Modulated Model Predictive Control with Branch and Band Scheme for Unbalanced Load Compensation by MMCC-STATCOM

This paper presents a novel modulated model- predictive control (MMPC) scheme for Modular Multilevel Cascaded Converter-based STATCOMs (MMCC-STATCOM) to compensate unbalanced load current and regulate reactive power flow. By adding a common mode voltage (CMV) to the phase-voltages of the star-connected MMCC current model, the method allows natural injection of a non-sinusoidal voltage to the neutral point of the converter, hence achieving inter-phase cluster voltage balance. Moreover the imposed CMV is shown to extend the operating ranges of MMCC STATCOMs when used for negative sequence current compensation. The proposed MMPC method incorporates a modified branch and bound (B&B) algorithm to optimize the per-phase switch duty ratios. It is shown to be computationally more efficient compared to model-predictive control schemes using optimal voltage level method combined with voltage sorting schemes. Experimental results with different weighting factors confirm the effectiveness of this control scheme, and compared favorably with the conventional scheme of injecting only a sinusoidal zero sequence voltage

Modulated Model Predictive Control with Common Mode Voltage Injection for MMCC-STATCOM Based Unbalanced Load Compensation

This paper presents a novel modulated model- predictive control (MMPC) scheme for Modular Multilevel Cascaded Converter-based STATCOMs (MMCC-STATCOM) to compensate unbalanced load current and regulate reactive power flow. By adding a common mode voltage (CMV) to the phase-voltages of the star-connected MMCC current model, the method allows natural injection of a non-sinusoidal voltage to the neutral point of the converter, hence achieving inter-phase cluster voltage balance. Moreover the imposed CMV is shown to extend the operating ranges of MMCC STATCOMs when used for negative sequence current compensation. The proposed MMPC method incorporates a modified branch and bound (B&B) algorithm to optimize the per-phase switch duty ratios. It is shown to be computationally more efficient compared to model-predictive control schemes using optimal voltage level method combined with voltage sorting schemes. Experimental results with different weighting factors confirm the effectiveness of this control scheme, and compared favorably with the conventional scheme of injecting only a sinusoidal zero sequence voltage

Model Predictive Control for Power Converters and Drives: Advances and Trends

Model predictive control (MPC) is a very attractive solution for controlling power electronic converters. The aim of this paper is to present and discuss the latest developments in MPC for power converters and drives, describing the current state of this control strategy and analyzing the new trends and challenges it presents when applied to power electronic systems. The paper revisits the operating principle of MPC and identifies three key elements in the MPC strategies, namely the prediction model, the cost function, and the optimization algorithm. This paper summarizes the most recent research concerning these elements, providing details about the different solutions proposed by the academic and industrial communitiesMinisterio de Economia y Competitividad TEC2016-78430-RConsejeria de Innovacion, Ciencia y Empresa (Junta de Andalucia) P11-TIC-707

Operation of STATCOM connected to a weak grid

The purpose of this thesis was to investigate the operational possibility of MMC STATCOM in weak grid conditions. Weak grid is a power system which has low short circuit capacity, high equivalent grid impedance, high dV/dQ sensitivity and higher volatility to voltage instability. Hence integration of power electronics based equipment due to their fast response is a big challenge in weak grid. Interaction with high grid impedance of weak grid leads to loss of synchronization and consequently unstable operation of VSC connected to weak grid. In this thesis first, the effects of high grid impedance on the PLL synchronization was investigated. It was observed that high grid impedance has high impact on PLL dynamics and introduces self-synchronization. However, it was noticed that the PLL remains stable if there is no power exchange between grid and VSC. Therefore, STATCOM performance is not deteriorated by the weak grid conditions. STATCOM model connected to weak grid was simulated in Matlab/Simulink environment to study the impact of the weak grid on STATCOM control system. Initially STATCOM was simulated as a constant current source to investigate the factors that impacts STATCOM stability. It was found that STATCOM operation in weak and very weak grid conditions is limited due to some factors that affects STATCOM stability. In capacitive mode the amount of the DC link voltage is main limiting factor and insufficient amount of DC link voltage results in the harmonic injection by STATCOM. In inductive mode high reactive power absorption results in high frequency oscillations in grid voltage which leads to loss of synchronization. Making PLL slower improves the synchronization with the grid; however, this modification deteriorates the DC link voltage performance which requires DC link voltage controller retuning. Second, STATCOM was simulated in the voltage regulation mode and it was noticed that STATCOM operation introduces high frequency ripple to grid voltage. The ripple frequency changes with the grid strengths and at very low short circuit levels system becomes unstable. To improve the system stability a notch filter tuned to the ripple frequency was added. Notch filter significantly improved STATCOM performance and extended the operational limits of STATCOM. However, it was noticed that at some short circuit levels resonance happens and in inductive mode high inductive current absorption makes system unstable. Further elaborations showed that interaction with the HF filter cause the system instability and reduction of the HF filter rating improves the system stability. Finally, STATCOM performance was tested under symmetrical and asymmetrical fault conditions. In case of asymmetrical fault due to unbalanced grid voltages part of the current is used to balance DC link voltage waveforms and the output current of the STATCOM was reduced

