Controlled transition bridge converter : operating principle, control and application in HVDC transmission systems

This paper employs an amplitude modulation with sinusoidal plus third harmonic injection instead of trapezoidal modulation to operate a controlled transition bridge (CTB) converter as ac/dc and dc/ac converter terminals. With such an operation, the CTB converter may require small ac filters; thus attractive for high-voltage direct current (HVDC) transmission systems. To facilitate ac voltage control over a wide range and black-start capability, the injected 3rd harmonic allows the cell capacitor voltages of the CTB converter to be regulated independent of the modulation index and power factor. The insertion of 3rd harmonic into modulating signals achieves two objectives: extends the regions around voltage zeros so that the total voltage unbalanced can be distributed between the cell capacitors, thereby exploiting the bipolar capability of the full-bridge cells in each limb; and to ensure that each limb can be clamped to the positive and negative dc rails every half fundamental period independent of the modulation index to allow recharge of the cell capacitors from the active dc link. The suitability of the CTB converter for HVDC type applications is demonstrated using a two-terminal HVDC link that employs a 21-cell CTB converter, considering normal operation and ac faults

Power-System Level Classification of Voltage-Source HVDC Converter Stations Based Upon DC Fault Handling Capabilities

status: publishe

Potential of Bipolar Full-Bridge MMC-HVdc Transmission for Link and Overlay Grid Applications

Bipolare HGÜ Systeme in Multi-Level-Ausführung stellen ein attraktives Lösungskonzept zur Bewältigung einer Vielzahl von Herausforderungen im Kontext heutiger Energiesysteme dar. Da dies jedoch auf Kosten einer deutlich erhöhten Systemkomplexität geschieht, ist ein tiefgreifendes Verständnis des transienten Verhaltens sowie der dynamischen Charakteristik von enormer Wichtigkeit. Diese Doktorarbeit beinhaltet eine detaillierte Analyse von grundlegenden Zusammenhängen bezogen auf bipolare HGÜ Systeme in Multi-Level-Ausführung und stellt ein generisches Regelungs-, Bilanzierungs- und Schutzkonzept vor. Die generelle Leistungsfähigkeit des Konzepts wird durch elektro-magnetische Transientensimulationen nachgewiesen

A new fast-acting backup protection strategy for embedded MVDC links in future distribution networks

This paper presents a new fast-acting backup protection strategy for future hybrid ac-dc distribution networks. By examining the impedance measured by a distance protection relay measuring from the “ac-side” of the network, a unique characteristic is established for faults occurring on the “dc-side” of an embedded medium-voltage dc (MVDC) link, interconnecting two 33 kV distribution network sections. Based on the identified impedance characteristic, appropriate settings are developed and deployed on a verified software model of a commercially available distance protection relay. To remain stable for ac-side faults, it is found that the tripping logic of the device must be altered to provide correct time grading between standard, ac, protection zones and the fast-acting dc region, which can identify faults on the dc system within 40 ms. An additional confirmatory check is also employed to reduce the likelihood of mal-operation. Trials on a test system derived from an actual distribution network, which employs distance protection, are shown to provide stable operation for both ac-side and dc-side pole-pole and pole-pole-ground fault

Analysis of heterogeneously configured converter stations in HVDC grids under asymmetrical DC operation

