Hybrid AC-High Voltage DC Grid Stability and Controls

abstract: The growth of energy demands in recent years has been increasing faster than the expansion of transmission facility construction. This tendency cooperating with the continuous investing on the renewable energy resources drives the research, development, and construction of HVDC projects to create a more reliable, affordable, and environmentally friendly power grid. Constructing the hybrid AC-HVDC grid is a significant move in the development of the HVDC techniques; the form of dc system is evolving from the point-to-point stand-alone dc links to the embedded HVDC system and the multi-terminal HVDC (MTDC) system. The MTDC is a solution for the renewable energy interconnections, and the MTDC grids can improve the power system reliability, flexibility in economic dispatches, and converter/cable utilizing efficiencies. The dissertation reviews the HVDC technologies, discusses the stability issues regarding the ac and HVDC connections, proposes a novel power oscillation control strategy to improve system stability, and develops a nonlinear voltage droop control strategy for the MTDC grid. To verify the effectiveness the proposed power oscillation control strategy, a long distance paralleled AC-HVDC transmission test system is employed. Based on the PSCAD/EMTDC platform simulation results, the proposed power oscillation control strategy can improve the system dynamic performance and attenuate the power oscillations effectively. To validate the nonlinear voltage droop control strategy, three droop controls schemes are designed according to the proposed nonlinear voltage droop control design procedures. These control schemes are tested in a hybrid AC-MTDC system. The hybrid AC-MTDC system, which is first proposed in this dissertation, consists of two ac grids, two wind farms and a five-terminal HVDC grid connecting them. Simulation studies are performed in the PSCAD/EMTDC platform. According to the simulation results, all the three design schemes have their unique salient features.Dissertation/ThesisDoctoral Dissertation Electrical Engineering 201

A novel backup protection scheme for hybrid AC/DC power systems

This thesis presents and demonstrates (both via simulation and hardware-based tests) a new protection scheme designed to safeguard hybrid AC/DC distribution networks against DC faults that are not cleared by the main MVDC (Medium Voltage DC) link protection. The protection scheme relies on the apparent impedance measured at the AC "side" of the MVDC link to detect faults on the DC system. It can be readily implemented on existing distance protection relays with no changes to existing measuring equipment. An overview of the literature in this area is presented and it is shown that the protection of MVDC links is only considered at a converter station level. There appears to be no consideration of protecting the MVDC system from the wider AC power system via backup - as would be the case for standard AC distribution network assets, where the failure of main protection would require a (usually remote) backup protection system to operate to clear the fault. Very little literature considers remote backup protection of MVDC links.;To address this issue, the research presented in this thesis characterises the apparent impedance as measured in the neighbouring AC system under various DC fault conditions on an adjacent MVDC link. Initial studies, based on simulations, show that a highly inductive characteristic, in terms of the calculations from the measured AC voltages and currents, is apparent on all three phases in the neighbouring AC system during DC-side pole-to-pole and pole-poleground faults. This response is confirmed via a series of experiments conducted at low voltage in a laboratory environment using scaled down electrical components. From this classification, a fast-acting backup protection methodology, which can detect pole-to-pole and pole-poleground faults within 40 ms, is proposed and trialled through simulation. The solution can be deployed on distance protection relays using a typically unused zone (e.g. zone 4).;New relays could, of course, incorporate this functionality as standard in the future. To maximise confidence and demonstrate the compatibility of the solution, the protection scheme is deployed under a real-time hardware-in-the-loop environment using a commercially available distance protection relay. Suggestions to improve the stability of the proposed solution are discussed and demonstrated. Future areas of work are identified and described. As an appendix, early stage work pertaining to the potential application and benefits of MVDC is presented for two Scottish distribution networks. The findings from this are presented as supplementary material at the end of the thesis.This thesis presents and demonstrates (both via simulation and hardware-based tests) a new protection scheme designed to safeguard hybrid AC/DC distribution networks against DC faults that are not cleared by the main MVDC (Medium Voltage DC) link protection. The protection scheme relies on the apparent impedance measured at the AC "side" of the MVDC link to detect faults on the DC system. It can be readily implemented on existing distance protection relays with no changes to existing measuring equipment. An overview of the literature in this area is presented and it is shown that the protection of MVDC links is only considered at a converter station level. There appears to be no consideration of protecting the MVDC system from the wider AC power system via backup - as would be the case for standard AC distribution network assets, where the failure of main protection would require a (usually remote) backup protection system to operate to clear the fault. Very little literature considers remote backup protection of MVDC links.;To address this issue, the research presented in this thesis characterises the apparent impedance as measured in the neighbouring AC system under various DC fault conditions on an adjacent MVDC link. Initial studies, based on simulations, show that a highly inductive characteristic, in terms of the calculations from the measured AC voltages and currents, is apparent on all three phases in the neighbouring AC system during DC-side pole-to-pole and pole-poleground faults. This response is confirmed via a series of experiments conducted at low voltage in a laboratory environment using scaled down electrical components. From this classification, a fast-acting backup protection methodology, which can detect pole-to-pole and pole-poleground faults within 40 ms, is proposed and trialled through simulation. The solution can be deployed on distance protection relays using a typically unused zone (e.g. zone 4).;New relays could, of course, incorporate this functionality as standard in the future. To maximise confidence and demonstrate the compatibility of the solution, the protection scheme is deployed under a real-time hardware-in-the-loop environment using a commercially available distance protection relay. Suggestions to improve the stability of the proposed solution are discussed and demonstrated. Future areas of work are identified and described. As an appendix, early stage work pertaining to the potential application and benefits of MVDC is presented for two Scottish distribution networks. The findings from this are presented as supplementary material at the end of the thesis