Operation of STATCOM connected to a weak grid

The purpose of this thesis was to investigate the operational possibility of MMC STATCOM in weak grid conditions. Weak grid is a power system which has low short circuit capacity, high equivalent grid impedance, high dV/dQ sensitivity and higher volatility to voltage instability. Hence integration of power electronics based equipment due to their fast response is a big challenge in weak grid. Interaction with high grid impedance of weak grid leads to loss of synchronization and consequently unstable operation of VSC connected to weak grid. In this thesis first, the effects of high grid impedance on the PLL synchronization was investigated. It was observed that high grid impedance has high impact on PLL dynamics and introduces self-synchronization. However, it was noticed that the PLL remains stable if there is no power exchange between grid and VSC. Therefore, STATCOM performance is not deteriorated by the weak grid conditions. STATCOM model connected to weak grid was simulated in Matlab/Simulink environment to study the impact of the weak grid on STATCOM control system. Initially STATCOM was simulated as a constant current source to investigate the factors that impacts STATCOM stability. It was found that STATCOM operation in weak and very weak grid conditions is limited due to some factors that affects STATCOM stability. In capacitive mode the amount of the DC link voltage is main limiting factor and insufficient amount of DC link voltage results in the harmonic injection by STATCOM. In inductive mode high reactive power absorption results in high frequency oscillations in grid voltage which leads to loss of synchronization. Making PLL slower improves the synchronization with the grid; however, this modification deteriorates the DC link voltage performance which requires DC link voltage controller retuning. Second, STATCOM was simulated in the voltage regulation mode and it was noticed that STATCOM operation introduces high frequency ripple to grid voltage. The ripple frequency changes with the grid strengths and at very low short circuit levels system becomes unstable. To improve the system stability a notch filter tuned to the ripple frequency was added. Notch filter significantly improved STATCOM performance and extended the operational limits of STATCOM. However, it was noticed that at some short circuit levels resonance happens and in inductive mode high inductive current absorption makes system unstable. Further elaborations showed that interaction with the HF filter cause the system instability and reduction of the HF filter rating improves the system stability. Finally, STATCOM performance was tested under symmetrical and asymmetrical fault conditions. In case of asymmetrical fault due to unbalanced grid voltages part of the current is used to balance DC link voltage waveforms and the output current of the STATCOM was reduced

Contributions to Modulation and Control Algorithms for Multilevel Converters

Las actuales tendencias de la red eléctrica han lanzado a la industria a la búsqueda de sistemas de generación, distribución y consumo de energía eléctrica más eficientes. Generación distribuida, reducción de componentes pasivos, líneas DC de alta tensión son, entre otras, las posibles líneas de investigación que están actualmente siendo consideradas como el futuro de la red eléctrica. Sin embargo, nada de esto sería posible si no fuera por los avances alcanzados en el campo de la electrónica de potencia. El trabajo aquí presentado comienza con una breve introducción a la electrónica de potencia, concretamente a los convertidores de potencia conectados a red, sus estrategias de control más comunes y enfoques ante redes desbalanceadas. A continuación, las contribuciones del autor sobre el control y modulación de una topología particular de convertidores, conocidos como convertidores multinivel, se presentan como el principal contenido de este trabajo. Este tipo de convertidores mejoran la eficiencia y ciertas prestaciones, en comparación con convertidores más tradicionales, a costa de una mayor complejidad en el control al incrementar la cantidad de los componentes hardware. A pesar de que existen numerosas topologías de convertidores multinivel y algunas de ellas son brevemente expuestas en este trabajo, la mayoría de las aportaciones están enfocadas para convertidores del tipo diode-clamped converter. Adicionalmente, se incluye una aportación para convertidores del tipo multinivel modular, y otra para convertidores en cascada. Se espera que el contenido de la introducción de este trabajo, junto a las contribuciones particulares para convertidores multinivel sirva de inspiración para futuros investigadores del campo

Advances and Technologies in High Voltage Power Systems Operation, Control, Protection and Security

