The inevitable choice for the automotive industry to suppress greenhouse gas emissions is zero-emission vehicles such as battery electric vehicles. Some of the main barriers regarding the adoption of electric vehicles are range anxiety, and lack of charging infrastructure, which can be addressed by ultra-fast chargers or charging stations. The conventional ultra-fast chargers are low-voltage (LV) connected through line-frequency transformers, which pose disadvantages such as low efficiency, high cost, and large footprints. The medium-voltage (MV) connected charging station is proposed by the researchers to overcome the challenges regarding the conventional chargers by eliminating the line-frequency transformer and direct connection to the medium voltage. The most challenging part of the medium-voltage ultra-fast chargers is the AC/DC stage connection to the medium voltage. Different medium-voltage multilevel converters have been proposed to facilitate the direct connection to the medium-voltage grid. However, disadvantages such as a high number of components and control complexity weaken the strength of medium-voltage connected stations. The main focus of this thesis is on novel advanced medium-voltage multilevel topologies and control techniques for medium-voltage connected ultra-fast EV charging applications. First, a novel controller based on SPWM is proposed to control the flying capacitor voltages of a four-level T-type Nested Neutral Point Clamped (NNPC) topology. Second, a new five-level T-type NNPC topology is proposed that has a minimum number of components in comparison to other existing five-level topologies. To extend the voltage and power rating, a novel seven-level topology is proposed that has the lowest number of components in comparison to other existing topologies. Moreover, three different controllers are developed to control the voltages of the seven-level topology based on Model Predictive Control, where the challenges regarding significant computational burden and weighting factor elimination are addressed. Finally, an MV-connected ultra-fast charging station architecture is proposed, where the proposed seven-level topology is considered as the AC/DC stage. Comparison of the proposed topology to the LV-connected stations shows that the efficiency, cost, and power quality of the charging stations can be improved significantly. / Thesis / Doctor of Philosophy (PhD)

Este trabalho apresenta as principais topologias de conversores multinível descritos na literatura. É feita uma revisão bibliográfica, apresentando as vantagens e as limitações de cada topologia. A seguir são apresentados os métodos mais usuais de modulação para conversores multinível, bem como os campos de aplicação na indústria e em sistemas de potência As associações de conversores baseadas em conversores do tipo ponte completa monofásica foram escolhidas para um estudo mais detalhado. Para estas topologias são apresentados detalhes sobre as respectivas estratégias de controle e modulação. Inicialmente são apresentados os detalhes da ponte completa monofásica com tensão de saída modulada em PWM com dois e três níveis. A seguir são estudadas as associações série e paralela com barramentos CC isolados. O estudo da associação paralela com barramento CC único é a principal contribuição desta dissertação. Para esta topologia são propostas algumas estratégias de controle. Para todos estes casos são apresentados resultados de simulação e experimentais obtidos com inversores operando com potência reduzida, controlados por DSP. / This work presents the main multilevel converter topologies described in the literature. A bibliography revision is presented, showing the advantages and limitations of each topology. The most usual multilevel modulation methods are also presented, as well as their fields of application in the industry as well as in power systems. The one phase full bridge and its associations were chosen for detailed study. For this family of topologies, this dissertation presents details about control and modulation strategies. Initially, details about the one phase full bridge with two and three levels PWM modulated output voltage are presented. Next, the series association is analyzed, followed by the parallel with isolated DC link. The study of parallel connection with a single DC Link is the mainly contribution of this dissertation. For this topology are proposed some control strategies. For all these cases, simulation and experimental results (with, small scale prototype controlled by DSP) are presented.

