Control of grid-connected three-phase three-wire voltage-sourced inverters under voltage disturbances

Tesi per compendi de publicacions, amb una secció retallada per drets de l'editorThe present doctoral thesis focuses on designing control schemes for three-phase three-wire voltage-sourced inverters connected to the grid under voltage disturbances. The research recognizes the large-scale integration of distributed power generation systems into the network and takes advantage of this circumstance to investigate and develop new control strategies in order to provide better support to the modern power grid. As a first contribution, a new algorithm to maximize power delivery capability of the inverter has been developed and experimentally tested under voltage imbalance conditions, i.e., during slight/shallow and deep asymmetrical sags. The algorithm of this control strategy meets grid code requirements, performs active power control, limits the maximum current injected by the inverter, and eliminates active power oscillations. As a result, six different cases of current injection were identified in this work, considering restrictions imposed by grid codes as well as different active-power production scenarios. The second contribution of this research work has provided an experimental analysis of a low-voltage ride-through strategy whose voltage support capability had not been tested when voltage sags occur. This study was performed considering a scenario of multiple grid-connected inverters, different profiles of active power injection, and the equivalent grid impedance seen from the output side of each converter. In the third contribution has been proposed a closed-loop controller for low-power distributed inverters that maximizes the current injection when voltage sag occurs. The control algorithm has been designed to meet grid code requirements and avoid overvoltage in non-faulty phases during grid faults. The controller is responsible for meeting coordinately several objectives and addressing the interactions that appear among them. In the last two chapters, the argument of this doctoral thesis is complemented, the obtained experimental results are globally analyzed, finally, the present research work is concluded.Esta tesis doctoral, presentada en la modalidad de compendio de publicaciones en cumplimiento parcial de los requisitos para optar al título de Doctor en Ingeniería Electrónica de la Universidad Politécnica de Cataluña, se centra en el diseño de esquemas de control para inversores trifásicos conectados a la red eléctrica durante perturbaciones de voltaje. La investigación reconoce la integración a gran escala de los sistemas de generación distribuida en la red y aprovecha esta circunstancia para estudiar y desarrollar nuevas estrategias de control con el propósito de brindar un mejor soporte a la red eléctrica moderna. Como primera contribución, se desarrolló un nuevo algoritmo para maximizar la capacidad de suministro de potencia del inversor en condiciones de desequilibrio de voltaje, es decir, durante caídas asimétricas de tensión leves, poco profundas y severas. El algoritmo de esta estrategia de control fue diseñado para cumplir los requerimientos de los vigentes códigos de red (grid codes), realizar control de la potencia activa, limitar la corriente máxima inyectada por el inversor y eliminar las oscilaciones de la potencia activa instantánea. Como resultado, en esta investigación se identificaron y validaron experimentalmente seis casos diferentes de inyección de corriente en la red, trabajo que tuvo en cuenta no solo las restricciones impuestas por los códigos de red, sino también los diferentes escenarios de producción de potencia activa. La segunda contribución de este trabajo de investigación ha proporcionado el análisis experimental de una estrategia de inyección de corriente cuya capacidad de soporte de voltaje no se había probado durante fallos de red. Este estudio se realizó sobre un escenario de múltiples inversores conectados a la red eléctrica, utilizando diferentes perfiles de inyección de potencia activa y considerando, como aspecto fundamental para el análisis experimental, la impedancia de red equivalente vista desde el lado de salida de cada convertidor. En la tercera contribución se diseñó un controlador en lazo cerrado para inversores distribuidos de baja potencia que maximiza la inyección de corriente cuando se produce una caída de tensión. Este algoritmo de control también satisface los requerimientos de los actuales códigos de red en cuanto a inyección de corriente reactiva durante fallos de red, pero cuenta con la capacidad adicional de evitar sobretensiones en las fases no defectuosas. De igual forma, este controlador es responsable de acometer coordinadamente varios objetivos y gestionar las interacciones que aparecen entre ellos. En los últimos dos capítulos se complementa la unidad temática de esta tesis doctoral, se analizan globalmente los resultados experimentales obtenidos y, finalmente, se concluye el presente trabajo de investigación agregando, también, futuros campos de estudio.Postprint (published version

Control of grid-connected three-phase three-wire voltage-sourced inverters under voltage disturbances