Modular multilevel converter based HVDC transmission system for offshore wind farms

This doctoral thesis falls within the scope of electronic power converters oriented to high voltage transmission applications, in particular the power generated in remote offshore wind farms by means of HVDC subsea cables. This research is focused on the Modular Multilevel Converter (MMC) with two level submodules but also with multilevel topology submodules such as 3L-FC (three level flying capacitors) and 3L-NPC (three level neutral point capacitors). The main contribution of this thesis is the developed PWM based modulation strategy which allows the balancing of the total amount of submodules capacitors. It is applicable to the aforementioned submodule topologies under different working conditions as evidenced by experimental results

Power Electronics Converter Interfaced Power System Emulation Platform Development and Research Demonstration

To provide a more realistic testing environment for power system studies and demonstrations, a hardware test-bed (HTB) platform has been developed by the CURENT at the University of Tennessee, Knoxville, to emulate power systems by programming interconnected three-phase voltage converters (VSCs) to behave like the intended power system components.In this dissertation, several platforms are developed to extend the HTB emulation capability, including a versatile power electronics converter based transmission line emulator, a hybrid emulation platform with the HTB and the real time digital simulator (RTDS), and a flexible modular multilevel converter (MMC) test-bed for HVDC applications.For the versatile power electronics converter based transmission line emulator, the algorithms for emulating transmission line lumped model, traveling wave model, transmission line with integrated compensation devices, and faults are proposed and implemented. A combined model is proposed to avoid the switching transients between the normal state and the fault state. Stability analysis is also conducted to locate the boundary conditions of emulating the transmission line stably.For the hybrid emulation with the HTB and the RTDS, two hybrid emulation interfaces are developed to emulate the subsystems of a large system in the HTB and RTDS separately and simultaneously to perform as a whole system. An interface model by combining two complementary ideal transformer model (ITM) interface algorithms are implemented to realize the hybrid emulation stably under different conditions. A time-delay correction method is proposed to compensate the interface time-delay to improve the hybrid emulation accuracy. An analysis of the hybrid emulation stability with two interfaces is also conducted to provide guidance on the interface algorithm selection.For the flexible MMC prototype development, a test-bed with 10 full-bridge submodules (SMs) in each arm and flexible topology, switching frequency, and passive component parameters reconfiguration capabilities is developed. A cost-effective method is also proposed and demonstrated to pre-charge the MMC by utilizing a low voltage dc source.In addition, the potential station transformer saturation issue of line-commutated converter (LCC) HVDC in hybrid ac/dc transmissions is evaluated. The dc fault impact on connected ac system stability is also evaluated by comparing with an equivalent ac fault

DC current flow controllers for meshed HVDC grids

Premi Extraordinari de Doctorat, promoció 2018-2019. Àmbit d’Enginyeria IndustrialMeshed 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-winningPostprint (published version

Design, Control and Protection of Modular Multilevel Converter (MMC)-Based Multi-Terminal HVDC System

