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

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

Virtual Friction for Oscillation Damping and Inertia Sharing from Multi-Terminal VSC-HVDC Grids

This paper proposes a control scheme for multi-terminal HVDC (MTDC) interconnections that introduces an effect equivalent to a mechanical coupling between asynchronous ac networks providing damping of frequency oscillations and frequency support. From the control systems perspective this virtual mechanical coupling is equivalent to a mechanical friction interconnecting the ac networks (modelled as equivalent rotating masses). Also, the control system provides an inertia sharing effect. These two properties can be used to effectively damp frequency oscillations, and provide frequency and inertia support to any ac network connected simultaneously. It is shown that the controller can effectively attenuate poorly damped oscillations that are observed at the MTDC terminals. The dynamic properties of the proposed controller are analysed and its impact of the stability of the three ac networks is evaluated by using a simplified model. The controller was validated by using detailed simulations. IEEE Keywords Mathematical model , Power system stability , HVDC transmission , Voltage control , Oscillators , Control systems , Power system dynamicsacceptedVersio

Virtual Friction Control for Power System Oscillation Damping with VSC-HVDC Links

This paper presents a technique for damping of oscillations in ac grids by control of VSC-HVDC links. The effect of the proposed controller is equivalent to a mechanical friction between two asynchronous networks (modelled as rotating masses) interconnected by the HVDC link. Therefore, the dynamics of the ac grids will be coupled, and the virtual friction gain can be utilised to effectively damp frequency oscillations in any of the ac networks. A centralised and a decentralised implementation of the proposed controller are presented. It is shown that both implementations of the virtual friction-based damping can effectively attenuate poorly damped frequency oscillations in both of the interconnected ac grids and that the decentralised implementation can ensure damping without relying on fast communication between the HVDC terminals. The impact of the proposed controller on the stability of the two grids is analysed with a simplified system model, and the performance is experimentally validated by a scaled laboratory setup.acceptedVersio

Virtual Synchronous Machine Control of VSC HVDC for Power System Oscillation Damping

Two different methods for implementing inertial damping on a Virtual Synchronous Machine (VSM) and their potential for attenuating power system oscillations when utilized in a VSC HVDC terminal are investigated in this paper. As a reference case, the VSM is considered with only a frequency droop providing damping in the virtual swing equation. Then, the effect of damping based on high-pass filtering of the virtual speed is compared to damping based on high-pass filtering of the measured grid frequency. A simplified model of a power system with two equivalent generators and a VSC HVDC terminal is introduced as a case study. Analysis of the small-signal dynamics indicates that damping based on the VSM speed has limited influence on the power system oscillations, while improved attenuation can be obtained by introducing damping based on the locally measured grid frequency. The presented analysis and the operation of the proposed VSM-based damping strategy is validated by numerical simulation of a 150 MVA VSM controlled VSC HVDC terminal connected to a dynamic model of the assumed grid configuration.Virtual Synchronous Machine Control of VSC HVDC for Power System Oscillation DampingacceptedVersio

Coupling of AC Grids via VSC-HVDC Interconnections for Oscillation Damping based on Differential and Common Power Control

This paper presents a control approach for HVDC interconnections that provides damping of frequency oscillations in asynchronous ac grids by introducing a virtual friction for coupling their inertial dynamics. The HVDC interconnection is modelled by using the concept of common and differential power flow, allowing for independent control of the dc voltage and the net power transfer between HVDC terminals, respectively. The proposed controller introduces a damping effect in the differential power flow which is equivalent to a mechanical friction between the generators connected to the ac grids at the two terminals. This virtual friction-based damping can effectively attenuate poorly damped frequency oscillations that can be observed where the HVDC interconnection is interfaced to either of the ac grids without relying on fast communication between the converter terminals. The impact of the proposed control technique on the stability and damping of two interconnected power systems is analysed first by using a simplified model. Then, the sensitivity to the frequency and damping of the oscillation modes appearing in the ac grid frequencies, as well as the effect of the dc line resistance on the oscillation damping are evaluated. Finally, the control system performance is experimentally validated on a scaled laboratory setup

Virtual Friction Control for Power System Oscillation Damping with VSC-HVDC Links

This paper presents a technique for damping of oscillations in ac grids by control of VSC-HVDC links. The effect of the proposed controller is equivalent to a mechanical friction between two asynchronous networks (modelled as rotating masses) interconnected by the HVDC link. Therefore, the dynamics of the ac grids will be coupled, and the virtual friction gain can be utilised to effectively damp frequency oscillations in any of the ac networks. A centralised and a decentralised implementation of the proposed controller are presented. It is shown that both implementations of the virtual friction-based damping can effectively attenuate poorly damped frequency oscillations in both of the interconnected ac grids and that the decentralised implementation can ensure damping without relying on fast communication between the HVDC terminals. The impact of the proposed controller on the stability of the two grids is analysed with a simplified system model, and the performance is experimentally validated by a scaled laboratory setup

Virtual Friction Control for Power System Oscillation Damping with VSC-HVDC Links

This paper presents a technique for damping of oscillations in ac grids by control of VSC-HVDC links. The effect of the proposed controller is equivalent to a mechanical friction between two asynchronous networks (modelled as rotating masses) interconnected by the HVDC link. Therefore, the dynamics of the ac grids will be coupled, and the virtual friction gain can be utilised to effectively damp frequency oscillations in any of the ac networks. A centralised and a decentralised implementation of the proposed controller are presented. It is shown that both implementations of the virtual friction-based damping can effectively attenuate poorly damped frequency oscillations in both of the interconnected ac grids and that the decentralised implementation can ensure damping without relying on fast communication between the HVDC terminals. The impact of the proposed controller on the stability of the two grids is analysed with a simplified system model, and the performance is experimentally validated by a scaled laboratory setup

Virtual Friction for Oscillation Damping and Inertia Sharing from Multi-Terminal VSC-HVDC Grids

This paper proposes a control scheme for multi-terminal HVDC (MTDC) interconnections that introduces an effect equivalent to a mechanical coupling between asynchronous ac networks providing damping of frequency oscillations and frequency support. From the control systems perspective this virtual mechanical coupling is equivalent to a mechanical friction interconnecting the ac networks (modelled as equivalent rotating masses). Also, the control system provides an inertia sharing effect. These two properties can be used to effectively damp frequency oscillations, and provide frequency and inertia support to any ac network connected simultaneously. It is shown that the controller can effectively attenuate poorly damped oscillations that are observed at the MTDC terminals. The dynamic properties of the proposed controller are analysed and its impact of the stability of the three ac networks is evaluated by using a simplified model. The controller was validated by using detailed simulations. IEEE Keywords Mathematical model , Power system stability , HVDC transmission , Voltage control , Oscillators , Control systems , Power system dynamic