Advanced control of multi-microgrids for grid integration

Thanks to tremendous growing interest, the significant number of microgrids form a system called Multi-Microgrid, where multiple microgrids are interconnected to support local loads and exchange power to or from grid. Industry demands for advanced control and optimal coordination among microgrids with consideration of high penetration of renewable energy and complex system architectures. This thesis focuses on different key aspects of power systems and microgrids to develop novel approaches targeting the problem. Firstly, different topologies of microgrids are studied from the literature review and most popular system architectures are considered in the study for proposing advanced control techniques. Distributed control systems with nested formation in the microgrids are proposed for improved power sharing strategy. The distributed control is designed to achieve self-healing capability of multi-microgrids during any contingency event. Local controllers of the inverters in each microgrid are interconnected through the nested formation. A nested optimization algorithm is designed to achieve power exchange between different microgrids. Multi-terminal HVDC network based multi-microgrids have been proposed for advanced control strategy due to its widespread application in power system. Adaptive droop control has been proposed based on consensus algorithm and matrix-based solutions to provide frequency support and power sharing between AC microgrids through the HVDC network. The proposed adaptive droop algorithm is featured to maintain frequency and voltage during contingency events and ensure efficient power sharing. Distributed hierarchical control system is proposed as well for multi-microgrids with nested formation-based optimization techniques to ensure proper power sharing in four-level based multi-microgrid topologies. The algorithm features energy management within the multi-microgrid through virtual controllers of primary and secondary frequency control. In addition, to the energy management issue, low system strength of grid has been considered to offer a wide range of areas under the advanced control of multi-microgrid. In that regard, single machine infinity bus model has been considered to implement control of grid forming inverters for integration with weak grid. Novel grid resynchronization and virtual synchronous generator control has been proposed to achieve multi-microgrids integration with weak grids. Then, various simulation studies are performed to test the effectiveness of the proposed controls. The time domain simulations are performed on EMT power system tool PSCAD under different operating conditions, such as loading variations, N-1 contingency events, grid frequency change disturbance, islanding conditions etc. In addition to the time domain simulation studies, stability analysis of the proposed control has been carried out. In the stability analysis, pole-zero map, Nyquist plots and Bode plots have been demonstrated to analyse the stable conditions of the proposed control. The optimization algorithms results are also included in the simulation studies to reflect the performance of the control. Finally, the advanced control solutions outcomes through time domain and stability results are compared with conventional control. It has been demonstrated that all proposed solutions perform better than conventional approaches and reflect significant improvement on the multi-microgrids. Furthermore, industry standards have been considered in the weak grid integration study and case studies are carried out based on power industry practices, including industry regulatory grid codes according to the power industry in Australia. The results indicate that the proposed controls are able to satisfy industry grid codes

Contributions to Modulation and Control Algorithms for Multilevel Converters

Las actuales tendencias de la red eléctrica han lanzado a la industria a la búsqueda de sistemas de generación, distribución y consumo de energía eléctrica más eficientes. Generación distribuida, reducción de componentes pasivos, líneas DC de alta tensión son, entre otras, las posibles líneas de investigación que están actualmente siendo consideradas como el futuro de la red eléctrica. Sin embargo, nada de esto sería posible si no fuera por los avances alcanzados en el campo de la electrónica de potencia. El trabajo aquí presentado comienza con una breve introducción a la electrónica de potencia, concretamente a los convertidores de potencia conectados a red, sus estrategias de control más comunes y enfoques ante redes desbalanceadas. A continuación, las contribuciones del autor sobre el control y modulación de una topología particular de convertidores, conocidos como convertidores multinivel, se presentan como el principal contenido de este trabajo. Este tipo de convertidores mejoran la eficiencia y ciertas prestaciones, en comparación con convertidores más tradicionales, a costa de una mayor complejidad en el control al incrementar la cantidad de los componentes hardware. A pesar de que existen numerosas topologías de convertidores multinivel y algunas de ellas son brevemente expuestas en este trabajo, la mayoría de las aportaciones están enfocadas para convertidores del tipo diode-clamped converter. Adicionalmente, se incluye una aportación para convertidores del tipo multinivel modular, y otra para convertidores en cascada. Se espera que el contenido de la introducción de este trabajo, junto a las contribuciones particulares para convertidores multinivel sirva de inspiración para futuros investigadores del campo

Grid Forming Wind Power Plants: Black start operation for HVAC grids and Diode Rectifier-based Wind Power Plant integration

