandeza controladora de cada equipamento com a respectiva tensão controlada (variável controlada), e a análise do sinal desses elementos estabelece se uma determinada ação de controle será adequada ou não, isto é, se terá efeito esperado ou oposto. Os elementos fora da diagonal representam a interdependência existente entre os equipamentos controladores de tensão. A matriz de sensibilidade QV, nomeada como [JSQV] é obtida a partir da matriz Jacobiana do sistema linearizado das equações de fluxo de carga. As áreas de controle de tensão determinadas da análise por autovalores e autovetores usando-se cada uma das matrizes de sensibilidade são coerentes. Adicionalmente, obtêm-se áreas de controle de tensão diretamente das matrizes de sensibilidade. Estas áreas foram comparadas encontrando-se resultados coerentes. / [en] After of the incidence of innumerable problems related to voltage instability phenomena, the control of reactive power in electrical power systems became an important issue in the last years. The adequate control of the voltage for a specific area can prevent this kind of problem. With this objective, voltage control areas are established from an eigenvalues and eigenvectors analysis of the sensitivity matrixes: VCS Voltage Control Sensitivity Matrix and QV. The sensitive matrix [VCS] is form by diagonal elements that relate to the controlling variables and to the controlled voltage (controlled variable), and the analysis of the sign of each diagonal element indicate if a specific control action is adequate or not. The off-diagonal elements represent the interdependence among the voltage controller equipments. The sensitivity matrix QV, called [JSQV] is obtained from the Jacobean matrix from the linear load flow equations. The voltage control areas recognized from the eigenvalues and eigenvectors analysis to each sensitivity matrix are coherent. Also, voltage control areas were identified directly from sensitivity matrixes. These areas were compared founded coherent results.

Distributed energy resources (DE) with power electronics interfaces and logic control using local measurements are capable of providing reactive power related to ancillary system services. In particular, local voltage regulation has drawn much attention in regards to power system reliability and voltage stability, especially from past major cascading outages. This dissertation addresses the challenges of controlling the DEs to regulate the local voltage in distribution systems. First, an adaptive voltage control method has been proposed to dynamically modify the control parameters of a single DE to respond to system changes such that the ideal response can be achieved. Theoretical analysis shows that a corresponding formulation of the dynamic control parameters exists; hence, the adaptive control method is theoretically solid. Also, the field experiment test results at the Distributed Energy Communications and Controls (DECC) Laboratory in single DE regulation case confirm the effectiveness of this method. Then, control methods have been discussed in the case of multiple DEs regulating voltages considering the availability of communications among all the DEs. When communications are readily available, a method is proposed to directly calculate the needed adaptive change of the DE control parameters in order to achieve the ideal response. When there is no communication available, an approach to adaptively and incrementally adjust the control parameters based on the local voltage changes is proposed. Since the impact from other DEs is implicitly considered in this approach, multiple DEs can collectively regulate voltages closely following the ideal response curve. Simulation results show that each method, with or without communications, can satisfy the fast response requirement for operational use without causing oscillation, inefficiency or system equipment interference, although the case with communication can perform even faster and more accurate. Since the proposed adaptive voltage regulation method in the case of multiple DEs without communication, has a high tolerance to real-time data shortage and can still provide good enough performance, it is more suitable for broad utility applications. The approach of multiple DEs with communication can be considered as a high-end solution, which gives faster and more precise results at a higher cost

In the future, renewable energy share, especially wind power share, in electricity generation is expected to increase. Due to nature of the wind, wind power generation pattern includes uncertainties which affects the energy prices in the electricity markets. New simulations are needed for efficient planning process for the resources in the power systems to address the uncertainties in demand, generation, legal, economical and technical limitations. In this study, the aspects of planning process for wind power generation is described and some example scenarios are implemented with the help of MATLAB software.