Search

© 2020 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, In meshed high voltage direct current grids, the current of each line cannot be controlled independently, since it depends on the resistances between nodes. Additional devices, such as current flow controllers (CFCs), may be needed to avoid bottlenecks or line overloads. This work presents a multiport dc-dc-based CFC topology to be connected to n lines with unidirectional current flows. The device is able to control the dc lines' currents to the desired value by inserting variable voltage sources in series. First, the modeling of the generic n-port topology is presented and then, its modulation and control strategy are described. In the first case study, the concept is validated considering a 5-port CFC by means of dynamic simulations using different control methods. Finally, in the second case study, a 3-port CFC prototype is built and tested in an experimental platform in the laboratory considering different control modes., Peer Reviewed, Postprint (published version)

© 2020 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, Flexible Alternating Current Transmission Systems (FACTS) have achieved to enhance the flexibility of modern AC power systems, by providing fast, reliable and controllable solutions to steer the power flows and voltages in the network. The proliferation of High Voltage Direct Current (HVDC) transmission systems is leading to the opportunity of interconnecting several HVDC systems forming HVDC Supergrids. Such grids can eventually evolve to meshed systems which interconnect a number of different AC power systems and large scale offshore wind (or other renewable sources) power plants and clusters. While such heavily meshed systems can be considered futuristic and will not certainly happen in the near future, the sector is witnessing initial steps in this direction. In order to ensure the flexibility and controllability of meshed DC grids, the shunt connected AC-DC converters can be combined with additional simple and flexible DC-DC converters which can directly control current and power through the lines. The proposed DC-DC converters can provide a range of services to the HVDC grid, including power flow control capability, ancillary services for the HVDC grid or adjacent grids, stability improvement, oscillation damping, pole balancing and voltage control. The present paper presents relevant developments from industry and academia in the direction of the development of these converters, considering technical concepts, converter functionalities and possible integration with other existing systems. The paper explores a possible vision on the development of future meshed HVDC grids and discusses the role of the proposed converters in such grids., Postprint (published version)

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, 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, Award-winning, Postprint (published version)

This paper is meant to show that an interline Current Flow Controller (CFC) is able to reduce the operating costs of hybrid AC/DC meshed grids by alleviating the congestion within the DC lines. The work considers a unidirectional DC/DC CFC with a simple structure and its average model is introduced into the mathematical formulation of the Optimal Power Flow (OPF) problem of an hybrid AC/DC meshed grid, which also considers the losses of the Voltage Source Converters (VSC). The CFC restrictions and the different connection arrangements of the device into the AC/DC grid are also included in the model and show an important effect on the OPF solution. The case studies are conducted in a 5-terminal AC/DC meshed grid and point out that the CFC has a reduced capability to reduce the operating costs of the system if there are no overloads in the DC lines. However, when the power limit of one of the lines is reached, the CFC can redirect part of the DC current and, therefore, reduce the operating costs compared to the case without CFC., Postprint (author's final draft)

Two key problems in meshed high-voltage direct current (HVDC) transmission grids are managing line power flows and protection against dc faults. Current flow controllers (CFCs) will be required to balance cable currents in meshed dc grids, in order to prevent individual line power capacity limits restricting overall power flow in the grid. Direct current circuit breakers (DCCBs) will be also required to protect HVDC grids from dc faults. This paper demonstrates that the current flow controller functionality can be added into a hybrid CB's design. This paper proposes integrating an interline CFC into the load commutation switch (LCS) of a hybrid DCCB. The integrated design LCS/CFC is analyzed and a state-space model is derived. The control of the CFC is designed and the performance of the LCS/CFC during normal operation is verified by means of MATLAB Simulink and PSCAD simulations. A comparison of the integrated LCS/CFC and the separate design is given. The case studies show a reduction in total power losses, and improved protection operation times can be achieved., Peer Reviewed, Postprint (author's final draft)

Peer Reviewed, Preprint

This study focuses on the operation and control of HVDC multi-terminal systems that transmit the power being generated in offshore wind farms to the terrestrial AC grids. The aim of the paper is to propose and validate an algorithm to ensure optimal operation of HVDC-HVAC systems. This algorithm is implemented in a central controller that, knowing the electrical characteristics of the DC and AC systems, the power generation from the wind farms and the power demand, executes periodically an AC/DC Optimal Power Flow (OPF) and sends the appropriate voltage references to the grid side converter's control. The voltage control of the DC grid is distributed and based on droop law, implemented in grid side converters. The droop offset is modified periodically so as to adapt to the actual operating conditions and ensure optimal operation according to a specified objective function. Dynamic simulations show the system optimal operation in terms of loss minimization under wind speed changes, loss of communications and demand variation. These results are validated experimentally after implementing the control scheme in an HVDC scaled experimental platform. Dynamic simulations are also performed to show that the system can still be operated based on the proposed strategy even during contingencies implying the disconnection of a power system element (converter and DC cable)., Postprint (author's final draft)

This paper presents two methods for frequency control of onshore AC grids from offshore wind power plants (OWPPs) connected to a multi-terminal DC (MTDC) grid. The first method is based on communicating the onshore frequency to the OWPP and the other voltage source converters (VSCs) in the MTDC grid. The second method is based on a coordinated strategy between the VSCs in the MTDC grid, where the onshore frequency is replicated at the offshore grid using additional control blocks implemented locally at the VSCs of the MTDC grid. The proposed control methods are first verified through simulations on a test set up with an OWPP connected to a three-terminal DC grid using DIgSILENT PowerFactory and then validated experimentally on a laboratory scaled three-terminal DC grid. The simulation and experimental results prove that, with the proposed control strategies, OWPPs and the VSCs in the MTDC grid can participate in frequency control and support the onshore grid frequency stability.

