Control of power· electronics interfaces for safeguarding stability of future power networks

Abstract

En el pasado, los sistemas eléctricos de potencia se han basado en la operación centralizada de un gran número de centrales eléctricas. Sin embargo, en los últimos años los sistemas de potencia se están avanzando hacia uno nuevo paradigma basado en la operación de un gran número de generadores distribuidos basados en fuentes renovables. Los convertidores electrónicos usados típicamente para conectar fuentes renovables a las redes eléctricas no proporcionan la estabilidad y apoyo que sí proveen los generadores convencionales. En muchos casos, las fuentes renovables se encuentran redes mal interconectadas, en localizaciones remotas o en redes pequeñas, que se suelen caracterizar por tener baja inercia y valores de impedancia elevados. Estas condiciones tienen un impacto significativo en la estabilidad de los convertidores electrónicos, por lo que se necesitan soluciones de control robustas que garanticen una operación estable y segura. A su vez, la integración de un gran número de convertidores electrónicos en las redes puede dar lugar a interacciones que comprometan la estabilidad del sistema eléctrico. La topología de convertidor Back-to-Back es una de las más populares para el control de máquinas eléctricas, en sistemas de transmisión de potencia o en aplicaciones de calidad de potencia. Esta tesis presenta un control por realimentación del estado completo del convertidor Back-to-Back basado en los novedosos conceptos de potencia común y diferencial. Este controlador permite simultáneamente un control rápido de potencia activa y reactiva, y una regulación rápida del bus de DC sin limitaciones de ancho de banda. Las oscilaciones electromecánicas de baja frecuencia y los métodos para su atenuación han sido ampliamente estudiadas en los sistemas de potencia tradicionales. La tecnología HVDC presenta numerosas ventajas frente a la tecnología HVAC, como la mejora de eficiencia y flexibilidad, o la interconexión de sistemas asíncronos. Esta tesis presenta una estructura de control para enlaces HVDC entre sistemas asíncronos que permite amortiguar oscilaciones de frecuencia. Este controlador un efecto electromecánico equivalente a una fricción mecánica acoplando las inercias de los dos sistemas asíncronos. En muchos casos, las fuentes renovables se encuentran en redes mal interconectadas, en localizaciones remotas que requieren de largas líneas de distribución. Estas interconexiones "débiles" tienen un impacto significativo en los sistemas de control de los convertidores electrónicos. Esta tesis presenta un novedoso modelo de pequeña señal para convertidores electrónicos conectados a redes débiles que incluye efectos como tiempos muertos y retardos, típicamente causados por la implementación digital de los controles y la conmutación. Los controladores basados en la emulación de generadores síncronos presentan ventajas a la hora de integrar fuentes renovables en redes débiles o mal interconectadas. Sin embargo, estas estrategias de control presentan algunos inconvenientes como son las dinámicas no-lineales, el diseño de los parámetros de control o su estabilidad. Esta tesis presenta una metodología de diseño de una máquina síncrona virtual con impedancia virtual conectada a una red débil. La intermitencia, variabilidad y falta de control propias de las fuentes renovables y la incertidumbre de la demanda traen consigo nuevos retos relacionados con el diseño y la operación de los sistemas eléctricos. Las redes multiterminales de DC resultan una solución prometedora dado que permiten integrar fuentes renovables, sistemas de almacenamiento y cargas electrónicas en un bus común de DC. En esta tesis se presenta el modelado de pequeña señal y análisis de estabilidad de redes multiterminales de DC con almacenamiento de energía para aplicaciones ferroviarias.

In the past, the concept of electric power systems have been based on the centralised operation of large power generation plants. However, in recent years, power systems have shifted away from such traditional paradigm towards a new one based on a large number of small and distributed generation units based on renewable energy sources (RES). The power electronics interfaces commonly used to connected RES to power networks do not inherently provide the stability and grid support the conventional generators do. In many cases, the RES are placed in poorly interconnected networks, in remote locations or in small networks that are characterised by low mechanical inertia and high network impedances. Such network conditions have a signi cant impact on the stability of power electronics converters and robust control solutions are required in order to guarantee their stable operation. Moreover, the integration of a large number of electronic interfaces allow a wide variety of possible interactions that can lead to the instability of the power network itself. The objective of this thesis is to evaluate the impact of a massive deployment of power electronics interfaces on the stability of power systems and improve the control systems of grid-connected converters to safeguard the stability of future power networks. Back-to-Back converter topology is frequently used for control of electrical machines, power transmission systems and power quality applications. The cascade control structure is commonly used for the this converter topology due to its simple design procedure even though its performance is limited because of the reduced bandwidth of the DC-link voltage regulator. This thesis introduces a full-state feedback controller based on the novel common- and di erential-power concepts. This controller achieves both - a fast control of active and reactive powers and a fast regulation of the DC-link voltage. Low-frequency electro-mechanical frequency oscillations and methods for their attenuation have been widely studied in conventional interconnected power systems. The HVDC technology introduces several advantages over the conventional HVAC technology, such as increased e ciency and exibility, or interconnection of asynchronous systems. This thesis proposes a control scheme for HVDC interconnections xiii xiv that provides damping of frequency oscillations in asynchronous ac grids. This controller introduces a virtual electro-mechanical e ect equivalent to mechanical friction between the inertial dynamics of the two coupled ac grids. In many cases, RES are located in small and poorly interconnected distribution networks, in locations requiring long distribution cables or in small networks designed to operate disconnected from the main grid (microgrids). Such \weak" interconnections have a signi cant impact on the control system of power electronics interfaces and may lead to undesired e ects. This thesis proposes a novel and detailed small-signal modelling procedure for voltage source converters connected to weak grids that includes e ects such as dead-time and time delays caused by the digital implementation and switching process. In order to facilitate the integration of RES to weakly interconnected grids, the controllers based on the emulation of synchronous generators have been increasingly used. This is an attractive approach since existing power systems are designed to operate based on the connection of synchronous generators and this approach successfully emulates most important features of conventional generators. However, the existing control methods for the emulation of synchronous generators still have a number of pending issues, such as the system non-linear dynamics, control parameter design or stability under weak grid conditions. A comprehensive methodology for the design of virtual synchronous machines with virtual impedance connected to weak grids is proposed in this thesis. The uncontrolled, variable and intermittent characteristics of RES generators and the uncertainty of the demand give rise to new challenges related to the design and operation of power systems. Solutions based on multi-terminal DC networks have been considered to overcome these issues since they are capable of integrating RES generators, energy storage systems and electronic loads to a common DC network that is then interfaced to an AC grid via a single converter. However, the modelling, analysis and control design of multi-terminal DC grids for di erent applications has not been further explored. This thesis presents a small-signal modelling and stability analysis of electronics interfaces in multi-terminal DC networks with energy storage for railway applications. For each of these di erent applications a state-of-the-art is presented and their respective control issues are studied in detail. The proposed control schemes and ndings from the stability analyses in this thesis were implemented and experimentally validated in the Smart Energy Integration Lab (SEIL) at IMDEA Energy. Finally, conclusions and guidelines for further research are presented.