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Premi Extraordinari de Doctorat, promoció 2018-2019. Àmbit d’Enginyeria Industrial Meshed High Voltage Direct Current (HVDC) grids are seen as solution to transmit and exchange high amounts of power across long distances or using submarine cables with high levels of flexibility and redundancy. Also, they can be especially suitable to integrate offshore energy resources such as offshore wind power plants. This thesis focuses on the DC Current Flow Controllers (CFC) for meshed HVDC grids. The CFCs are being thought as power electronics based devices that may be installed in future meshed HVDC grids to aid in the current flow regulation. The concept is similar to Flexible Alternating Current Transmission Systems (FACTS) but applied to HVDC grids. First, an overview of the different CFC concepts proposed in the literature is presented. Then, the modelling and control of a DC/DC CFC converter is developed and the benefits of installing it in a meshed HVDC grid are analysed. The functionality of the previous CFC is also integrated into a DC Circuit Breaker, in order to have a single device with both capability to interrupt DC faults and provide DC current regulation. Afterwards, an interline DC/DC CFC topology is proposed, which has the advantage of a simplified converter structure. It is validated using dynamic simulations and a prototype is built and tested in a meshed DC grid experimental platform. A single CFC may not be enough to regulate the current flows in complex meshed HVDC grids, thus, this work also considers the concept of Distributed CFCs (DCFC) in a meshed HVDC grid, which are being operated selectively, allowing more flexibility when regulating the current flows. Also, multiple lines can be connected to a certain HVDC node. Therefore, the proposed CFC is extended to be connected to any number of HVDC lines and so, be able to control the current circulating through any of them. The obtained multi-port CFC is validated through simulations. Other devices can help to the current regulation in meshed HVDC grids, for example already installed DC/DC converters that adapt the different voltages of the HVDC systems. A transformerless DC/DC topology is analysed in this work and the design of its AC filter addressed. Finally, taking into account that some HVDC links based on Line Commutated Converters (LCC-HVDC) are installed near to potential offshore wind power resources, this work studies the operation and control of a Current Source Converter (CSC) based tapping station connected in series with the HVDC link to integrate offshore wind power. Les xarxes d'alta tensió mallades en contínua, meshed High voltatge Direct Current (HVDC) grids, es presenten com una solució per transportar grans quantitats d'energia a través de llargues distàncies o mitjançant cables submarins amb alts nivells de flexibilitat i redundància. També, són especialment adequades per la captació d'energia de parcs eòlics marins. Aquesta tesi se centra en els controladors del flux de corrent, Current Flow Controllers (CFC), per a xarxes HVDC mallades. Els CFC es plantegen com dispositius d'electrònica de potència que es podrien instal·lar en les futures xarxes HVDC mallades per tal d'ajudar en la regulació dels fluxos de corrent de les línies. Aquest concepte és similar als dispositius FACTS (Flexible AC Transmission Systems), però aplicat a xarxes HVDC. Primer, es realitza un recull de les diferents propostes de CFCs a la literatura. Després, es modelitza i es dissenya el control d'un convertidor DC/DC CFC i s'analitzen els beneficis d'instal·lar-lo en una xarxa HVDC mallada. La funcionalitat de l'anterior CFC s'inclou en els interruptors de contínua, DC Circuit Breakers (DCCB), per tal de tenir un dispositiu amb capacitat d'interropre faltes DC i també controlar corrents. A continuació, es proposa una topologia de CFC simplicada, que es valida per mitjà de simulacions i se'n construeix un prototip que es prova experimentalment al laboratori. Un únic CFC pot no ser suficient per a controlar els fluxos de corrent en xarxes HVDC mallades d'una certa complexitat. És per això, que també s'introdueix el concepte de CFCs distribuïts en diferents nodes de la xarxa i que s'operen de forma selectiva. Vàries línies HVDC poden estar connectades a un node, per aquest motiu, la topologia de CFC anteriorment presentada s'actualitza per tal de poder ser connectada a un nombre qualsevol de línies. La topologia multi-port obtinguda es valida per mitjà de simulacions. Altres dispositius que poden ajudar a controlar els fluxos de corrent són els propis convertidors DC/DC que s'encarreguen d'adaptar la tensió dels sistemes HVDC. S'analitza també un convertidor DC/DC sense transformador AC i es realitza el disseny del seu filtre AC. Finalment, algunes de les línies HVDC basades en tecnologia Line Commutated Converter (LCC) es troben a prop de zones amb energia eòlica potencial. Per aquest motiu, s'estudia l'operació i control d'un convertidor Current Source Converter (CSC) que actua com una estació de tapping per tal d'injectar l'energia d'un parc eòlic marí a la línia LCC-HVDC. Award-winning

Increased penetration of offshore wind power plants (OWPPs) demands frequency control services from them. Overloading the wind turbine, for few seconds after the under frequency event, to utilize its kinetic energy seems promising option for fast primary freque ncy control. Two methods of overloading the wind turbine (WT), with and without considering the impact of WT dynamics and variation of WT output power during the overload, are proposed in the literature. In this paper, these two methods are applied for fast primary frequency control from OWPPs connected through multi-terminal DC grid consideri ng the operation of the WT at below rated wind speed. Moreover, the impact of release of overload on the dynamics of the wind turbine, therefore on the associated AC and DC grids are studied in this paper. Finally, the suitable overloading method is proposed based on the simulation and experimental results. The time domain simulations for fast primary fre quency control are performed on an OWPP connected through a 3-terminal DC grid using DIgSILENT PowerFactory. The experiments are performed on OWPP model integrated to a laboratory scale 3-terminal DC grid test set up. Based on the simulations and experimental results, overloading method whic h considers the variation of WT output power during the overload provides better performance during and after release of the overload.

This project is oriented to the development of a Supervisory, Control and Data Acquisition (SCADA) software to control and supervise electrical variables from a scaled platform that represents a meshed HVDC grid employing National Instruments hardware and LabVIEW logic environment. The objective is to obtain real time visualization of DC and AC electrical variables and a lossless data stream acquisition. The acquisition system hardware elements have been configured, tested and installed on the grid platform. The system is composed of three chassis, each inside of a VSC terminal cabinet, with integrated Field-Programmable Gate Arrays (FPGAs), one of them connected via PCI bus to a local processor and the rest too via Ethernet through a switch. Analogical acquisition modules were A/D conversion takes place are inserted into the chassis. A personal computer is used as host, screen terminal and storing space. There are two main access modes to the FPGAs through the real time system. It has been implemented a Scan mode VI to monitor all the grid DC signals and a faster FPGA access mode VI to monitor one converter AC and DC values. The FPGA application consists of two tasks running at different rates and a FIFO has been implemented to communicate between them without data loss. Multiple structures have been tested on the grid platform and evaluated, ensuring the compliance of previously established specifications, such as sampling and scanning rate, screen refreshment or possible data loss. Additionally a turbine emulator was implemented and tested in Labview for further testing.