Este trabalho apresenta as principais topologias de conversores multinível descritos na literatura. É feita uma revisão bibliográfica, apresentando as vantagens e as limitações de cada topologia. A seguir são apresentados os métodos mais usuais de modulação para conversores multinível, bem como os campos de aplicação na indústria e em sistemas de potência As associações de conversores baseadas em conversores do tipo ponte completa monofásica foram escolhidas para um estudo mais detalhado. Para estas topologias são apresentados detalhes sobre as respectivas estratégias de controle e modulação. Inicialmente são apresentados os detalhes da ponte completa monofásica com tensão de saída modulada em PWM com dois e três níveis. A seguir são estudadas as associações série e paralela com barramentos CC isolados. O estudo da associação paralela com barramento CC único é a principal contribuição desta dissertação. Para esta topologia são propostas algumas estratégias de controle. Para todos estes casos são apresentados resultados de simulação e experimentais obtidos com inversores operando com potência reduzida, controlados por DSP. / This work presents the main multilevel converter topologies described in the literature. A bibliography revision is presented, showing the advantages and limitations of each topology. The most usual multilevel modulation methods are also presented, as well as their fields of application in the industry as well as in power systems. The one phase full bridge and its associations were chosen for detailed study. For this family of topologies, this dissertation presents details about control and modulation strategies. Initially, details about the one phase full bridge with two and three levels PWM modulated output voltage are presented. Next, the series association is analyzed, followed by the parallel with isolated DC link. The study of parallel connection with a single DC Link is the mainly contribution of this dissertation. For this topology are proposed some control strategies. For all these cases, simulation and experimental results (with, small scale prototype controlled by DSP) are presented.

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This dissertation is dedicated to a comprehensive study of static synchronous compensator (STATCOM) systems utilizing cascaded-multilevel converters (CMCs). Among flexible AC transmission system (FACTS) controllers, the STATCOM has shown feasibility in terms of cost-effectiveness in a wide range of problem-solving abilities from transmission to distribution levels. Referring to the literature reviews, the CMC with separated DC capacitors is clearly the most feasible topology for use as a power converter in the STATCOM applications. The controls for the CMC-based STATCOM were, however, very complicated. The intricate control design was begun without well-defined system transfer functions. The control compensators were, therefore, randomly selected. The stability of the system was achieved by trial and error processes, which were time-consuming and ineffective. To be able to operate in a high-voltage application, a large number of DC capacitors are utilized in a CMC-based STATCOM. All DC capacitor voltages must be balanced in order to avoid over-voltages on any particular link. Not only do these uneven DC voltages introduce voltage stress on the semiconductor switches, but they also lower the quality of the synthesized output waveforms of the converter. Previous researches into DC capacitor voltage-balancing techniques were very straightforward, in that individual voltage compensators were added into the main control loop. However, the compensator design for these individual loops is very problematic because of the complexity of the voltage-loop transfer functions. Basically, the trial and error technique again provides the simplest way to achieve acceptable compensators. Moreover, the greater number of voltage levels, the more complex the control design, and the main controller must perform all of the feedback control procedures. As a result, this approach potentially reduces the reliability of the controller. The goal of this dissertation is to achieve high-performance, reliable, flexible, cost-effective power stages and controllers for the CMC-based STATCOM. Major contributions are addressed as follows: 1) optimized design for the CMC-based STATCOM power stages and passive components, 2) accurate models of the CMC for reactive power compensations in both ABC and DQ0 coordinates, 3) an effective decoupling power control technique, 4) DC-link balancing strategies; and 5) improvements in the CMC topology. To enhance the modularity and output voltage of the CMC, the high-switching-frequency, high-power H-bridge building block (HBBB) and the optimized design for its power stage and snubber circuits are first proposed. The high-switching-frequency feature is achieved by utilizing the Virginia Tech-patented emitter turn-off (ETO) thyristor. Three high-power HBBB prototypes were implemented, and their performance was experimentally verified. To simplify the control system design, well-defined models of the CMC in both ABC and DQ0 coordinates are proposed. The proposed models are for the CMC with any number of voltage levels. The key system transfer functions are achieved and used in the control design processes. To achieve independent power control capability, the control technique, called the decoupling power control, is proposed. By applying this control technique, real and reactive power components can be controlled separately. In order to balance the DC capacitor voltages, a new, effective pulse width modulation (PWM) technique, which is suitable for any number of H-bridge converters, is proposed. The proposed cascaded PWM algorithm can be practically realized into the field programmable gate arrays (FPGA), and its complexity is not affected by the number of voltage levels. In addition, the complexity of the main controller, which is essentially based on the digital signal processor (DSP), is no longer a function of the number of the output voltage levels. The basic structure of the cascaded PWM is modular, which, in general, enhances the modularity of the CMC power stages. With the combination of the decoupling power control and the cascaded PWM, a CMC with any number of voltage levels can be simply modeled as a three-level cascaded converter, which is the simplest topology to deal with. This significantly simplifies and optimizes the control design process. To verify the accuracy of the proposed models and the performance of the control system for the CMC-based STATCOM, a low-power, seven-level cascaded-based STATCOM hardware prototype is implemented. The key control procedures are performed by a main controller, which consists of a DSP and an FPGA. The simulation and experimental results indicate the superior performance of the proposed control system, as well as the precision of the proposed models. / Ph. D.