The electrical demands in several countries around the world are increasing due to the huge energy requirements of prosperous economies and the human activities of modern life. In order to economically transfer electrical powers from the generation side to the demand side, these powers need to be transferred at high-voltage levels through suitable transmission systems and power substations. To this end, high-voltage transmission systems and power substations are in demand. Actually, they are at the heart of interconnected power systems, in which any faults might lead to unsuitable consequences, abnormal operation situations, security issues, and even power cuts and blackouts. In order to cope with the ever-increasing operation and control complexity and security in interconnected high-voltage power systems, new architectures, concepts, algorithms, and procedures are essential. This book aims to encourage researchers to address the technical issues and research gaps in high-voltage transmission systems and power substations in modern energy systems

Contrôle avancé des convertisseurs de puissance multi-niveaux pour applications sur réseaux faibles

139 p.El advenimiento progresivo de las microrredes que incorporan fuentes de energía renovable está dando lugar a un nuevo paradigma de distribución de la electricidad. Este nuevo planteamiento sirve de interfaz entre consumidores no controlados y fuentes intermitentes, implicando desafíos adicionales en materia de conversión, almacenamiento y gestión de la energía.Los convertidores de potencia se adaptan en consecuencia, en particular con el desarrollo de los convertidores multinivel, que integrando los mismos componentes que sus predecesores y un control más complejo, soportan potencias más altas y aseguran una mejor calidad de la energía.Debido al carácter híbrido de los convertidores de potencia, su control se divide comúnmente en dos partes: por un lado, el control de los objetivos continuos vinculados a la función principal de los convertidores de servir de interfaz, y, por otro, el control discreto de los interruptores de potencia, conocido con el nombre de modulación.En este contexto, las exigencias crecientes en términos de eficiencia, fiabilidad, versatilidad y rendimiento hacen necesaria una mejora de la inteligencia de la estructura de control. Para cumplir conestos requisitos, se propone tratar mediante un solo controlador ambas problemáticas, la vinculada a la función de interfaz de los convertidores y la relacionada con su naturaleza discreta. Esta decisión implica incorporar la no-linealidad de los convertidores de potencia en el controlador, lo que equivale a suprimir el bloque de modulación, que constituye la solución tradicional para linealizar el comportamiento interno de los convertidores. Se adopta un planteamiento de Control Predictivo basado en Modelos (MPC) para abordar la no-linealidad y la gran diversidad de objetivos de control que acompañan a los convertidores de potencia.El algoritmo desarrollado combina teoría de grafos ¿con algoritmos de Dijkstra, A* y otros¿ con un modelo de estado especial para sistemas conmutados al objeto de proporcionar una herramienta potente y universal, capaz de manipular simultáneamente el carácter cuantificado de los interruptores de potencia y el continuo de las entidades interconectadas por el convertidor. Se han obtenido resultados sobre la estabilidad y la controlabilidad de los modelos de estado conmutados aplicados al caso particular de los convertidores de potencia.El controlador así desarrollado y descrito se ha examinado en simulación frente a varios casos y aplicaciones: inversor aislado o conectado a la red, rectificador y convertidor bidireccional. Se ha empleado la misma estructura de control para tres topologías de convertidor multinivel: Neutral-Point Clamped, Flying Capacitor y Cascaded H-Bridge. Al objeto de adaptarse a los cambios citados, lo único que varía en el controlador es el modelo del convertidor adoptado para la predicción, así como la función de coste, que traduce los requisitos de control en un problema de optimización a solucionar por el algoritmo. Un cambio de topología resulta en una modificación del modelo interno, sin impacto sobre la función de coste, mientras que variaciones de esta función son suficientes para adaptarse a la aplicación.Los resultados muestran que el controlador logra actuar directamente sobre los interruptores de potencia en función de diversos requisitos. Los desempeños de la estructura de control propuesta son similares a los de las numerosas estructuras dedicadas a cada uno de los casos estudiados, excepto en el caso de operación en modo rectificador, en el que la versatilidad y rapidez de control obtenidos son particularmente interesantes.En definitiva, el controlador planteado puede emplearse para diferentes aplicaciones, topologías, objetivos y limitaciones. Si bien las estructuras de control lineal tradicionales han de modificarse, a menudo en profundidad, para afrontar diferentes modos de operación o requisitos de control, dichas alteraciones no tienen ningún impacto sobre la arquitectura del controlador MPC obtenido, lo que pone de manifiesto su versatilidad, así como su universalidad, también demostrada por su capacidad para adaptarse a diferentes convertidores de potencia sin modificaciones importantes. Finalmente, la solución propuesta elude por completo la complejidad de la modulación, ofreciendo simplicidad y flexibilidad al diseño del control