Tesis por compendio[ES] La Unión Europea plantea una serie de retos en el GREEN DEAL para conseguir un escenario donde sus miembros sean climáticamente neutros en 2050. Para ello se plantean unas acciones entre las que destaca la descarbonización del sector de la energía. Por otra parte, se ha puesto como objetivo conseguir una capacidad de energía eólica de 1200 GW en 2050, desde los 190 GW que se tenía en 2019. El cierre de centrales eléctricas basadas en grandes generadores síncronos junto con el aumento en la construcción de centrales eléctricas con fuentes basadas en electrónica de potencia como parques eólicos o plantas fotovoltaicas, hace necesario la incorporación de controladores grid forming en energías renovables basadas en electrónica de potencia. La integración de estrategias de control grid forming en turbinas eólicas debe considerar las funciones de reparto de potencia activa y reactiva (control droop en generadores síncronos), así como un sistema de protección ante faltas que permita una recuperación rápida cuando se despeja la falta. Por otra parte, en la transición de los actuales parques eólicos grid following que funcionan como fuentes de corriente a parques eólicos grid forming que funcionan como fuentes de tensión se debe considerar que ambas tecnologías van a coexistir conjuntamente por mucho tiempo. Por tanto, se hace necesario el estudio de estabilidad de parques eólicos con ambas tecnologías, así como el estudio de cuanta generación grid forming es necesaria para mantener el parque eólico estable en cualquier situación. Además, el uso de parques eólicos grid foming permite diferentes aplicaciones como la energización de redes eléctricas después de un apagón desde estos parques eólicos, o el uso de diodos rectificadores en enlaces HVDC para la conexión de parques eólicos marinos. Para facilitar el estudio y diseño de aerogeneradores grid forming, en la presente tesis se propone una técnica de agregación de aerogeneradores tipo-4. Esta técnica permite reducir la complejidad del parque eólico para su estudio y análisis. Esta tesis incluye las siguientes contribuciones: Para el funcionamiento de aerogeneradores grid forming en paralelo con aerogeneradores grid following se propone un método de diseño analizando la estabilidad del sistema completo. Además, se propone una estrategia de control ante faltas para asegurar una recuperación rápida y segura. Se propone el uso de técnicas de control H∞ para la sintonización de controladores grid forming. El uso de estas técnicas para el diseño de controladores puede mejorar la robustez de los controladores, así como el rendimiento de estos. Considerando la aplicación de la energización de redes HVAC desde parques eólicos, se ha propuesto como llevar a cabo dicha maniobra a partir de parque con aerogeneradores grid forming y grid following. Los resultados obtenidos validan el funcionamiento de la operación. Además, muestran que la cantidad de generación grid forming está relacionada con la dimensión de cargas que tienen que aguantar dichas turbinas más que con la estabilidad del sistema con un porcentaje bajo de generación grid forming. Finalmente, se han propuesto estrategias de protección para la integración de parque eólicos en redes malladas HVDC utilizando rectificadores de diodos. Los parques eólicos grid forming pueden ayudar a gestionar faltas disminuyendo los requisitos en las protecciones necesarias en el enlace HVDC. Esto permite una reducción del coste de la instalación, además de aumentar la robustez del sistema.[CA] La Unió Europea planteja uns reptes al GREEN DEAL per a aconseguir un escenario on els seus membres siguen climàticament neutrals al 2050. Per a aconseguir aquest objectiu es plantegen unes accions entre les quals destaca la de descarbonitzar el sector energètic. D'altra banda, s'ha posat com a objectiu aconseguir una capacitat d'energia eòlica de 1200 GW al 2050, des dels 190 GW que es tenia al 2019. El tancament de centrals elèctriques basades en grans generadors síncrons juntament amb l'augment en la construcció de centrals elèctriques basades en electrónica de potència com a parcs eòlics o plantes fotovoltaiques, fa necessari la incorporació de controladors grid forming en energies renovables que es basan en electrònica de potència. La integració d'estratègies de control grid forming per a turbines eòliques ha de considerar les funcions de repartiment de potència activa i reactiva, així com un sistema de protecció davant faltes que permeta una recuperació ràpida quan s'aïlla la falta. D'altra banda, en la transició dels actuals parcs eòlics grid following que funcionen com a fonts de corrent a parcs eòlics grid forming que funcionen com a fonts de tensió, s'ha de considerar que totes dues tecnologies coexistiran conjuntament per molt de temps. Per tant, es fa necessari l'estudi d'estabilitat d'un parc eòlic amb totes dues tecnologies, així com l'estudi del percentatge de generació grid forming necessari per a mantindre el parc eòlic estable en qualsevol situació. A més, l'ús de parcs eòlics grid foming