This thesis is dedicated to a comprehensive study of static series synchronous compensator (SSSC) systems utilizing cascaded-multilevel converters (CMCs). Among flexible AC transmission system (FACTS) controllers, the SSSC has shown feasibility in terms of cost-effectiveness in a wide range of problem-solving abilities from transmission to distribution levels. Referring to the literature reviews, the CMC with separated DC capacitors is clearly the most feasible topology for use as a power converter in the SSSC applications. The control for the CMC-Based SSSC is complicated. The design of the complicated control strategy was begun with well-defined system transfer functions. The stability of the system was achieved by trial and error processes, which were time-consuming and ineffective. The goal of this thesis is to achieve a reliable controller design for the CMC-based SSSC. Major contributions are addressed as follows: 1) accurate models of the CMC for reactive power compensations in both ABC and DQ0 coordinates, and 2) an effective decoupling power control technique. To simplify the control system design, well-defined models of the CMC-Based SSSC in both ABC and DQ0 coordinates are proposed. The proposed models are for the CMC-Based SSSC focus on only three voltage levels but can be expanded for any number of voltage levels. The key system transfer functions are derived and used in the controller design process. To achieve independent power control capability, the control technique, called the decoupling power control used in the design for the CMC-Based STATCOM is applied. This control technique allows both the real and reactive power components to be independently controlled. With the combination of the decoupling power control and the cascaded PWM, a CMC with any number of voltage levels can be simply modeled as a three-level cascaded converter, which is the simplest topology to deal with. This thesis focuses on the detailed design process needed for a CMC-Based SSSC. / Master of Science

With the deployment of sensors in hardware equipment and advanced metering infrastructure, system operators have access to unprecedented amounts of data. Simultaneously, grid-connected power electronics technology has had a large impact on the way electrical energy is generated, transmitted, and delivered to consumers. Artificial intelligence and machine learning can help address the new power grid challenges with enhanced computational abilities and access to large amounts of data. This thesis discusses the fundamentals of neural networks and their applications in power systems such as load forecasting, power system stability analysis, and fault diagnosis. It extends application of neural networks to inverter-based resources by studying the implementation and performance of a neural network controller emulator for voltage-sourced converters. It delves into how neural networks could enhance cybersecurity of a component through multiple hardware and software implementations of the same component. This ensures that vulnerabilities inherent in one form of implementation do not affect the system as a whole. The thesis also proposes a comprehensive support vector classifier (SVC)--based submodule open-circuit fault detection and localization method for modular multilevel converters. This method eliminates the need for extra hardware. Its efficacy is discussed through simulation studies in PSCAD/EMTDC software. To ensure efficient usage of neural networks in power system simulation softwares, this thesis entails the step by step implementation of a neural network custom component in PSCAD/EMTDC. The custom component simplifies the process of recreating a neural network in PSCAD/EMTDC by eliminating the manual assembly of predefined library components such as summers, multipliers, comparators, and other miscellaneous blocks. / Master of Science / Data analytics and machine learning play an important role in the power grids of today, which are continuously evolving with the integration of renewable energy resources. It is expected that by 2030 most of the electric power generated will be processed by some form of power electronics, e.g., inverters, from the point of its generation. Machine learning has been applied to various fields of power systems such as load forecasting, stability analysis, and fault diagnosis. This work extends machine learning applications to inverter-based resources by using artificial neural networks to perform controller emulation for an inverter, provide cybersecurity through heterogeneity, and perform submodule fault detection in modular multilevel converters. The thesis also discusses the step by step implementation of a neural network custom component in PSCAD/EMTDC software. This custom component simplifies the process of creating a neural network in PSCAD/EMTDC by eliminating the manual assembly of predefined library components.

