Modeling and enhanced control of hybrid full bridge–half bridge MMCs for HVDC grid studies

Modular multilevel converters (MMCs) are expected to play an important role in future high voltage direct current (HVDC) grids. Moreover, advanced MMC topologies may include various submodule (SM) types. In this sense, the modeling of MMCs is paramount for HVDC grid studies. Detailed models of MMCs are cumbersome for electromagnetic transient (EMT) programs due to the high number of components and large simulation times. For this reason, simplified models that reduce the computation times while reproducing the dynamics of the MMCs are needed. However, up to now, the models already developed do not consider hybrid MMCs, which consist of different types of SMs. In this paper, a procedure to simulate MMCs having different SM topologies is proposed. First, the structure of hybrid MMCs and the modeling method is presented. Next, an enhanced procedure to compute the number of SMs to be inserted that takes into account the different behavior of full-bridge SMs (FB-SMs) and half-bridge submodules (HB-SMs) is proposed in order to improve the steady-state and dynamic response of hybrid MMCs. Finally, the MMC model and its control are validated by means of detailed PSCAD simulations for both steady-state and transients conditions (AC and DC faults)

Control and Protection of MMC-Based HVDC Systems: A Review

The voltage source converter (VSC) based HVDC (high voltage direct current system) offers the possibility to integrate other renewable energy sources (RES) into the electrical grid, and allows power flow reversal capability. These appealing features of VSC technology led to the further development of multi-terminal direct current (MTDC) systems. MTDC grids provide the possibility of interconnection between conventional power systems and other large-scale offshore sources like wind and solar systems. The modular multilevel converter (MMC) has become a popular technology in the development of the VSC-MTDC system due to its salient features such as modularity and scalability. Although, the employment of MMC converter in the MTDC system improves the overall system performance. However, there are some technical challenges related to its operation, control, modeling and protection that need to be addressed. This paper mainly provides a comprehensive review and investigation of the control and protection of the MMC-based MTDC system. In addition, the issues and challenges associated with the development of the MMC-MTDC system have been discussed in this paper. It majorly covers the control schemes that provide the AC system support and state-of-the-art relaying algorithm/ dc fault detection and location algorithms. Different types of dc fault detection and location algorithms presented in the literature have been reviewed, such as local measurement-based, communication-based, traveling wave-based and artificial intelligence-based. Characteristics of the protection techniques are compared and analyzed in terms of various scenarios such as implementation in CBs, system configuration, selectivity, and robustness. Finally, future challenges and issues regarding the development of the MTDC system have been discussed in detail

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

A novel multilevel DC - AC converter from green energy power generators using step-square waving and PWM technique

Green energy sources or renewable energy system generally utilize modular approach in their design. This sort of power sources are generally in DC form or in single cases AC. Due to high fluctuation in the natural origin of this energy (wind & solar) source they are stored as DC. DC power however are difficult to transfer over long distances hence DC to AC converters and storage system are very important in green energy system design. In this work we have designed a novel multilevel DC to AC converter that takes into account the modular design of green energy systems. A power conversion efficiency of 99% with reduced total harmonic distortion (THD) was recorded from our simulated system design

DC fault detection and location in meshed multi-terminal HVDC systems based on DC reactor voltage change rate

The change rate of the DC reactor voltage with predefined protection voltage thresholds is proposed to provide fast and accurate DC fault detection in a meshed multi-terminal HVDC system. This is equivalent to the measurement of the second derivative of the DC current but has better robustness in terms of EMI noise immunization. In addition to fast DC fault detection, the proposed scheme can also accurately discriminate the faulty branch from the healthy ones in a meshed DC network by considering the voltage polarities and amplitudes of the two DC reactors connected to the same converter DC terminal. Fast fault detection leads to lower fault current stresses on DC circuit breakers and converter equipment. The proposed method requires no telecommunication, is independent of power flow direction, and is robust to fault resistance variation. Simulation of a meshed three-terminal HVDC system demonstrates the effectiveness of the proposed DC fault detection scheme

Enhanced control of offshore wind farms connected to MTDC network using partially selective DC fault protection