Even though today’s transmission grids are predominantly based on the high voltage alternating current (HVAC) scheme, interests on high voltage direct current (HVDC) are growing rapidly during the past decade, due to the increased penetration of remote renewable energy. Voltage source converter (VSC) type is preferred over the traditional line-commutated converter (LCC) for this application, due to the advantages like smaller station footprint and no need for strong interfacing ac grid. As the state-of-the-art VSC topology, modular multilevel converter (MMC) is mostly considered. Most renewable energy sources, such as wind and solar, is usually sparsely located. Multi-terminal HVDC (MTDC) provides better use of transmission infrastructure, higher transmission flexibility and reliability, than building multiple point-to-point HVDCs. This dissertation studies the MMC-based MTDC system, including design, control and protection. Passive components design methodology in MMC is developed, with practical consideration. The developed arm inductance selection criterion considers the implementation of circulating current suppression control. And the unbalanced voltage among submodule capacitor is taken into account for submodule capacitance design. Circulating current suppression control is found to impact the MMC operating range. The maximum modulation index reduction is calculated utilizing a decoupled MMC model. A four-terminal HVDC testbed is developed, with similar control and communication architectures of the practical projects implemented. Several most typical operation scenarios and controls are demonstrated or proposed. In order to allow HVDC disconnects to online trip a line, dc line current control is proposed through station control. Utilizing the dc line current control, an automatic dc line current limiting control is proposed. Both controls have been verified in the developed testbed. A systematic dc fault protection strategy of MTDC utilizing hybrid dc circuit breaker is developed, including a new fast and selective fault detection method taking advantage of the hybrid dc circuit breaker special operation mechanism. Detailed criteria and control methods to assist system recovery are presented. A novel fault tolerant MMC topology is proposed with a hybrid submodule by adding an ultra-fast mechanical switch. The converter power loss can be almost the same as the half-bridge MMC, and 1/3 reduction compared to the similar clamp-double topology

Convertisseurs modulaires multiniveaux pour le transport d'énergie électrique en courant continu haute tension