Additional technologies different from classical high voltage alternating current (HVAC) transmission are necessary to deal with the higher renewable energy integration in the current energetic framework. High voltage direct current (HVDC) transmission based on modular multilevel voltage source converters (MMC-VSC) is a promising alternative for some applications. Thus, the number of HVDC projects is increasing worldwide. This makes possible their future gradual interconnection to constitute an overlay DC grid that offers numerous additional advantages but still many challenges. Even if the development of the HVDC technology overcomes all the present challenges in the future, the lack of standardisation will lead to a DC grid integrated by different HVDC station topologies, grounding schemes, DC-DC converters, or control strategies. During normal operation, the DC grid is assumed to work symmetrically, and some aspects, such as the topology or the grounding scheme, do not intervene in the system response. However, in case of working asymmetrically due to a fault or outage affecting a single pole of the DC network, all the aspects mentioned above affect the system operation. However, such a heterogeneous DC grid under asymmetrical DC operation has yet to be addressed in the literature. Thus, it constitutes the general objective of this thesis. To achieve this objective, the asymmetrical DC operation in different heterogeneous DC systems is studied using load flow, dynamic EMT simulation, and small-signal stability analysis. The analysis of a system of these characteristics under asymmetrical DC operation is an original contribution of the thesis. First, a DC grid connecting different AC zones and formed by different HVDC station topologies and DC-DC converters is modelled to perform the load-flow assessment. The asymmetrical DC operation is examined by causing an asymmetrical contingency in the DC network. The analysis is carried out considering different grounding resistances, control strategies, control parameters, and galvanic isolation ability of the DC-DC converters. The results obtained regarding DC current and voltage asymmetry, which are related to the overloading of elements and excessive voltage deviation, allow for assessing the impact of the asymmetrical operation under different circumstances. Second, the dynamic assessment aims to identify the main aspects involved in the transient response during asymmetrical DC operation. The connection of a symmetrical monopolar station to a bipolar system is modelled, and the outage of one of the converters of a bipolar station is simulated. The effect of the grounding impedance and the control strategy on the dynamic response of the system is assessed. Therefore, the main system parameters and issues that may appear are identified. Furthermore, the effect of the connection of the symmetrical monopole station over the existing protections of the bipolar system is assessed by considering different grounding impedances in the monopolar station. Finally, the small-signal analysis of a system composed of different topologies focuses on the asymmetrical DC operation. A new suitable model is developed and validated against EMT simulations. The small-signal analysis is carried out, and the main aspects that impact the small-signal stability during asymmetrical operation are identified. Furthermore, a new controller that enhances the system stability during asymmetrical DC operation is developed.Para hacer frente a la mayor integración de energías renovables en el marco energético actual se necesitan tecnologías adicionales distintas de la transmisión clásica en corriente alterna en alta tensión (HVAC). La transmisión de corriente continua en alta tensión (HVDC) basada en convertidores multinivel modulares de fuente de tensión (MMCVSC) es una alternativa