In recent years, several DC fault clearance schemes have emerged, in which reduced number of fast acting DC circuit breakers (DCCBs) and AC circuit breakers (ACCBs) are used to clear DC faults. In offshore DC grids, such approach entails opening of the ACCBs that connect the wind farms to the offshore HVDC stations which control offshore AC voltages and frequencies, potentially leading to uncontrolled offshore voltage and frequency. Existing studies show that the loss of offshore converter due to blocking or sudden opening of ACCBs can cause significant over-voltage and over-frequency in the offshore AC grid, which could necessitate immediate shutdown of the wind farm. An enhanced control for wind turbine converters (WTCs) of the offshore wind farm is proposed to enable retention of AC voltage and frequency control when the offshore converter is lost, in which seamless transition of the WTCs between grid following and forming modes is facilitated. The viability of the proposed control is demonstrated in wider context of partially selective DC fault protection in an illustrative meshed DC grid, which includes detailed implementations of DC fault clearance, system restart and power transfer resumption. The presented simulation results confirm the effectiveness of the proposed WTC control in preventing excessive rise of offshore AC voltage and frequency and facilitating DC fault ride-through using reduced number of DCCBs

Small signal stability analysis of proportional resonant controlled VSCs connected to AC grids with variable X/R characteristic

Capítuos 2,3 y 4 confidenciales por patente.-- Tesis completa 237.p. Tesis censurada 120 p.Para garantizar un futuro energético sostenible, es fundamental la incorporación de energías renovables en la red eléctrica. Sin embargo, con su creciente integración, las redes eléctricas AC se están volviendo cada vez más débiles, más complejas y caóticas. Por ello, se hace imprescindible el estudio de los retos técnicos que dicha integración plantea. Fenómenos como la desconexión de líneas AC, el bloqueo de convertidores, o variaciones de carga debidas a las intermitencias de la generación renovable, están comenzando a producir cambios en los valores de impedancia y en las características inductivo-resistivas de incluso las redes fuertes. Conforme una red AC se debilita su impedancia equivalente aumenta, y esto provoca cambios indeseados en las magnitudes de potencia activa y reactiva, que derivan en variaciones repentinas de tensión en diferentes puntos de la red AC. Esto también conlleva el deterioro de los convertidores y empeoramiento de la calidad de onda. Una solución parcial a este problema es limitar la potencia allí donde se genera, en perjuicio de aumentar las pérdidas locales. Otra solución es introducir controles de convertidores más robustos, para que sean capaces de sortear estos escenarios cada vez más frecuentes. En este contexto, los convertidores de fuente de tensión (VSC), y en especial los convertidores modulares multinivel, presentan una serie de prestaciones que los hacen idóneos para esta clase de escenarios, dado su mejor comportamiento dinámico frente a los convertidores de fuente de corriente, al operar a una frecuencia de conmutación mayor, y presentando capacidades LVRT y control desacoplado de potencia activa y reactiva. Entre los controles internos de corriente de los VSCs, los controladores proporcional resonantes han aparecido como alternativa a los proporcional integrales, debido a su capacidad de manejar operación tanto equilibrada como desequilibrada y a que eliminan lanecesidad de utilizar un phase-locked loop y las transformadas de Park. Muy pocos estudios se han realizado con VSCs con control proporcional resonante sujetos a cambios en la fortaleza de la red AC, y menos aun considerando la variación de su característica inductivo-resistiva. Por lo tanto, en esta tesis doctoral se propone una metodología de parametrización del control proporcional resonante de un VSC conectado a una red AC con fortaleza y característica inductivo-resistiva variables, que asegure su estabilidad en pequeña señal. Con el objetivo de caracterizar dicha estabilidad, se construye un modelo de pequeña señal del sistema compuesto por el VSC conectado a red AC. Posteriormente se valida con simulaciones EMT y se procede con el análisis de escenarios. Los resultados del análisis demuestran que tan solo una desviación del 20% en el ratio X/R de la red AC con respecto a su valor habitual puede hacer perder al sistema su estabilidad en pequeña señal cuando la red AC es débil. La metodología propone nuevas parametrizaciones del control proporcional resonante del VSC que devuelven la estabilidad al sistema en estos escenarios. La validación y verificación de la metodología se realiza a través de un caso de estudio en DIgSILENT PF: una planta de generación eólica marina que evacúa energía a la red AC por medio de un enlace de alta tensión en continua