Les travaux présentés dans ce mémoire ont été réalisés dans le cadre d’une collaboration entre le LAboratoire PLAsma et Conversion d’Énergie (LAPLACE), Université de Toulouse, et la Seconde Université de Naples (SUN). Ce travail a reçu le soutien de la société Rongxin Power Electronics (Chine) et traite de l’utilisation des convertisseurs multi-niveaux pour le transport d’énergie électrique en courant continu Haute Tension (HVDC). Depuis plus d’un siècle, la génération, la transmission, la distribution et l’utilisation de l’énergie électrique sont principalement basées sur des systèmes alternatifs. Les systèmes HVDC ont été envisagés pour des raisons techniques et économiques dès les années 60. Aujourd’hui il est unanimement reconnu que ces systèmes de transport d’électricité sont plus appropriés pour les lignes aériennes au-delà de 800 km de long. Cette distance limite de rentabilité diminue à 50 km pour les liaisons enterrées ou sous-marines. Les liaisons HVDC constituent un élément clé du développement de l’énergie électrique verte pour le XXIème siècle. En raison des limitations en courant des semi-conducteurs et des câbles électriques, les applications à forte puissance nécessitent l’utilisation de convertisseurs haute tension (jusqu’à 500 kV). Grâce au développement de composants semi-conducteurs haute tension et aux architectures multicellulaires, il est désormais possible de réaliser des convertisseurs AC/DC d’une puissance allant jusqu’au GW. Les convertisseurs multi-niveaux permettent de travailler en haute tension tout en délivrant une tension quasi-sinusoïdale. Les topologies multi-niveaux classiques de type NPC ou « Flying Capacitor » ont été introduites dans les années 1990 et sont aujourd’hui couramment utilisées dans les applications de moyenne puissance comme les systèmes de traction. Dans le domaine des convertisseurs AC/DC haute tension, la topologie MMC (Modular Multilevel Converter), proposée par le professeur R. Marquardt (Université de Munich, Allemagne) il y a dix ans, semble particulièrement intéressante pour les liaisons HVDC. Sur le principe d’une architecture de type MMC, le travail de cette thèse propose différentes topologies de blocs élémentaires permettant de rendre le convertisseur AC/DC haute tension plus flexible du point de vue des réversibilités en courant et en tension. Ce document est organisé de la manière suivante. Les systèmes HVDC actuellement utilisés sont tout d’abord présentés. Les configurations conventionnelles des convertisseurs de type onduleur de tension (VSCs) ou de type onduleur de courant (CSCs) sont introduites et les topologies pour les systèmes VSC sont ensuite plus particulièrement analysées. Le principe de fonctionnement de la topologie MMC est ensuite présenté et le dimensionnement des éléments réactifs est développé en considérant une commande en boucle ouverte puis une commande en boucle fermée. Plusieurs topologies de cellules élémentaires sont proposées afin d’offrir différentes possibilités de réversibilité du courant ou de la tension du côté continu. Afin de comparer ces structures, une approche analytique de l’estimation des pertes est développée. Elle permet de réaliser un calcul rapide et direct du rendement du système. Une étude de cas est réalisée en considérant la connexion HVDC d’une plateforme éolienne off-shore. La puissance nominale du système étudié est de 100 MW avec une tension de bus continu égale à 160 kV. Les différentes topologies MMC sont évaluées en utilisant des IGBT ou des IGCT en boitier pressé. Les simulations réalisées valident l’approche analytique faite précédemment et permettent également d’analyser les modes de défaillance. L’étude est menée dans le cas d’une commande MLI classique avec entrelacement des porteuses. Enfin, un prototype triphasé de 10kW est mis en place afin de valider les résultats obtenus par simulation. Le système expérimental comporte 18 cellules de commutations et utilise une plate-forme DSP-FPGA pour l’implantation des algorithmes de commande. ABSTRACT : This work was performed in the frame of collaboration between the Laboratory on Plasma and Energy Conversion (LAPLACE), University of Toulouse, and the Second University of Naples (SUN). This work was supported by Rongxin Power Electronic Company (China) and concerns the use of multilevel converters in High Voltage Direct Current (HVDC) transmission. For more than one hundred years, the generation, the transmission, distribution and uses of electrical energy were principally based on AC systems. HVDC systems were considered some 50 years ago for technical and economic reasons. Nowadays, it is well known that HVDC is more convenient than AC for overhead transmission lines from 800 - 1000 km long. This break-even distance decreases up to 50 km for underground or submarine cables. Over the twenty-first century, HVDC transmissions will be a key point in green electric energy development. Due to the limitation in current capability of semiconductors and electrical cables, high power applications require high voltage converters. Thanks to the development of high voltage semiconductor devices, it is now possible to achieve high power converters for AC/DC conversion in the GW power range. For several years, multilevel voltage source converters allow working at high voltage level and draw a quasi-sinusoidal voltage waveform. Classical multilevel topologies such as NPC and Flying Capacitor VSIs were introduced twenty years ago and are nowadays widely used in Medium Power applications such as traction drives. In the scope of High Voltage AC/DC converters, the Modular Multilevel Converter (MMC), proposed ten years ago by Professor R. Marquardt from the University of Munich (Germany), appeared particularly interesting for HVDC transmissions. On the base of the MMC principle, this thesis considers different topologies of elementary cells which make the High Voltage AC/DC converter more flexible and easy suitable respect to different voltage and current levels. The document is organized as follow. Firstly, HVDC power systems are introduced. Conventional configurations of Current Source Converters (CSCs) and Voltage Source Converters (VSCs) are shown. The most attractive topologies for VSC-HVDC systems are analyzed. The operating principle of the MMC is presented and the sizing of reactive devices is developed by considering an open loop and a closed loop control. Different topologies of elementary cells offer various properties in current or voltage reversibility on the DC side. To compare the different topologies, an analytical approach on the power losses evaluation is achieved which made the calculation very fast and direct. A HVDC link to connect an off-shore wind farm platform is considered as a case study. The nominal power level is 100 MW with a DC voltage of 160 kV. The MMC is rated considering press-packed IGBT and IGCT devices. Simulations validate the calculations and also allow analyzing fault conditions. The study is carried out by considering a classical PWM control with an interleaving of the cells. In order to validate calculation and the simulation results, a 10kW three-phase prototype was built. It includes 18 commutation cells and its control system is based on a DSP-FGPA platform

