Control strategy for distribution generation inverters to maximize the voltage support in the lowest phase during voltage sags

IEEE Voltage sags are considered one of the worst perturbations in power systems. Distributed generation power facilities are allowed to disconnect from the grid during grid faults whenever the voltage is below a certain threshold. During these severe contingencies, a cascade disconnection could start, yielding to a blackout. To minimize the risk of a power outage, inverter-based distributed generation systems can help to support the grid by appropriately selecting the control objective. Which control strategy performs better when supporting the grid voltage is a complex decision that depends on many variables. This paper presents a control scheme that implements a smart and simple strategy to support the fault: the maximum voltage support for the lowest phase voltage. Therefore, the faulted phase that is more affected by the sag can be better supported since this phase voltage increases as much as possible, reducing the risk of under-voltage disconnection. The proposed controller has the following features: a) maximizes the voltage in the lowest phase, b) injects the maximum rated current of the inverter, and c) balances the active and reactive power references to deal with resistive and inductive grids. The control proposal is validated by means of experimental results in a laboratory prototype.Postprint (author's final draft

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

Optimal voltage-support control for distributed generation inverters in RL grid-faulty networks

© 2019 IEEE. Personal use of this material is permitted. Permission from IEEE must be obtained for all other uses, in any current or future media, including reprinting/republishing this material for advertising or promotional purposes, creating new collective works, for resale or redistribution to servers or lists, or reuse of any copyrighted component of this work in other works.During grid faults, the stability and reliability of the network are compromised, and the risk of a widespread disconnection of distributed generation power facilities is increased. Distributed generation inverters must support the power system to prevent this issue. Voltage support depends substantially on the currents injected into the grid and the equivalent grid impedance. This paper considers these two aspects and proposes an optimal voltage-support strategy in RL grids. The control algorithm guarantees a safe operation of the inverter during voltage sags by calculating the appropriate reference currents according to the equivalent impedance and the voltage sag characteristics, avoiding active power oscillations, and limiting the injected current to the maximum allowed by the inverter. Consequently, the grid can be better supported since the voltage at the point of common coupling is improved and the voltage support objectives are achieved. The proposed control strategy is validated through experimental tests in different grid scenarios. Throughout the work, it is assumed that the grid impedance is known, but the proposed solution requires calculating the grid impedance angle.Peer ReviewedPostprint (author's final draft

Flexible power control strategy for elliptical trajectory based dynamic voltage restorer during voltage sags

Abstract Dynamic voltage restorer (DVR) plays an essential role in achieving high‐quality voltage restoration in the distribution system. This paper presents an elliptical‐trajectory‐based approach to guarantee continuous power delivery by considering the active and reactive power limits during voltage sags. Besides, two parameters are proposed in the accurate mathematical analysis of current references. Thus, the control objective can be fulfilled by the flexible current injection strategy that combines a precise balance between the positive and negative sequences. The proposed control method can determine the required power injections by selecting the two proper values to support that the converter output current is smaller than the maximum rated value. Under the presented control, the maximum power limit and flexible power dispatch are combined together to develop four operation schemes. The main advantage is the smooth transition process during changes in the operation schemes and the disappearance of active and reactive power oscillations. Also, these four control states are applied to reveal the flexibility of the proposed designs. Finally, the hardware tests in a laboratory prototype are implemented to validate the feasibility of the proposed method

A flexible experimental laboratory for distributed generation networks based on power inverters

In the recently deregulated electricity market, distributed generation based on renewable sources is becoming more and more relevant. In this area, two main distributed scenarios are focusing the attention of recent research: grid-connected mode, where the generation sources are connected to a grid mainly supplied by big power plants, and islanded mode, where the distributed sources, energy storage devices, and loads compose an autonomous entity that in its general form can be named a microgrid. To conduct a successful research in these two scenarios, it is essential to have a flexible experimental setup. This work deals with the description of a real laboratory setup composed of four nodes that can emulate both scenarios of a distributed generation network. A comprehensive description of the hardware and software setup will be done, focusing especially in the dual-core DSP used for control purposes, which is next to the industry standards and able to emulate real complexities. A complete experimental section will show the main features of the system.Peer ReviewedPostprint (published version