This thesis presents the real-time simulation of a realistic-size Modular Multilevel Converter (MMC) based High-Voltage Direct Current (HVDC) transmission system. Based on the concept of time-varying Thevenin equivalent, a computationally efficient model of the MMC is developed and deployed on an FPGA platform. The salient features of the developed MMC model are: (i) The decoupling of the solutions of the MMC model and the of the rest of the system, (ii) it provides an equivalent representation of the full MMC, (iii) it is suitable for parallel implementation. The model is used as part of the simulation of the 401-level France/Spain MMC-HVDC link between, in which 2 separate MMCs are included, showing the expandability of the designed system to larger DC grid scenarios. Hardware in the loop (HIL) testing capabilities of the system are also demonstrated with the addition of an external controller to the simulation system.

This thesis presents the real-time simulation of a realistic-size Modular Multilevel Converter (MMC) based High-Voltage Direct Current (HVDC) transmission system. Based on the concept of time-varying Thevenin equivalent, a computationally efficient model of the MMC is developed and deployed on an FPGA platform. The salient features of the developed MMC model are: (i) The decoupling of the solutions of the MMC model and the of the rest of the system, (ii) it provides an equivalent representation of the full MMC, (iii) it is suitable for parallel implementation. The model is used as part of the simulation of the 401-level France/Spain MMC-HVDC link between, in which 2 separate MMCs are included, showing the expandability of the designed system to larger DC grid scenarios. Hardware in the loop (HIL) testing capabilities of the system are also demonstrated with the addition of an external controller to the simulation system.

The recent introduction of a new converter topology, the modular multilevel converter (MMC) is a major step forward in voltage sourced converter (VSC) technology for high voltage, high power applications. To obtain a multilevel ac output waveform, a large number of semiconductor switches has to be used in the converter. The number of switches in the MMC for HVDC transmission is typically two orders of magnitudes larger than that in a two or three level VSC used in earlier generation. This large device count creates a computational challenge for electromagnetic transients (EMT) simulation programs, as it significantly increases the simulation time. The purpose of this research is to investigate whether the simulation can be speeded up. This research develops an efficient, time-varying Thévenin's equivalent model for the MMC converter based on partitioning the system’s admittance matrix. EMT simulation results show that the proposed equivalent model can drastically reduce the computational time without loss of accuracy. The use of the proposed equivalent method is demonstrated by simulating a point to point MMC based HVDC transmission system successfully with more than 100 levels. This approach enables what was hitherto not practical; the modeling of large MMC based HVDC systems on personal computers. With the assumption of ideal switch operation and using an equivalent average capacitor value based approach, an average valued model of MMC is also proposed in this thesis. The average model can be accurately used in most of the system level studies. The control algorithms and other modeling aspects of MMC applications are also presented in this thesis. One of the advantages of multilevel converters is the low operating losses as the smaller switching frequency of each individual power electronics switch and the low voltage step change during each switching. Using a recently developed, time domain simulation approach, the operating losses of the MMC converter are estimated in this thesis. When comparing the MMC operating losses against the losses of two-level VSC, the power loss for the two-level VSC is found to be significantly higher than the power loss of the MMC.