prometedora para algunas aplicaciones. Por tanto, el número de proyectos HVDC está aumentando en todo el mundo. Esto hace posible que se interconecten gradualmente en el futuro para formar una red de corriente continua (CC) que ofrece numerosas ventajas adicionales, pero todavía muchos retos. Aunque el desarrollo de la tecnología HVDC supere todos los retos actuales en el futuro, la falta de normalización dará lugar a una red de CC integrada por diferentes topologías de estaciones HVDC, esquemas de puesta a tierra, convertidores CC-CC o estrategias de control. Durante el funcionamiento normal, la red de CC funciona simétricamente y algunos aspectos, como la topología o el esquema de puesta a tierra, no intervienen en la respuesta del sistema. Sin embargo, en caso de funcionamiento asimétrico, debido a una falta o desconexión que afecte a un solo polo de la red de CC, todos los aspectos mencionados anteriormente afectan al funcionamiento del sistema. Este tipo de red de CC heterogénea en funcionamiento asimétrico aún no se ha abordado en el estado del arte. Por ello, constituye el objetivo general de esta tesis. Para lograr este objetivo, se estudia el funcionamiento asimétrico de CC en diferentes sistemas heterogéneos de CC utilizando diferentes enfoques como el flujo de cargas, la simulación dinámica EMT y el análisis de estabilidad de pequeña señal. El análisis de un sistema de estas características en funcionamiento asimétrico en CC constituye la principal contribución de la tesis. Para realizar la evaluación del flujo de cargas, se modela una red de CC que conecta diferentes zonas de CA y está formada por diferentes topologías de estaciones HVDC y convertidores CC-CC. A continuación, se examina el funcionamiento asimétrico de CC provocando una contingencia asimétrica en la red de CC. El análisis se lleva a cabo considerando diferentes resistencias de puesta a tierra, estrategias de control, parámetros de control y capacidad de aislamiento galvánico de los convertidores CC-CC. Los resultados obtenidos sobre la asimetría de corriente y tensión en CC, relacionados con la sobrecarga de los elementos y la desviación excesiva de la tensión, permiten evaluar el impacto del funcionamiento asimétrico en distintas circunstancias. La evaluación dinámica pretende identificar los principales aspectos que intervienen en la respuesta transitoria durante el funcionamiento asimétrico en CC. En primer lugar, se modela la conexión de una estación monopolar simétrica a un sistema bipolar. A continuación, se simula la interrupción de uno de los convertidores de una estación bipolar y se evalúa el efecto de la impedancia de puesta a tierra y de la estrategia de control en la respuesta dinámica del sistema. Por último, se identifican los principales parámetros del sistema y los problemas que pueden aparecer. Además, se evalúa el efecto de la conexión de la estación monopolar simétrica sobre las protecciones existentes del sistema bipolar, considerando diferentes impedancias de puesta a tierra en la estación monopolar. Por último, se realiza el análisis de pequeña señal de un sistema compuesto por diferentes topologías centrándose en el funcionamiento asimétrico en CC. Para ello, primero se desarrolla un nuevo modelo adecuado para este análisis y se valida con simulaciones EMT. A continuación, se lleva a cabo el análisis de pequeña señal y se identifican los principales aspectos que afectan a la estabilidad de pequeña señal durante el funcionamiento asimétrico. Además, se desarrolla un nuevo controlador que mejora la estabilidad del sistema durante el funcionamiento asimétrico en CC.Programa de Doctorado en Ingeniería Eléctrica, Electrónica y Automática por la Universidad Carlos III de MadridPresidente: José Luis Rodríguez Amenedo.- Secretario: Eduardo Prieto Araujo.- Vocal: Dunixe Marene Larruskain Escoba