A Modular IGBT Based Current Flow Controller for Multi-terminal HVDC Grids

Offshore wind turbines are preferred rather than onshore ones for their numerous advantages, such as land saving, higher wind speeds and higher power generation. However, AC power transmission would fail to deliver the generated power economically over distances longer than 80 kilometres using submarine cables. The more feasible option is to use High Voltage DC (HVDC) power transmission for offshore wind generation. Unlike AC transmission systems that have established power and current flow control methods, DC power transmission systems have only reliable power flow control techniques for point to point systems, which makes it one of the challenges preventing realisation of Multi Terminal HVDC grids (MT-HVDC) as cables may be subjected to higher currents causing overloading and thermal problems. Different HVDC power flow control schemes are suggested by controlling the AC/DC converters such as voltage droop control and voltage margin control. Other methods of power and current flow control based on the connection of new power electronic equipment to the grid have been also proposed. This thesis presents operation and control of an IGBT based Current Flow Controller (CFC) for MT-HVDC grid applications. The CFC is studied in its preliminary two-port configuration and possible modes of operation and dynamic models are produced. An extended topology is proposed to allow the CFC to be connected to more than two cables at a time. Although the proposed extended CFC topology is simple in construction and gave acceptable results in most case studies, it has shown some drawbacks in certain case studies where controlled currents have significant differences in magnitudes. To resolve this problem, a generalized Modular CFC (MCFC) topology is proposed which allows each current to be controlled independently and overcome the extended topology’s drawback. Moreover, a reduced count switch count topology is proposed which reduces the MCFC cost by half in cases of unidirectional current flow control. All proposed control strategies and topologies are validated using both computer simulation through MATLAB/SIMULINK and PSCAD/EMTDC software packages and experimental validation through Rapid Control Prototyping (RCP) with the aid of Opal RT real time simulator. Studies carried throughout this thesis show that the proposed MCFC may play an important role in current flow control applications in MT-HVDC grids due to its low cost, small footprint and accurate performance

Modeling of Direct Current Grid Equipment for the Simulation and Analysis of Electromagnetic Transients