permet diferents aplicacions com l'energització de xarxes elèctriques després d'una apagada des d'aquests parcs eòlics, o l'ús de díodes rectificadors en enllaços HVDC per a la connexió de parcs eòlics marins. Per a facilitar l'estudi i disseny d'aerogeneradors grid forming, en la present tesi es proposa una tècnica d'agregació d'aerogeneradors tipus-4. Aquesta técnica permet reduir la complexitat del parc eòlic per al seu estudi i anàlisi. La present tesi inclou les següents contribucions: Per al funcionament d'aerogeneradors grid forming en paral¿lel amb aerogeneradors grid following es proposa un mètode de disseny analitzant l'estabilitat del sistema complet. A més, es proposa una estratègia de control davant faltes per a assegurar una recuperació ràpida i segura. Es proposa l'ús de tècniques de control H∞ per a la sintonització de controladors grid forming. L'ús d'aquestes tècniques per al disseny de controladors pot millorar la robustesa dels controladors, a mé del rendiment d'aquestos. Considerant l'aplicació de l'energització de xarxes HVAC des de parcs eòlics, s'ha proposat com dur a terme aquesta maniobra a partir d'un parc eòlic amb aerogeneradors grid forming i grid following. Els resultats obtinguts validen el funcionament de l'operació. A més, mostren que la quantitat de generació grid forming està més relacionada amb la dimensió de les càrregues que ha d'aguantar el parc eòlic, que amb l'estabilitat del sistema que permet un menr percentatge de generació grid forming. Finalment, s'han proposat estratègies de protecció per a la integració de parcs eòlics marins en xarxes multi-punt HVDC utilitzant rectificadors de díodes. Els parcs eòlics grid forming poden ajudar a gestionar faltes disminuint els requisits de les proteccions necessàries en l'enllaç HVDC. Això permet una reducció del cost de la instal¿lació, a més d'augmentar la robustesa del sistema.[EN] The European Union GREEN DEAL aims to make its 27 members climate-neutral by 2050. The decarbonization of the energy sector stands out as one of the proposed actions. To achieve that goal, the target for wind power generation is set at 1,200 GW in 2050, from the 190 GW that was had in 2019. The closure of power plants based on large synchronous generators and the increase of power electronics based generation such as Wind Power Plants (WPPs) or photovoltaic plants, leads to the use of grid forming controllers for power electronics based renewable energy. Grid forming control strategies for wind turbines generators (WTGs) must consider active and reactive power sharing control (droop control in synchronous generators) as well as a protection system that allows a quick recovery after fault clearance. Moreover, a transition from the conventional grid following WPPs to new grid forming WPPs is required. The transition must consider the parallel operation of both technologies for a long time. Thus, it is necessary to study the stability of Wind Power Plants with both technologies, as well as the study of how much grid forming generation is required to keep a mixed grid forming and grid following WPP stable in any situation. In addition, the use of grid forming WPPs allows different applications such as the energization of HVAC and HVDC grids after a blackout from these WPPs, or the use of diode rectifiers in HVDC links for the off-shore WPPs connection. An aggregation technique for type-4 WTGs has been proposed in this thesis in order to facilitate the study and design of grid forming WPPs. The aggregation technique allows to reduce the WPP complexity for its study and analysis. The main contributions of this theses are: A design methodology has been proposed for the parallel operation of grid forming and grid following WTGs. Including the stability analysis of the complete system. Additionally, a fault control strategy is proposed to ensure a fast and safe recovery. The use of H∞ control techniques is also proposed for grid forming controller tuning. Using H∞ control techniques for controller design may improve the robustness of the controllers as well as the performance of the controllers. A procedure to carry out black start operation of HVAC grid from mixed grid forming and grid following WPPs has been proposed. The obtained results validate that the procedure works as expected. Moreover, the results show that the amount of grid-forming generation usually is determined by the load size steps as stability limits are usually less stringent. Finally, protection strategies have been proposed for the integration of off-shore WWPs in multi-terminal HVDC grids using diode rectifiers. Grid forming WPPs are able to help managing faults. Their use allow lower requirements of the HVDC protection equipment, leading to overall cost reduction and an increment of the system robustness.Martínez Turégano, J. (2022). Grid Forming Wind Power Plants: Black start operation for HVAC grids and Diode Rectifier-based Wind Power Plant integration [Tesis doctoral]. Universitat Politècnica de València. https://doi.org/10.4995/Thesis/10251/181529TESISCompendi