Developing A Medium-Voltage Three-Phase Current Compensator Using Modular Switching Positions

The objective of this thesis is to present the context, application, theory, design, construction, and testing of a proposed solution to unbalanced current loading on three-phase four-wire systems. This solution, known as the Medium-Voltage Unbalanced Current Static Compensator or MV-UCSC, is designed to recirculate currents between the three phases of adistribution system. Through this redistribution of the currents negative- and zero-sequence current components are eliminated and a balanced load is seen upstream from the point of installation. The MV-UCSC as it operates in the distribution system is presented followed by its effect on traditional compensation equipment. The construction of the MV-UCSC as well as 13.8 kV simulations are then shown. Development of the switching positions required by the MVUCSC is then given followed by a variation on this switching position with the intent to reduce part count. Finally, the testing the 13.8 kV three-phase four-wire, neutral-point-clamped, elevenlevel, flying-capacitor-based MV-UCSC connected directly to the grid is presented

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

Méthodes scalables de commande par allocation pour le convertisseur modulaire multiniveaux : de la modélisation à l'implémentation temps réel

Dans le cadre de la montée en puissance des convertisseurs statiques, les différents avantages qu’il y a à utiliser les Convertisseurs Modulaires Multiniveaux (MMC) ont mené à leur popularisation. Cependant, à mesure que le nombre de niveaux de tension et le nombre de phase augmentent, ces convertisseurs présentent un nombre de plus en plus important de degrés de liberté pour en effectuer la commande. Ainsi les MMC représentent un défi pour la commande car le nombre de variables de commande est alors supérieur aux contraintes à satisfaire, faisant d’eux des systèmes redondants ou encore sous-déterminés ce qui ouvre la voie de l’optimisation. D’abord apparues dans les années 1980 dans l’aéronautique pour tirer profit de la multiplicité des surfaces aérodynamiques et des redondances associées que présente un avion afin d’en contrôler sa trajectoire (volets, ailerons, gouvernes…), les méthodes de commande par allocation ont fait leurs preuves en étant progressivement appliquées dans différents domaines technologiques. En parallèle ces algorithmes ont fait l’objet de travaux pour améliorer les performances obtenues et notamment s’adapter aux systèmes commandés. Le sujet de la thèse concerne donc le développement et l’implémentation en temps réel de méthodes de commande par allocation, avec un souci d’optimisation en ligne, pour un système de conversion d’énergie à base de MMC. La première partie de la thèse portent sur la modélisation du convertisseur MMC en vue de sa commande à partir de méthodes d’allocation. Ce qui implique le développement de différents modèles de commande avec différents niveaux de détails et de complexité. Un résultat fort issu de cette première partie est un modèle de commande dont la complexité n’est plus influencée par le nombre de phases du système électrique considéré. La deuxième étape des travaux concerne le développement d’une nouvelle méthode d’allocation qui met à profit les avantages des méthodes présentes dans l’état de l’art pour en concevoir une nouvelle plus adaptée. Ainsi cette démarche a conduit à la programmation d’un nouvel algorithme d’allocation présentant des caractéristiques dynamiques et statiques réglables et adaptables simplement, son intégration aux méthodes déjà existantes est aisée et presque immédiat. La troisième étape des travaux combine les travaux précédents. Tout d’abord en simulation, la méthode de commande par allocation du convertisseur est programmée puis testée pour finalement être validée. Pour la commande différentes architectures sont conçues permettant de réaliser des comparatifs afin d’évaluer leur capacité à atteindre les performances requises pour le bon fonctionnement du système. Il en découle une analyse des différents algorithmes de commande proposés. Le résultat principal de cette partie est la conception d’un nouvel algorithme d’allocation permettant de contrôler les tensions aux bornes des condensateurs ainsi que les tous les courants du convertisseur dans chacune des branches et ce indépendamment du nombre de phases. La quatrième étape porte sur la validation expérimentale des méthodes développées. Pour se faire, le convertisseur MMC disponible au laboratoire LAPLACE est utilisé ainsi qu’un ensemble d’outils de prototypage rapide (OPAL-RT) permettant de tester et mettre au point les algorithmes de façon sûre et efficace. La cinquième partie des travaux concerne l’extension, hors de la zone de fonctionnement nominale du convertisseur, des algorithmes de commande développés. En effet une ouverture est proposée mettant en exergue les capacités des méthodes d’allocation à reconfigurer le fonctionnement du MMC lorsqu’un défaut apparait dans l’un des sous-modules. Les résultats obtenus en simulation montrent une amélioration de la disponibilité du convertisseur, c’est-à-dire une continuité de fonctionnement en présence de défauts ce qui justifie l’intérêt de poursuivre les travaux dans cette direction

Protection and fault location schemes suited to large-scale multi-vendor high voltage direct current grids