The present doctoral thesis focuses on designing control schemes for three-phase three-wire voltage-sourced inverters connected to the grid under voltage disturbances. The research recognizes the large-scale integration of distributed power generation systems into the network and takes advantage of this circumstance to investigate and develop new control strategies in order to provide better support to the modern power grid. As a first contribution, a new algorithm to maximize power delivery capability of the inverter has been developed and experimentally tested under voltage imbalance conditions, i.e., during slight/shallow and deep asymmetrical sags. The algorithm of this control strategy meets grid code requirements, performs active power control, limits the maximum current injected by the inverter, and eliminates active power oscillations. As a result, six different cases of current injection were identified in this work, considering restrictions imposed by grid codes as well as different active-power production scenarios. The second contribution of this research work has provided an experimental analysis of a low-voltage ride-through strategy whose voltage support capability had not been tested when voltage sags occur. This study was performed considering a scenario of multiple grid-connected inverters, different profiles of active power injection, and the equivalent grid impedance seen from the output side of each converter. In the third contribution has been proposed a closed-loop controller for low-power distributed inverters that maximizes the current injection when voltage sag occurs. The control algorithm has been designed to meet grid code requirements and avoid overvoltage in non-faulty phases during grid faults. The controller is responsible for meeting coordinately several objectives and addressing the interactions that appear among them. In the last two chapters, the argument of this doctoral thesis is complemented, the obtained experimental results are globally analyzed, finally, the present research work is concluded.Esta tesis doctoral, presentada en la modalidad de compendio de publicaciones en cumplimiento parcial de los requisitos para optar al título de Doctor en Ingeniería Electrónica de la Universidad Politécnica de Cataluña, se centra en el diseño de esquemas de control para inversores trifásicos conectados a la red eléctrica durante perturbaciones de voltaje. La investigación reconoce la integración a gran escala de los sistemas de generación distribuida en la red y aprovecha esta circunstancia para estudiar y desarrollar nuevas estrategias de control con el propósito de brindar un mejor soporte a la red eléctrica moderna. Como primera contribución, se desarrolló un nuevo algoritmo para maximizar la capacidad de suministro de potencia del inversor en condiciones de desequilibrio de voltaje, es decir, durante caídas asimétricas de tensión leves, poco profundas y severas. El algoritmo de esta estrategia de control fue diseñado para cumplir los requerimientos de los vigentes códigos de red (grid codes), realizar control de la potencia activa, limitar la corriente máxima inyectada por el inversor y eliminar las oscilaciones de la potencia activa instantánea. Como resultado, en esta investigación se identificaron y validaron experimentalmente seis casos diferentes de inyección de corriente en la red, trabajo que tuvo en cuenta no solo las restricciones impuestas por los códigos de red, sino también los diferentes escenarios de producción de potencia activa. La segunda contribución de este trabajo de investigación ha proporcionado el análisis experimental de una estrategia de inyección de corriente cuya capacidad de soporte de voltaje no se había probado durante fallos de red. Este estudio se realizó sobre un escenario de múltiples inversores conectados a la red eléctrica, utilizando diferentes perfiles de inyección de potencia activa y considerando, como aspecto fundamental para el análisis experimental, la impedancia de red equivalente vista desde el lado de salida de cada convertidor. En la tercera contribución se diseñó un controlador en lazo cerrado para inversores distribuidos de baja potencia que maximiza la inyección de corriente cuando se produce una caída de tensión. Este algoritmo de control también satisface los requerimientos de los actuales códigos de red en cuanto a inyección de corriente reactiva durante fallos de red, pero cuenta con la capacidad adicional de evitar sobretensiones en las fases no defectuosas. De igual forma, este controlador es responsable de acometer coordinadamente varios objetivos y gestionar las interacciones que aparecen entre ellos. En los últimos dos capítulos se complementa la unidad temática de esta tesis doctoral, se analizan globalmente los resultados experimentales obtenidos y, finalmente, se concluye el presente trabajo de investigación agregando, también, futuros campos de estudio

Voltage support experimental analysis of a low-voltage ride-through strategy applied to grid-connected distributed inverters

In recent decades, different control strategies have been designed for the increasing integration of distributed generation systems. These systems, most of them based on renewable energies, use electronic converters to exchange power with the grid. Capabilities such as low-voltage ride-through and reactive current injection have been experimentally explored and reported in many research papers with a single inverter; however, these capabilities have not been examined in depth in a scenario with multiple inverters connected to the grid. Only few simulation works that include certain methods of reactive power control to solve overvoltage issues in low voltage grids can be found in the literature. Therefore, the overall objective of the work presented in this paper is to provide an experimental analysis of a low-voltage ride-through strategy applied to distributed power generation systems to help support the grid during voltage sags. The amount of reactive power will depend on the capability of each inverter and the amount of generated active power. The obtained experimental results demonstrate that, depending on the configuration of distributed generation, diverse inverters could have different control strategies. In the same way, the discussion of these results shows that the present object of study is of great interest for future research.Peer ReviewedPostprint (published version

On the optimal reactive power control for grid-connected photovoltaic distributed generation systems

The increasing deployment of distributed generation (DG) such as photovoltaic panels (PV) connected to low-voltage (LV) grids is becoming a common trend in urban areas. The advances of information and communication technology (ICT) facilitates the collaborative operation of DG systems to achieve collective benefits. The fusion of these two trends creates a new scenario where reactive power control methods can offer additional features and benefits beyond the conventional voltage regulation provided by the droop method. Taking advantage of this new scenario, this paper formulates the application of reactive power control as an optimization problem where simple and ideal settings are imposed by design in order to facilitate the exploration search as well as to avoid over-constraining the optimization space. By appropriately using the reactive power capacity of inverters, the desired collective benefit is to minimize power losses while individual voltages at each inverter should be kept within the statutory limits. The simulated solution of the optimization problem is applied to a real-inspired PV-LV grid subject to an over-voltage situation, which may typically occur during periods of high production but low consumption. Simulation results reveal unexpected optimal settings for reactive power control set-points at each inverter, which calls for a final discussion to review the applicability of the optimization approach.Postprint (author's final draft

On the optimal reactive power control for grid-connected photovoltaic distributed generation systems

The increasing deployment of distributed generation (DG) such as photovoltaic panels (PV) connected to low-voltage (LV) grids is becoming a common trend in urban areas. The advances of information and communication technology (ICT) facilitates the collaborative operation of DG systems to achieve collective benefits. The fusion of these two trends creates a new scenario where reactive power control methods can offer additional features and benefits beyond the conventional voltage regulation provided by the droop method. Taking advantage of this new scenario, this paper formulates the application of reactive power control as an optimization problem where simple and ideal settings are imposed by design in order to facilitate the exploration search as well as to avoid over-constraining the optimization space. By appropriately using the reactive power capacity of inverters, the desired collective benefit is to minimize power losses while individual voltages at each inverter should be kept within the statutory limits. The simulated solution of the optimization problem is applied to a real-inspired PV-LV grid subject to an over-voltage situation, which may typically occur during periods of high production but low consumption. Simulation results reveal unexpected optimal settings for reactive power control set-points at each inverter, which calls for a final discussion to review the applicability of the optimization approach