New phase-changing soft open point and impacts on optimising unbalanced power distribution networks

Three-phase unbalanced conditions in distribution networks are conventionally caused by load imbalance, asymmetrical fault conditions of transformers and impedances of three phases. The uneven integration of single-phase distributed generation (DG) worsens the imbalance situation. These unbalanced conditions result in financial losses, inefficient utilisation of assets and security risks to the network infrastructure. In this study, a phase-changing soft open point (PC-SOP) is proposed as a new way of connecting soft open points (SOPs) to balance the power flows among three phases by controlling active power and reactive power. Then an operational strategy based on PC-SOPs is presented for three-phase four-wire unbalanced systems. By optimising the regulation of SOPs, optimal energy storage systems dispatch and DG curtailment, the proposed strategy can reduce power losses and three-phase imbalance. Second-order cone programming (SOCP) relaxation is utilised to convert the original non-convex and non-linear model into an SOCP model which can be solved efficiently by commercial solvers. Case studies are conducted on a modified IEEE 34-node three-phase four-wire system and the IEEE 123-node test feeder to verify the effectiveness, efficiency and scalability of the proposed PC-SOP concept and its operational strategy

An enhanced Series-Connected Offshore Wind Farm (SC-OWF) system considering fault resiliency

The series-connected offshore wind farm (SC-OWF) is a promising offshore wind generation solution to mitigate the need of centralized offshore high-voltage/power converter stations. Predominantly, researchers have focused on the steady-state operation and control of SC-OWFs, without considering the system-level characteristics and ability to ride-through dc side and ac network faults. This paper proposes an enhanced system for SC-OWF applications with fault-resilient capability, where comprehensive circuit configuration and protection strategies are articulated to minimize the negative effects caused by various types of dc and ac faults. For the offshore wind farm architecture, a grouping scheme is adopted where a substation based on disconnectors and diodes is proposed to realize prompt fault bypass/isolation and protection functions in the event of offshore system faults. Additionally, an onshore fault-tolerant modular multilevel converter (MMC) with modified dc-system-oriented control is employed to enable smooth and secure operation under steady-state and fault conditions. The proposed SC-OWF system is quantitatively substantiated by time-domain simulations where four ac/dc fault cases are considered, and the results consolidate the feasibility of the proposed configuration and control, indicating fault resilience of the SC-OWF system. Additionally, size, weight and cost estimations of the proposed offshore substation are presented and compared to a conventional MMC offshore station, to further highlight the merits of the proposed solutio

HVDC transmission : technology review, market trends and future outlook

HVDC systems are playing an increasingly significant role in energy transmission due to their technical and economic superiority over HVAC systems for long distance transmission. HVDC is preferable beyond 300–800 km for overhead point-to-point transmission projects and for the cable based interconnection or the grid integration of remote offshore wind farms beyond 50–100 km. Several HVDC review papers exist in literature but often focus on specific geographic locations or system components. In contrast, this paper presents a detailed, up-to-date, analysis and assessment of HVDC transmission systems on a global scale, targeting expert and general audience alike. The paper covers the following aspects: technical and economic comparison of HVAC and HVDC systems; investigation of international HVDC market size, conditions, geographic sparsity of the technology adoption, as well as the main suppliers landscape; and high-level comparisons and analysis of HVDC system components such as Voltage Source Converters (VSCs) and Line Commutated Converters (LCCs), etc. The presented analysis are supported by practical case studies from existing projects in an effort to reveal the complex technical and economic considerations, factors and rationale involved in the evaluation and selection of transmission system technology for a given project. The contemporary operational challenges such as the ownership of Multi-Terminal DC (MTDC) networks are also discussed. Subsequently, the required development factors, both technically and regulatory, for proper MTDC networks operation are highlighted, including a future outlook of different HVDC system components. Collectively, the role of HVDC transmission in achieving national renewable energy targets in light of the Paris agreement commitments is highlighted with relevant examples of potential HVDC corridors

Modeling and Control of Power Electronics Interfaced Load for Transmission Power Network Analysis

