Operation and control of a current source converter series tapping of an LCC-HVDC link for integration of offshore wind power plants

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 ReviewedPostprint (author's final draft

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

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

Fully Decou pled Controller Models for Voltage Source Converter based High Voltage DC Transmission

VSC-HVDC has two distinct advantages over its earlier generation thyristor based High Voltage DC transmission. Synchronous voltage source is not required to commutate against, for its operation and it does not suffer from commutation failures under adverse conditions in interfacing ac system. These two properties make it amenable to wider application areas. To make it adapt to operational conditions imposed on it in various applications, its controller parameters need to be assessed and tuned through extensive simulation studies. To facilitate this, two alternative controllers viz. a fully decoupled controller model and also an instantaneous theory based fully decoupled hybrid controller model are developed in the thesis. The decoupling is achieved by exploiting similarity transformation in both the controllers. In the first controller model, the Park's currents and voltages are directly obtained from the measured network variables and the reference park's currents for the inner current loop are obtained from the instantaneous measured power. In the second one, both the feedback as well as reference Park's currents for the inner current loop are obtained from the Clarke's variables. An AC system interfacing electronics based power transmission or distribution network experiences non-sinusoidal voltage and current waveforms. Instantaneous power theory being suitable for steady as well as transient states, is used for handling measured inputs. The performance of the models is assessed through SIMULINK Power system Blockset aided simulations on a VSC-HVDC link interfacing an ac system, having normal fault level, low fault level and also witnessing a single line to ground fault on its rectifier transformer primary side

Coordinated Control Of A Back-ToBack HVDC Link

This thesis will present the mathematical model, controller design, simulation, and analysis of a complete back-to-back VSC HVDC system. There will be two stations of Voltage Source Converter (VSC) located back-to-back. One station shall act as a rectifier and another one will be the inverter. This will allow power transfer from one station (rectifier) to other (inverter) through a DC link. A back-to-back VSC HVDC has the advantages compare to the AC transmission because it can allow power transfer without having to synchronize both of the station or networks. In order to complete a back-to-back VSC HVDC system, it will require a d-q current controller, DC voltage controller, active/reactive power controller and DC power (DC current controller). A simulation and analysis shall be executed for each of the controller and response of voltage and current for a complete system. The simulation result will show the dynamic control of the rectifier and inverter is achieved with fast response

High Voltage Direct Current Energy Transmission Using Modular Multilevel Converters

This thesis focus on high voltage direct current (HVdc) energy transmission using modular multilevel converter (MMC) based terminals. It provides a brief comparison between different HVdc technologies, focusing on voltage source converters (HVdc-VSC) with the MMC-based terminal emerging as the topology of choice for ratings less than 1 GW. The controllers for a two-terminal HVdc-link are analyzed and Matlab/SimulinkTM simulation models are presented. The simplified models and full Matlab/SimulinkTM based model are used to select the gains for the MMC controllers. Simulation results carried out on the full model validated the proposed methodologies. A new control technique that eliminates the voltage sensors on the grid side normally used to synchronize the MMC-based terminal with the grid is proposed. The performance of proposed technique was evaluated through Matlab/SimulinkTM simulations by considering inverter operation. The sensorless technique is able to synchronize a MMC-based inverter terminal to a grid under non-ideal conditions as well to accurately detect changes in the grid voltages. Finally, an analysis of the impact that a 15-kV SiC IGBT would have on HVdc MMC-based terminals is presented. The analysis evaluates parasitic inductances within the sub-module (SM) of an MMC, changes on the required SM capacitance, and impact on the voltage waveform THD. The evaluations showed that the 15-kV SiC IGBT would be only suitable if the module is rated 400 A or greater

A New MMC Topology Which Decreases the Sub Module Voltage Fluctuations at Lower Switching Frequencies and Improves Converter Efficiency

Modular Multi-level inverters (MMCs) are becoming more common because of their suitability for applications in smart grids and multi-terminal HVDC transmission networks. The comparative study between the two classic topologies of MMC (AC side cascaded and DC side cascaded topologies) indicates some disadvantages which can affect their performance. The sub module voltage ripple and switching losses are one of the main issues and the reason for the appearance of the circulating current is sub module capacitor voltage ripple. Hence, the sub module capacitor needs to be large enough to constrain the voltage ripple when operating at lower switching frequencies. However, this is prohibitively uneconomical for the high voltage applications. There is always a trade off in MMC design between the switching frequency and sub module voltage ripple

A PWM current source-based DC transmission system for multiple wind turbine interfacing

A pulsewidth modulation (PWM) current source wind energy conversion system based on a parallel configuration for high voltage direct current application is proposed. A comparison between the parallel and series configurations for current source-based systems is investigated, which shows the merits of the proposed system. A new control technique for the PWM current source inverter is proposed. It can effectively control the average dc-link voltage with a feed-forward loop, while independently controlling reactive power according to grid code requirements. The system simulation confirms the performance of the proposed system with no interaction between wind turbine modules and satisfying performance with grid integration. Practical implementation further verifies the proposed inverter control. Finally, a brief comparison between conventional line-commutated converter-based systems and the proposed PWM current source converter-based system is presented

Modeling of multi-terminal VSC-based HVDC systems

Improving the efficiency and operation of power transmission is important due to the continual increase in demand for electric power. In addition, many remote areas throughout the world lack sufficient access to electricity. Unfortunately, utilities cannot satisfy the high demand of power by building new power stations because of economic and environmental reasons. However, utilities can increase generation and transmission line efficiencies by controlling the power flow through their systems. One new attractive technology that enables the control of power flow in the system is Voltage-Source-Converter High Voltage Direct Current (VSC-HVDC) transmission. Multi-terminal-HVDC (M-HVDC) can be built using VSC technology. A model of a three-terminal VSC-HVDC system is presented in this thesis. One of the converters is used to regulate the DC voltage while the others converters control the active power independently and bi-directionally. The vector control strategy and pulse width modulation (PWM) technique are described and implemented in PSCAD/EMTDC. In addition, the region of controllability as a function of power flow has been analyzed. Furthermore, the steady-state and dynamic response characteristics as a function of capacitor size has been investigated --Abstract, page iii