DC/DC autotransformers present several advantages compared to Front-To-Front (F2F) topologies. This work presents the Transformerless (TL) DC/DC autotransformer topology, which consists on the substitution of the AC transformer in the DC/DC autotransformer for an AC filter, with the objective of reducing its cost. The TL topology is modelled, its control presented and its operation validated by means of simulations. Also, the losses of the converter are estimated for partial and full load. © 2018 IEEE., Postprint (published version)

In meshed High Voltage Direct Current (HVDC) grids, Current Flow Controllers (CFC) may be required to avoid bottlenecks or line overloads. The present paper proposes the concept of installing Distributed Current Flow Controllers (DCFCs) in different nodes of a meshed HVDC grid that are operated selectively. This concept allows to use simplified DCFC devices to prevent the overloads, with a reduction of the required CFC voltage compared with a single CFC in the grid. First, a methodology to investigate the effect of installing DCFCs in a generic meshed HVDC grid is presented. Several case studies with a 3-terminal and a 7-terminal meshed HVDC grids are conducted and the most suitable topology for the DCFCs is derived based on the previous analysis. Then, this work analyses the increase in the operational area brought by installing DCFCs in a 3-terminal HVDC grid and it is compared with the increase achieved by a Dual H-bridge CFC in one node. The CFC losses and voltage requirement depending on the location of the DCFCs are investigated. Finally, the operation and control of the DCFCs are designed and tested in a 3-terminal meshed HVDC grid using dynamic simulations., Postprint (published version)

This paper is focused in the control design of a DC/DC Current Flow Controller (CFC) for meshed HVDC grids. It proposes a control design methodology based on an outer current loop to regulate DC grid currents and an inner voltage loop to keep under control the CFC voltage. This approach is able to limit the CFC voltage to avoid exceeding the device limitations. A linear model of the HVDC grid including the CFC is obtained to apply linear control techniques to design the regulator. The system performance is validated by means of simulations considering different case studies., Postprint (published version)

This article is focused on the modelling and control of an interline Current Flow Controller (CFC) for meshed HVDC grids. The operation states of the CFC are presented and an average model is derived. The average model is used to perform steady-state analysis on a 3-terminal meshed grid, showing the current change capabilities and the benefits on the operation area. The converter control is designed using a linearised model. The system performance of the CFC is tested by means of simulation in a 3-terminal grid and in a 5-terminal grid., Postprint (author's final draft)

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., Postprint (published version)

This paper proposes a DC/DC Current Flow Controller (CFC) for meshed High Voltage Direct Current (HVDC) grids. The CFC has the advantage of a simplified structure that allows to use minimum number of switches when unidirectional current flows through the DC lines are expected. An extended CFC topology able to operate with all the possible current flows is also presented. Then, the operating principle of the CFC is discussed and its structure is compared with a dual H-bridge analysing advantages and disadvantages. The work also details the applied modulation strategy and the control methodology. Finally, a 5-terminal meshed HVDC grid is used to validate the CFC by means of simulations., Peer Reviewed, Postprint (published version)

El present treball presenta l’anàlisi, la modelització, el control i la simulació de xarxes mallades High Voltage Direct Current (HVDC) utilitzades per transmetre a la xarxa elèctrica l’energia generada a parcs eòlics marins. La xarxa HVDC utilitza convertidors Voltage Source Converters (VSC) per tal de mantenir la tensió constant i per transformar el corrent altern a continu i viceversa. A més, a la xarxa HVDC estudiada, s’ha considerat que un dels convertidors VSC és a la costa, mentre que els altres es troben connectats a parcs eòlics marins. Addicionalment, es realitza un anàlisi, una modelització i un estudi del control de diferents tipologies de current flow controllers (CFC) per gestionar els fluxos de corrent a la xarxa mallada HVDC. També es realitza la modelització i el control d’una turbina eòlica; i s’utilitza un model agregat per tal de treballar amb dos parcs eòlics sencers. Finalment, es duu a terme la simulació de la xarxa mallada HVDC amb els models agregats dels parcs eòlics i amb els diferents tipus de current flow controllers; emprant models ideals per estudiar els VSCs. Tots els models, simulacions i càlculs del present treball s’han realitzat utilitzant el programari MATLAB SIMULINK.