RÉSUMÉ Les transmissions à base de courant continu sont capables de répondre mieux que les transmissions traditionnelles à base de courant alternatif aux enjeux de nos jours tels que l’intégration des énergies renouvelables, les difficultés avec l’installation des nouvelles lignes aériennes pour les raisons socio-environnementaux, la gestion des flux de puissance sur le réseau électrique. Ceci est grâce aux systèmes de contrôle performants et rapides, à un niveau de fiabilité accrue des composants utilisés, à l’efficacité énergétique des technologies de pointe, telles que les convertisseurs modulaires multiniveaux (Modular Multilevel Converter ou MMC en anglais). Ces avantages ont contribué à une croissance rapide du nombre de transmissions à courant continu à travers le monde dans les dernières années, avec les plans d’établir des réseaux multi-terminaux d’un niveau supérieur aux réseaux électriques traditionnels dans le but de les renforcer. Les outils de simulation numériques sont nécessaires pour faciliter et accélérer la mise en œuvre de ce type de projets d’envergure. Ils permettent d’analyser et d’étudier les systèmes électriques de plus en plus complexes et par conséquent d’éviter les problèmes opérationnels, d’augmenter la fiabilité et l’efficacité des réseaux électriques. La complexité accrue des réseaux électriques modernes qui contiennent les composants à base de l’électronique de puissance tels que les liaisons à courant continu exige une recherche sur les outils de simulation et les modèles avancés. Ainsi, cette thèse se focalise sur le développement d’un cadre pour les simulations précises et rapides des liaisons à courant continu. À la suite d’une revue de la littérature il est démontré que la modélisation des MMCs a un impact particulièrement important sur la précision et l’accélération des simulations et par conséquent une grande partie de cette thèse est dédiée aux différentes méthodes pour réduire le temps de simulation et améliorer la précision des résultats dans les études avec les MMCs. Le cœur du sujet commence par la présentation de la modélisation des MMC hybrides et leurs systèmes de contrôle. Les modèles sont classés en quatre catégories selon le niveau de précision : le modèle détaillé permet de représenter les non-linéarités au niveau des composants semiconducteurs.----------ABSTRACT Compared to the traditional alternating current technology-based electrical grids, High-Voltage Direct Current (HVDC) transmission systems can more effectively respond to the challenges of the modern power grid related to the integration of renewable energy sources, difficulty to install new overhead lines due to socio-environmental reasons, and power flow management. This is mainly due to high performance of control systems, fast response times, reliable components and energy efficiency of the state-of-the-art HVDC technologies of today, such as the Modular Multilevel Converter (MMC). These advantages have contributed to the rapid growth in the number of HVDC projects in recent years with plans of having overlay HVDC grids that can reinforce the existing electrical grids. To facilitate and accelerate the implementation of large-scale HVDC projects, it is required to use numerical simulation tools. Such tools allow to perform advanced analysis of involved electrical systems for preventing operating problems, increasing robustness and efficiency in power grids. The increased level of complexity of modern power grids with power electronics-based components, such as HVDC, requires research on advanced simulation tools and models. Therefore, this thesis aims to develop a framework allowing for accurate modeling and fast simulations of HVDC projects. After analysis of existing literature, the areas with high potential impact on accuracy and acceleration of electromagnetic transient simulations are found, and it is the modeling of MMCs that is considered in this thesis. Thus, a significant part of this thesis is dedicated to research on efficient modeling techniques that allow to reduce simulation time and improve accuracy for MMC-based HVDC systems. The modeling aspects and control systems of hybrid MMCs are presented first. The MMC models used in electromagnetic transient simulations are grouped into four categories. The detailed model represents the nonlinear current-voltage characteristics of semiconductor switches. The detailed equivalent model represents the switches as two-value resistances: a small value for the closed state and a large value for the open state. The arm equivalent model assumes all capacitors in each arm have identical voltages, so a single equivalent capacitor is used to represent the whole arm, thus greatly reducing the computational burden of the model

Modular Multilevel Converter-Based Hvdc Transmission Systems

High-Voltage Direct Current (HVDC) transmission systems based on Voltage Source Converter (VSC) technology has attracted significant interest recently for transmitting large amounts of power over long distances using back-to-back or point-to-point configurations. VSC-HVDC has been addressed for various HV applications such as DC interconnections, Multi-Terminal HVDC Transmission (MT-HVDC), installation of offshore wind power generation such as Europe super DC grid and installation of other renewable energy sources. Several classes of VSC topologies can be employed in HVDC systems including the conventional two and three-level converters, multilevel converters, and Modular Multilevel Converters (MMCs) that has been recently introduced and investigated for HVDC applications. MMC is penetrating the modern HVDC transmission market, due to its inherent features such as scalability, modularity, and fault ride through capability. Therefore, this thesis investigates and models a point-to-point VSC-based HVDC transmission system using nine-level MMC transient model, and 25-level MMC averaged model using MATLAB/Simulink platform to meet the requirements of HVDC systems such as HV requirements and fault ride through capability. However, a point-topoint HVDC system using conventional two-level converter is modeled and simulated using MATLAB/Simulink as a starting and benchmarking model. MMC transient model employed in this study is based on Half-Bridge Sub-Modules (HB-SMs) due to its simple structure, yet, other structures are discussed. Nevertheless, balancing of the floating capacitors is one of the challenges associated with MMCs. Therefore, capacitor voltage balancing and its modeling is addressed. Then the average model of the MMC-based HVDC system is investigated. Moreover, the behavior during DC side faults is investigated, and the employment of hybrid DC circuit breakers and Hybrid Current Limiting Circuit (HCLC) are introduced for protection and limiting the DC fault current. This introduces a platform for studying large MMC-based HVDC systems in normal operation and during faults