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

Implementation of control schemes on a DC-AC terminal based on a HVDC-MMC

Los sistemas de transmisión de energía eléctrica a alta tensión en corriente directa o High Voltage Direct Current (HVDC) se han convertido en una solución atractiva para la integración de fuentes de energía renovables cómo las granjas eólicas que se encuentran alejadas de los centros de consumo.Tradicionalmente esta energía es transmitida mediante enlaces de corriente alterna (ac) lo que acarrea costos técnicos y económicos muy elevados. Los sistemas HVDC reducen las pérdidas que están asociadas a los efectos capacitivos de las líneas de transmisión ya que no dependen de la frecuencia y la distancia a la cual se transmite la energía por lo tanto no existe la necesidad de sistemas de compensación, utilizan un espacio más reducido para la ubicación de torres y las pérdidas de energía por conducción se reducen considerablemente. Los sistemas HVDC en su estructura topológica cuentan con estaciones convertidoras que realizan el proceso de conversión de energía ac-dc y dc-ac respectivamente, estas estaciones convertidoras se basan en la electrónica de potencia cuya configuración permite realizar la conversión de la energía y ser transmitida a largas distancias. Los sistemas de control requeridos por las estaciones convertidoras son de alta complejidad y no es una tarea sencilla diseñarlos debido a que el número de variables a regular es bastante alto, cómo las corrientes circulantes por los brazos, corrientes de salida, tensión en los capacitores entre otras. El objetivo principal de este proyecto es diseñar una técnica de control adecuada para un terminal HVDC basado en Modular Multilevel Converter (MMC), que garantice la estabilidad del sistema frente a pequeñas perturbaciones en las principales variables eléctricas, cómo por ejemplo cuando se presentan desbalances en la tensión y corriente de la red eléctrica, desbalance de tensión en los capacitores, reducción de las corrientes circulantes y el balance de energía en el terminal MMC. En el proyecto se realizará el modelado de un terminal MMC teniendo en cuenta el comportamiento dinámico del sistema, también se desarrollarán simulaciones del modelo en MATLAB Simulink para realizar la validación del modelo y diseñar las diferentes técnicas de control requeridas para cada tipo de variable.High voltage direct current or High Voltage Direct Current (HVDC) electric power transmission systems have become an attractive solution for the integration of renewable energy sources such as wind farms that are far from consumption centers. Traditionally, this energy is transmitted through alternating current (ac) links, which entails very high technical and economic costs. HVDC systems reduce the losses that are associated with the capacitive effects of the transmission lines since they do not depend on the frequency and the distance at which the energy is transmitted, therefore there is no need for compensation systems, they use a space smaller for the location of towers and energy losses by conduction are considerably reduced. HVDC systems in their topological structure have converter stations that carry out the AC-DC and DC-AC energy conversion process respectively, these converter stations are based on power electronics whose configuration allows energy to be converted and transmitted. a long distance. The control systems required by the converter stations are highly complex and it is not an easy task to design them because the number of variables to regulate is quite high, such as the circulating currents through the arms, output currents, voltage in the capacitors between others. The main objective of this project is to design a suitable control technique for an HVDC terminal based on the Modular Multilevel Converter (MMC), which guarantees the stability of the system against small disturbances in the main electrical variables, such as when there are imbalances in the voltage and current of the electrical network, voltage imbalance in the capacitors, reduction of circulating currents and the energy balance in the MMC terminal. In the project, the modeling of an MMC terminal will be carried out taking into account the dynamic behavior of the system, simulations of the model will also be developed in MATLAB Simulink to carry out the validation of the model and design the different control techniques required for each type of variable. The design will be implemented in a digital signal processor or Digital Signal Processing (DSP) to be validated in a prototype of the proposed system.MaestríaMagíster en Ingeniería EléctricaContents pág. 1 Introduction 2 Objectives 3 2.1 General Objective . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3 2.2 Specific Objectives . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3 3 Literature Review 5 3.1 High Voltage Direct Current (HVDC) . . . . . . . . . . . . . . . . . . . 5 3.1.1 Advantages . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6 3.1.2 Disadvantages . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7 3.2 Most essential projects of HVDC systems . . . . . . . . . . . . . . . . . 8 3.2.1 ABB implementations [1] . . . . . . . . . . . . . . . . . . . . . . 8 3.2.2 Siemens Implementation [2] . . . . . . . . . . . . . . . . . . . . 8 3.3 HVDC Topologies . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9 3.3.1 Bipolar Set-Up . . . . . . . . . . . . . . . . . . . . . . . . . . . 10 3.3.2 Homopolar Set-Up . . . . . . . . . . . . . . . . . . . . . . . . . 10 3.3.3 Back to Back Set-Up . . . . . . . . . . . . . . . . . . . . . . . . 11 3.3.4 Pole to Pole Set-Up . . . . . . . . . . . . . . . . . . . . . . . . . 11 3.4 Power Electronics Converters . . . . . . . . . . . . . . . . . . . . . . . 12 3.5 Line Commutated Converter (LCC) . . . . . . . . . . . . . . . . . . . . 12 3.5.1 LCC Components . . . . . . . . . . . . . . . . . . . . . . . . . . 13 3.6 Voltage Source Converter (VSC) . . . . . . . . . . . . . . . . . . . . . . 15 3.6.1 VSC Components . . . . . . . . . . . . . . . . . . . . . . . . . . 15 4 Modular Multilevel Converter 19 4.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19 4.2 Submodule Operation . . . . . . . . . . . . . . . . . . . . . . . . . . . 19 4.3 Operating Principle . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20 4.3.1 Currents Relation . . . . . . . . . . . . . . . . . . . . . . . . . . 22 4.3.2 Circulating Current . . . . . . . . . . . . . . . . . . . . . . . . . 23 4.4 System Model . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24 4.4.1 MMC Averaged Dynamic Model . . . . . . . . . . . . . . . . . 24 4.4.2 Dynamic Performance of the MMC . . . . . . . . . . . . . . . . 26 4.4.3 Selection of the Mean Sum Capacitor Voltages . . . . . . . . . . 27 4.4.4 Averaging Principle . . . . . . . . . . . . . . . . . . . . . . . . . 28 4.5 Design Considerations of the MMC . . . . . . . . . . . . . . . . . . . . 31 4.5.1 Design of The Submodule Capacitance CSM . . . . . . . . . . . 31 4.5.2 Arm Inductance Design . . . . . . . . . . . . . . . . . . . . . . 35 4.6 Modulation Methods . . . . . . . . . . . . . . . . . . . . . . . . . . . . 36 4.6.1 Carrier Disposition PWM . . . . . . . . . . . . . . . . . . . . . 37 4.6.2 Carrier Phase Shifted PWM . . . . . . . . . . . . . . . . . . . . 38 5 Control Schemes Applied to an MMC Terminal 41 5.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 41 5.2 Proportional Integral Control (PI) . . . . . . . . . . . . . . . . . . . . . 41 5.2.1 Active and Reactive Power Control . . . . . . . . . . . . . . . . 43 5.2.2 Internal Current Control . . . . . . . . . . . . . . . . . . . . . . 44 5.2.3 Average Voltage Control . . . . . . . . . . . . . . . . . . . . . . 46 5.2.4 Single Voltage Control . . . . . . . . . . . . . . . . . . . . . . . 48 5.2.5 Reference Signal . . . . . . . . . . . . . . . . . . . . . . . . . . 48 5.3 Proportional Integral Resonant Control (PIR) . . . . . . . . . . . . . . 49 5.3.1 Time Delays . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 51 5.3.2 Output Current Control . . . . . . . . . . . . . . . . . . . . . . 52 5.3.3 Circulating Current Control . . . . . . . . . . . . . . . . . . . . 56 5.3.4 Reference Signal . . . . . . . . . . . . . . . . . . . . . . . . . . 59 5.3.5 Hybrid Voltage Control . . . . . . . . . . . . . . . . . . . . . . . 60 5.3.6 Phase Locked Loop (PLL) . . . . . . . . . . . . . . . . . . . . . 63 5.4 Model Predictive Control (MPC) . . . . . . . . . . . . . . . . . . . . . 65 6 Simulation and Results 71 6.1 Output Voltage Response . . . . . . . . . . . . . . . . . . . . . . . . . 73 6.2 Output Current Response . . . . . . . . . . . . . . . . . . . . . . . . . 73 6.3 SM Capacitor Voltage Response . . . . . . . . . . . . . . . . . . . . . . 76 6.4 Circulating current Response . . . . . . . . . . . . . . . . . . . . . . . . 76 6.5 Small Signal Analysis . . . . . . . . . . . . . . . . . . . . . . . . . . . . 76 7 Experimental Results on a Scale Down MMC Prototype 83 7.1 Design Considerations for the Experimental MMC . . . . . . . . . . . . 85 7.1.1 MMC Submodule . . . . . . . . . . . . . . . . . . . . . . . . . . 85 7.2 Measurement Stage . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 88 7.2.1 Current Transducer . . . . . . . . . . . . . . . . . . . . . . . . . 88 7.2.2 Voltage Transducer . . . . . . . . . . . . . . . . . . . . . . . . . 89 7.3 Control Stage . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 90 7.4 Experimental Waveforms of the MMC . . . . . . . . . . . . . . . . . . 94 8 Conclusion and Remarks 99 References 10