En aquest treball es presenta la modelització, disseny i control d’un generador eòlic d’inducció de tipus gàbia d’esquirol (SCIG) considerant diferents metodologies de control sobre aquest. Primerament, es realitza la modelització d’una turbina eòlica, des de la captació del vent fins a l’obtenció de potència elèctrica a través del generador, connectant aquest a la xarxa. Posteriorment, aquest primer model es simula i s’estudia el seu comportament. A continuació, s’introdueix un convertidor estàtic de potència del tipus Voltage Source Converter (VSC) entre la màquina d’inducció i la xarxa elèctrica i s’implementen dues metodologies de control sobre aquest convertidor: el control Tensió-Freqüència i el Control Vectorial. Tot seguit, s’estudia el comportament cada un dels models obtinguts (un per a cada tipus de control). Finalment, es realitza una comparativa entre els tres models estudiats. L’estudi de cada model es realitza mitjançant simulació amb un software del tipus EMTP (ElectroMagnetic Transients Program).

This work presents a series tapping station for integrating Offshore Wind Power Plants (OWPP) into a (Line Commutated Converter High Voltage Direct Current) LCC-HVDC transmission system. The tapping station allows to integrate wind power resources without building a new HVDC link and it is based on a Current Source Converter (CSC). However, the CSC requires a minimum DC current to extract the power coming from the OWPP which may not be guaranteed depending on the power conditions of the HVDC corridor. For this reason, this paper proposes a coordinated operation and control of the CSC and the OWPP. A steady-state analysis is performed to determine the appropriate AC voltage level of the CSC. A power reduction algorithm is presented to limit power extraction during a reduction in the current of the HVDC transmission system and under loss of communications between the CSC and the OWPP. The proposed algorithm and the performance of the system are validated through simulation results., Peer Reviewed, Postprint (author's final draft)

Aquest TFM presenta l’an`alisi, modelitzaci´o, control i simulaci´o de convertidors de tipologia Line Commutated Converters (LCC), basats en tiristors. Aquest tipus de convertidors s´on actualment els m´es utilitzats en aplicacions High Voltage Direct Current (HVDC), donat que es troben en un estat d’alta maduresa tecnol`ogica i permeten transportar elevades pot`encies a grans dist`ancies, tant a`eries com submarines. Es realitza un an´alisi del convertidor LCC , del qual es dedueix un model simplificat: l’anomenat model promitjat. Aquest model negligeix o promitja l’efecte de la commutaci´o dels tiristors. El principal avantatge d’aquest model ´es la reducci´o del temps de simulaci´o, especialment per sistemes amb molts convertidors. Finalment, donat que les equacions que representen aquest sistema s´on no lineals, es presenten les equacions linealitzades del sistema mitjan¸cant la representaci´o en espai d’estats. Aquest model linealitzat ´es especialment ´util per l’aplicaci´o de t`ecniques de control lineal, per tant es dissenya el control del sistema mitjan¸cant el model lineal i es valida en el model complet.

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.

Aquest treball presenta l’anàlisi, la modelització, el control i la simulació d’un parc eòlic marí connectat amb un enllaç HVDC (High Voltage Direct Current). Els convertidors estudiats i emprats són els VSC (Voltage Source Converter), formant un enllaç punt a punt en contínua entre la costa i un parc eòlic marí. Es realitza també la modelització i el control d’un conjunt de turbines eòliques, com el control de l’angle de Pitch per limitar la potència de la turbina, i es fa ús del model agregat per tal d’estudiar un parc eòlic sencer. Els convertidors utilitzats dins del parc són VSC de tipus back-to-back. Finalment, es duu a terme la simulació de l’enllaç HVDC amb el model agregat del parc, utilitzant els models promitjats dels VSC, sota diferents condicions de treball per tal de validar el model emprat. La simulació es fa mitjançant el programa MATLAB SIMULINK.

Aquest projecte presenta l'anàlisi, modelització, el control i la simulació de dues tipologies de convertidors per a sistemes HVDC (High Voltage Direct Current). Els convertidors estudiats són els VSC (Voltage Source Converters) i els CSC (Current Source Converters), els quals formen un enllaç punt a punt en contínua entre la costa i un parc eòlic marí. També es realitza la modelització i control d'una turbina eòlica, i es fa ús d'un model agregat per tal de treballar amb un parc eòlic sencer. Es duu a terme la simulació dels enllaços HVDC amb el model agregat del parc, utilitzant els models promitjats dels VSC i CSC. Finalment, s'addiciona l'emulació d'una turbina eòlica a una plataforma experimental real. La plataforma en qüestió representa una xarxa multiterminal en contínua i consta de quatre màquines asíncrones d'inducció; dues de les quals emulen el vent i les altres dues, el generador. Els convertidors emprats són quatre VSC, dos d'ells actuen com a rectificadors enllaçats amb els generadors i els altres, com a inversors connectats a la xarxa alterna. L'emulació es duu a terme mitjançant l'entorn de programació LABVIEW de NATIONAL INSTRUMENTS. Es realitzen proves experimentals per tal de validar-ne la implementació.