Recent developments in voltage source converter (VSC) technology have led to an increased interest in high voltage direct current (HVDC) transmission to support the integration of massive amounts of renewable energy sources (RES) and especially, offshore wind energy. VSC-based HVDC grids are considered to be the natural evolution of existing point-to-point links and are expected to be one of the key enabling technologies towards expediting the integration and better utilisation of offshore energy, dealing with the variable nature of RES, and driving efficient energy balance over wide areas and across countries. Despite the technological advancements and the valuable knowledge gained from the operation of the already built multi-terminal systems, there are several outstanding issues that need to be resolved in order to facilitate the deployment of large-scale meshed HVDC grids. HVDC protection is of utmost importance to ensure the necessary reliability and security of HVDC grids, yet very challenging due to the fast nature of development of DC faults and the abrupt changes they cause in currents and voltages that may damage the system components. This situation is further exacerbated in highly meshed networks, where the effects of a DC fault on a single component (e.g. DC cable) can quickly propagate across the entire HVDC grid. To mitigate the effect of DC faults in large-scale meshed HVDC grids, fast and fully selective approaches using dedicated DC circuit breaker and protection relays are required. As the speed of DC fault isolation is one order of magnitude faster than typical AC protection (i.e. less than 10 ms), there is a need for the development of innovative approaches to system protection, including the design and implementation of more advanced protection algorithms. Moreover, in a multi-vendor environment (in which different or the same type of equipment is supplied by various manufacturers), the impact of the grid elements on the DC fault signature may differ considerably from case to case, thus increasing the complexity of designing reliable protection algorithms for HVDC grids. Consequently, there is a need for a more fundamental approach to the design and development of protection algorithms that will enable their general applicability. Furthermore, following successful fault clearance, the next step is to pinpoint promptly the exact location of the fault along the transmission medium in an effort to expedite inspection and repair time, reduce power outage time and elevate the total availability of the HVDC grid. Successful fault location becomes increasingly challenging in HVDC grids due to the short time windows between fault inception and fault clearance that limit the available fault data records that may be utilised for the execution of fault location methods. This thesis works towards the development of protection and fault location solutions, designed specifically for application in large-scale multi-vendor HVDC grids. First, a methodology is developed for the design of travelling wave based non-unit protection algorithms that can be easily configured for any grid topology and parameters. Second, using this methodology, a non-unit protection algorithm based on wavelet transform is developed that ensures fast, discriminative and enhanced protection performance. Besides offline simulations, the efficacy of the wavelet transform based algorithm is also demonstrated by means of real-time simulation, thereby removing key technical barriers that have impeded the use of wavelet transform in practical protection applications. Third, in an effort to reinforce the technical and economic feasibility of future HVDC grids, a thorough fault management strategy is presented for systems that employ efficient modular multilevel converters with partial fault tolerant capability. Finally, a fault location scheme is developed for accurately estimating the fault location in HVDC grids that are characterised by short post-fault data windows due to the utilisation of fast acting protection systems.Recent developments in voltage source converter (VSC) technology have led to an increased interest in high voltage direct current (HVDC) transmission to support the integration of massive amounts of renewable energy sources (RES) and especially, offshore wind energy. VSC-based HVDC grids are considered to be the natural evolution of existing point-to-point links and are expected to be one of the key enabling technologies towards expediting the integration and better utilisation of offshore energy, dealing with the variable nature of RES, and driving efficient energy balance over wide areas and across countries. Despite the technological advancements and the valuable knowledge gained from the operation of the already built multi-terminal systems, there are several outstanding issues that need to be resolved in order to facilitate the deployment of large-scale meshed HVDC grids. HVDC protection is of utmost importance to ensure the necessary reliability and security of HVDC grids, yet very challenging due to the fast nature of development of DC faults and the abrupt changes they cause in currents and voltages that may damage the system components. This situation is further exacerbated in highly meshed networks, where the effects of a DC fault on a single component (e.g. DC cable) can quickly propagate across the entire HVDC grid. To mitigate the effect of DC faults in large-scale meshed HVDC grids, fast and fully selective approaches using dedicated DC circuit breaker and protection relays are required. As the speed of DC fault isolation is one order of magnitude faster than typical AC protection (i.e. less than 10 ms), there is a need for the development of innovative approaches to system protection, including the design and implementation of more advanced protection algorithms. Moreover, in a multi-vendor environment (in which different or the same type of equipment is supplied by various manufacturers), the impact of the grid elements on the DC fault signature may differ considerably from case to case, thus increasing the complexity of designing reliable protection algorithms for HVDC grids. Consequently, there is a need for a more fundamental approach to the design and development of protection algorithms that will enable their general applicability. Furthermore, following successful fault clearance, the next step is to pinpoint promptly the exact location of the fault along the transmission medium in an effort to expedite inspection and repair time, reduce power outage time and elevate the total availability of the HVDC grid. Successful fault location becomes increasingly challenging in HVDC grids due to the short time windows between fault inception and fault clearance that limit the available fault data records that may be utilised for the execution of fault location methods. This thesis works towards the development of protection and fault location solutions, designed specifically for application in large-scale multi-vendor HVDC grids. First, a methodology is developed for the design of travelling wave based non-unit protection algorithms that can be easily configured for any grid topology and parameters. Second, using this methodology, a non-unit protection algorithm based on wavelet transform is developed that ensures fast, discriminative and enhanced protection performance. Besides offline simulations, the efficacy of the wavelet transform based algorithm is also demonstrated by means of real-time simulation, thereby removing key technical barriers that have impeded the use of wavelet transform in practical protection applications. Third, in an effort to reinforce the technical and economic feasibility of future HVDC grids, a thorough fault management strategy is presented for systems that employ efficient modular multilevel converters with partial fault tolerant capability. Finally, a fault location scheme is developed for accurately estimating the fault location in HVDC grids that are characterised by short post-fault data windows due to the utilisation of fast acting protection systems