The penetration level of power electronics (PE) interfaced loads has been gradually increasing in recent years. It is beneficial to equip the electric load with a PE interface since it allows for more advanced control of the load performance. Furthermore, the increasing penetration of PE interfaced loads will bring both challenges and opportunities to power network resilience and reliability. However, the lack of modeling and control design for PE interfaced load units in the transmission-level power network analysis, especially for these high-penetrated high-power-rating load applications, limits the accuracy of evaluating the dynamic performance and stability status of the power network. Additionally, the complex configuration and high bandwidth dynamic performance of the PE interfaced load computationally prohibit the model development in transient stability (TS) simulation programs. Therefore, the dynamic PE interfaced load model can be characterized considering the following aspects: 1) Utilize the real-time experimental platform to represent the PE load dynamic performance since the power testbed can reflect the power grid operation with more robustness. 2) Adapt the simplified PE-based model to TS simulation tools, which focus on grid electromechanical transients and oscillations between 0.1 and 3 Hz. Research of the PE interfaced load towards its modeling and control design in different simulation environments and the flexible contribution to the grid operation has been conducted. First, the variable speed drive (VSD) based motor load is studied as a typical PE interfaced load, which can actively interact with power grid operation. The model of VSD load is introduced and applied to the power emulator for the multi-converter-based hardware testbed (HTB) in the Center of Ultra-wide-area Resilient Electric Energy Transmission Network (CURENT). Second, the aggregated performance of multiple VSD load units with grid frequency support function is characterized. Third, the fast electric vehicle (EV) charging unit is studied as a typical PE interfaced load with high power consumption. The generic model of EV charger load is developed based on the detailed switching model. The accuracy of the proposed EV charger load TS model has been verified by comparing it to simulation results of the equivalent electromagnetic (EMT) model

Control of distributed renewable energy generation systems in converter-dominated microgrid applications

Mención Internacional en el título de doctorThere is a growing interest in the use of renewable Distributed Energy Resources (DERs) that increase the efficiency of the transmission system and reduce the ecological impact of renewable energy infrastructures. At the same time, they reduce the associated capital requirements, thus increasing the potential installation of renewable energy. Microgrids have been proposed as a solution to improve the integration of renewable DERs. By the use of advanced control techniques, they provide a reliable frame for DERs to support the power system operation. As such, Microgrids can be a promising solution to increase renewable energy penetration. However, since renewable DERs are usually interfaced by Power Electronic Converters (PECs), they do not provide the common stabilization characteristics of traditional generation interfaced by Synchronous Generators (SGs). Therefore, there are concerns about the stability of converter-dominated Microgrids. This Thesis focus on the specific requirements of PEC-interfaced renewable DERs operating in Microgrids. An overview of available solutions show that, for PECs to support the Microgrid operation in both grid-connected and islanded modes, they require a synchronizing mechanism that does not rely on the measurement of an external frequency. A promising alternative is to emulate the behavior of traditional SGs in the PEC control system with the so-called Virtual Synchronous Machine (VSM) solutions. The synchronization system underlying to these proposals is analyzed. A comparison with the use of traditional frequency measurement systems, namely Phase-Locked Loops (PLLs), in the support of the Microgrid power balance is addressed, showing that the PEC synchronization system has a direct effect on the Microgrid stability. The Thesis includes a new proposal to ensure synchronous operation based on the use reactive power, instead of active power as in VSMs, that does not require frequency measurements. A dynamic model of a grid-connected PEC is used to demonstrate that reactive power can be used to ensure synchronism. This Reactive Power Synchronization system is used to propose a solution for the black-start of Wind Energy Conversion Systems (WECSs), so that they can contribute to the restoration of the power system following a blackout. The proposed control systems are validated with experimental results of a grid connected PEC and an isolated WECS.Programa Oficial de Doctorado en Ingeniería Eléctrica, Electrónica y Automática por la Universidad Carlos III de MadridPresidente: Luis Rouco Rodríguez.- Secretario: Emilio José Bueno Peña.- Vocal: Roberto Alves Baraciart

Protection of Future Electricity Systems

The electrical energy industry is undergoing dramatic changes: massive deployment of renewables, increasing share of DC networks at transmission and distribution levels, and at the same time, a continuing reduction in conventional synchronous generation, all contribute to a situation where a variety of technical and economic challenges emerge. As the society’s reliance on electrical power continues to increase as a result of international decarbonisation commitments, the need for secure and uninterrupted delivery of electrical energy to all customers has never been greater. Power system protection plays an important enabling role in future decarbonized energy systems. This book includes ten papers covering a wide range of topics related to protection system problems and solutions, such as adaptive protection, protection of HVDC and LVDC systems, unconventional or enhanced protection methods, protection of superconducting transmission cables, and high voltage lightning protection. This volume has been edited by Adam Dyśko, Senior Lecturer at the University of Strathclyde, UK, and Dimitrios Tzelepis, Research Fellow at the University of Strathclyde