PI-based controller for low-power distributed inverters to maximise reactive current injection while avoiding over voltage during voltage sags

This paper is a postprint of a paper submitted to and accepted for publication in IET Power Electronics and is subject to Institution of Engineering and Technology Copyright. The copy of record is available at the IET Digital Library.In the recently deregulated power system scenario, the growing number of distributed generation sources should be considered as an opportunity to improve stability and power quality along the grid. To make progress in this direction, this work proposes a reactive current injection control scheme for distributed inverters under voltage sags. During the sag, the inverter injects, at least, the minimum amount of reactive current required by the grid code. The flexible reactive power injection ensures that one phase current is maintained at its maximum rated value, providing maximum support to the most faulted phase voltage. In addition, active power curtailment occurs only to satisfy the grid code reactive current requirements. As well as, a voltage control loop is implemented to avoid overvoltage in non-faulty phases, which otherwise would probably occur due to the injection of reactive current into an inductive grid. The controller is proposed for low-power rating distributed inverters where conventional voltage support provided by large power plants is not available. The implementation of the controller provides a low computational burden because conventional PI-based control loops may apply. Selected experimental results are reported in order to validate the effectiveness of the proposed control scheme.Peer ReviewedPostprint (updated version

International White Book on DER Protection : Review and Testing Procedures

This white book provides an insight into the issues surrounding the impact of increasing levels of DER on the generator and network protection and the resulting necessary improvements in protection testing practices. Particular focus is placed on ever increasing inverter-interfaced DER installations and the challenges of utility network integration. This white book should also serve as a starting point for specifying DER protection testing requirements and procedures. A comprehensive review of international DER protection practices, standards and recommendations is presented. This is accompanied by the identifi cation of the main performance challenges related to these protection schemes under varied network operational conditions and the nature of DER generator and interface technologies. Emphasis is placed on the importance of dynamic testing that can only be delivered through laboratory-based platforms such as real-time simulators, integrated substation automation infrastructure and fl exible, inverter-equipped testing microgrids. To this end, the combination of fl exible network operation and new DER technologies underlines the importance of utilising the laboratory testing facilities available within the DERlab Network of Excellence. This not only informs the shaping of new protection testing and network integration practices by end users but also enables the process of de-risking new DER protection technologies. In order to support the issues discussed in the white paper, a comparative case study between UK and German DER protection and scheme testing practices is presented. This also highlights the level of complexity associated with standardisation and approval mechanisms adopted by different countries

EcoBlock: Grid Impacts, Scaling, and Resilience

Widespread deployment of EcoBlocks has the potential to transform today's electricity system into one that is more resilient, flexible, efficient and sustainable. In this vision, the system will consist of self- su cient, renewable-powered, block-scale entities that can deliberately adjust their net power exchange and can optimize performance, maintain stability, support each other, or disconnect entirely from the grid as needed. This report is intended as an independent analysis of the potential relationships, both constructive and adverse, between EcoBlocks and the grid

Distribution System Voltage Management and Optimization for Integration of Renewables and Electric Vehicles: Research Gap Analysis

California is striving to achieve 33% renewable penetration by 2020 in accordance with the state’s Renewable Portfolio Standard (RPS). The behavior of renewable resources and electric vehicles in distribution systems is creating constraints on the penetration of these resources into the distribution system. One such constraint is the ability of present-­‐‑day voltage management methodologies to maintain proper distribution system voltage profiles in the face of higher penetrations of PV and electric vehicle technologies. This white paper describes the research gaps that have been identified in current Volt/VAR Optimization and Control (VVOC) technologies, the emerging technologies which are becoming available for use in VVOC, and the research gaps which exist and must be overcome in order to realize the full promise of these emerging technologies

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

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

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