A cascaded converter interfacing long distance HVDC and back-to-back HVDC systems

This paper proposes a cascaded converter dedicated to long-distance HVDC infeed and asynchronous back-to-back interconnection of receiving grids. The cascaded converter is consisted of MMCs in series and parallel connection, meeting the high DC voltage and power demand of HVDC system. It realizes hierarchical feeding and asynchronous interconnection of receiving grids, optimizing the multi-infeed short circuit ratio and improving the flexibility of the receiving grids. The topology and operating characteristics of the cascaded converter are introduced in detail. The multi-infeed short-circuits ratio (MISCR) and the maximum power infeed of the cascaded converter based HVDC systems are analyzed. Various feasible operating modes with online switching strategies of the cascaded converter are studied to improve the operational flexibility of the system. The simulation results verify the effectiveness of the control strategy of the HVDC system embedding the cascaded converter. The DC faults clearing strategy and operating modes switching strategies are also validated

Feasibility and reliability analysis of LCC DC grids and LCC/VSC hybrid DC grids

Power system interconnections using high-voltage direct-current (HVDC) technologies between different areas can be an effective solution to enhance system efficiency and reliability. Particularly, the multi-terminal DC grids, that could balance and ensure resource adequacy, increase asset utilization and reduce costs. In this paper, the technical feasibility of building DC grids using the line commutated converter based (LCC) and voltage source converter based (VSC) HVDC technologies are discussed. Apart from presenting the technical challenges of building LCC DC grids and LCC/VSC hybrid DC grids, the reliability modeling and analysis of these DC grids are also presented. First, the detailed reliability model of the modular multi-level converters (MMCs) with series connected high-voltage and low-voltage bridges are developed. The active mode redundancy design is considered for the reliability model. To this end, a comprehensive whole system reliability model of the studied systems is developed. The reliability model of each subsystem is modeled in detail. Various reliability indices are calculated using this whole system reliability model. The impacts of the redundancy design of the MMCs on these indices are presented. The studies of this paper provide useful guidance for DC grid design and reliability analysis

Solid state transformer technologies and applications: a bibliographical survey

This paper presents a bibliographical survey of the work carried out to date on the solid state transformer (SST). The paper provides a list of references that cover most work related to this device and a short discussion about several aspects. The sections of the paper are respectively dedicated to summarize configurations and control strategies for each SST stage, the work carried out for optimizing the design of high-frequency transformers that could adequately work in the isolation stage of a SST, the efficiency of this device, the various modelling approaches and simulation tools used to analyze the performance of a SST (working a component of a microgrid, a distribution system or just in a standalone scenario), and the potential applications that this device is offering as a component of a power grid, a smart house, or a traction system.Peer ReviewedPostprint (published version

DC voltage control for MMC-based railway power supply integrated with renewable generation

The full controllable electronic device based railway feeder station offers better power quality and more flexible configurations than conventional transformer based stations. This study investigates a modular multilevel converter (MMC)-based static frequency converter station with renewable energy access. Wind power generation is coupled into the station via DC link of the back to back converter. The dynamic single-phase traction load and intermittent renewable generation bring double frequency oscillation and large deviation problems to the DC link voltage. Special design considerations and control schemes are proposed for the MMC to stabilise DC link voltage by controlling the total number of total inserted modules. The proposed control scheme resolves the voltage oscillation issue caused by single-phase load and reduces the DC link voltage deviation under 10 MW step change. A series of device-based simulations validate the control scheme which realises a reliable coupling interface for connecting the renewable generation to the DC bus

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

Theoretical Evaluation of Semiconductor Loss Components Behavior in ISOP-DAB Converters

Input-Series Output-Parallel (ISOP) connected Dual-Active-Bridges (DABs) form an interesting topology which enable step down MVDC to LVDC by employing medium frequency transformers. Si-IGBT semiconductor devices are commonly used in power distribution grid applications such as Smart Transformers (STs) where both MVDC and LVDC energy ports can be presented. In this paper, Si-IGBT semiconductor loss components such as conduction, switching and gate driver losses behavior are theoretically evaluated in a wide range of blocking voltage, switching frequency and phase-shift angle of DAB converters. In addition, gate driver losses are approximated as a continuous function of blocking voltage based on available datasheets. The behavior of the loss components are analyzed for two 50 and 500 kW ISOP-DAB converters where DC voltage ratio is 10 kV/400V

Emerging Converter Topologies and Control for Grid Connected Photovoltaic Systems

Continuous cost reduction of photovoltaic (PV) systems and the rise of power auctions resulted in the establishment of PV power not only as a green energy source but also as a cost-effective solution to the electricity generation market. Various commercial solutions for grid-connected PV systems are available at any power level, ranging from multi-megawatt utility-scale solar farms to sub-kilowatt residential PV installations. Compared to utility-scale systems, the feasibility of small-scale residential PV installations is still limited by existing technologies that have not yet properly address issues like operation in weak grids, opaque and partial shading, etc. New market drivers such as warranty improvement to match the PV module lifespan, operation voltage range extension for application flexibility, and embedded energy storage for load shifting have again put small-scale PV systems in the spotlight. This Special Issue collects the latest developments in the field of power electronic converter topologies, control, design, and optimization for better energy yield, power conversion efficiency, reliability, and longer lifetime of the small-scale PV systems. This Special Issue will serve as a reference and update for academics, researchers, and practicing engineers to inspire new research and developments that pave the way for next-generation PV systems for residential and small commercial applications

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

An overview of HVDC technology

There is a growing use of High Voltage Direct Current (HVDC) globally due to the many advantages of Direct Current (DC) transmission systems over Alternating Current (AC) transmission, including enabling transmission over long distances, higher transmission capacity and efficiency. Moreover, HVDC systems can be a great enabler in the transition to a low carbon electrical power system which is an important objective in today’s society. The objectives of the paper are to give a comprehensive overview of HVDC technology, its development, and present status, and to discuss its salient features, limitations and applications.</jats:p