HVDC grids with heterogeneous configuration stations under DC asymmetrical operation

As a response to the future challenges that power systems will have to face, a gradual growth of present High Voltage Direct Current (HVDC) links is expected, leading to an HVDC supergrid integrating different configurations of HVDC stations. Such stations may suffer contingencies affecting just one pole (asymmetrical contingencies). The effects of these contingencies on an HVDC grid are very dependent on the configuration of the HVDC stations and their earthing system. Furthermore, when symmetrical monopolar stations exist in the DC grid, asymmetrical contingencies will also affect its healthy pole. For that reason, this paper focuses on the influence of the earthing system resistance of symmetrical monopolar stations on the performance of a heterogeneous HVDC grid during asymmetrical operation. The impact on the protection system is also investigated. The analysis concludes that an inappropriate earthing resistance magnitude may lead to a poor performance of heterogenous HVDC grids during asymmetrical operation. In addition, the study also indicates that the decisive factors for the selection of the grounding impedance of a symmetrical monopolar station are the asymmetrical operation of the whole grid and its contribution to pole-to-ground fault currents

System configuration, fault detection, location, isolation and restoration: a review on LVDC Microgrid protections

Low voltage direct current (LVDC) distribution has gained the significant interest of research due to the advancements in power conversion technologies. However, the use of converters has given rise to several technical issues regarding their protections and controls of such devices under faulty conditions. Post-fault behaviour of converter-fed LVDC system involves both active converter control and passive circuit transient of similar time scale, which makes the protection for LVDC distribution significantly different and more challenging than low voltage AC. These protection and operational issues have handicapped the practical applications of DC distribution. This paper presents state-of-the-art protection schemes developed for DC Microgrids. With a close look at practical limitations such as the dependency on modelling accuracy, requirement on communications and so forth, a comprehensive evaluation is carried out on those system approaches in terms of system configurations， fault detection, location, isolation and restoration

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

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

Management and Protection of High-Voltage Direct Current Systems Based on Modular Multilevel Converters

The electrical grid is undergoing large changes due to the massive integration of renewable energy systems and the electrification of transport and heating sectors. These new resources are typically non-dispatchable and dependent on external factors (e.g., weather, user patterns). These two aspects make the generation and demand less predictable, facilitating a larger power variability. As a consequence, rejecting disturbances and respecting power quality constraints gets more challenging, as small power imbalances can create large frequency deviations with faster transients. In order to deal with these challenges, the energy system needs an upgraded infrastructure and improved control system. In this regard, high-voltage direct current (HVdc) systems can increase the controllability of the power system, facilitating the integration of large renewable energy systems. This thesis contributes to the advancement of the state of the art in HVdc systems, addressing the modeling, control and protection of HVdc systems, adopting modular multilevel converter (MMC) technology, with focus in providing services to ac systems. HVdc system control and protection studies need for an accurate HVdc terminal modeling in largely different time frames. Thus, as a first step, this thesis presents a guideline for the necessary level of deepness of the power electronics modeling with respect to the power system problem under study. Starting from a proper modeling for power system studies, this thesis proposes an HVdc frequency regulation approach, which adapts the power consumption of voltage-dependent loads by means of controlled reactive power injections, that control the voltage in the grid. This solution enables a fast and accurate load power control, able to minimize the frequency swing in asynchronous or embedded HVdc applications. One key challenge of HVdc systems is a proper protection system and particularly dc circuit breaker (CB) design, which necessitates fault current analysis for a large number of grid scenarios and parameters. This thesis applies the knowledge developed in the modeling and control of HVdc systems, to develop a fast and accurate fault current estimation method for MMC-based HVdc system. This method, including the HVdc control, achieved to accurately estimate the fault current peak value and slope with very small computational effort compared to the conventional approach using EMT-simulations. This work is concluded introducing a new protection methodology, that involves the fault blocking capability of MMCs with mixed submodule (SM) structure, without the need for an additional CB. The main focus is the adaption of the MMC topology with reduced number of bipolar SM to achieve similar fault clearing performance as with dc CB and tolerable SM over-voltage

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

HVDC transmission : technology review, market trends and future outlook

HVDC systems are playing an increasingly significant role in energy transmission due to their technical and economic superiority over HVAC systems for long distance transmission. HVDC is preferable beyond 300–800 km for overhead point-to-point transmission projects and for the cable based interconnection or the grid integration of remote offshore wind farms beyond 50–100 km. Several HVDC review papers exist in literature but often focus on specific geographic locations or system components. In contrast, this paper presents a detailed, up-to-date, analysis and assessment of HVDC transmission systems on a global scale, targeting expert and general audience alike. The paper covers the following aspects: technical and economic comparison of HVAC and HVDC systems; investigation of international HVDC market size, conditions, geographic sparsity of the technology adoption, as well as the main suppliers landscape; and high-level comparisons and analysis of HVDC system components such as Voltage Source Converters (VSCs) and Line Commutated Converters (LCCs), etc. The presented analysis are supported by practical case studies from existing projects in an effort to reveal the complex technical and economic considerations, factors and rationale involved in the evaluation and selection of transmission system technology for a given project. The contemporary operational challenges such as the ownership of Multi-Terminal DC (MTDC) networks are also discussed. Subsequently, the required development factors, both technically and regulatory, for proper MTDC networks operation are highlighted, including a future outlook of different HVDC system components. Collectively, the role of HVDC transmission in achieving national renewable energy targets in light of the Paris agreement commitments is highlighted with relevant examples of potential HVDC corridors