Review of technologies for DC grids - power conversion, flow control and protection

Abstract: This article reviews dc transmission technologies for future power grids. The article emphasizes the attributes that each technology offers in terms of enhance controllability and stability, resiliency to ac and dc faults, and encourage increased exploitations of renewable energy resources (RERs) for electricity generation. Discussions of ac/dc and dc/dc converters reveal that the self-commutated dc transmission technologies are critical for better utilization of large RERs which tend to be dispersed over wide geographical areas, and offer needed controllability for operation of centralized and decentralized power grids. It is concluded that the series power flow controllers have potential to restrict the expensive isolated dc/dc converters to few applications, in which the prevention of dc fault propagation is paramount. Cheaper non-isolated dc/dc converters offer dc voltage tapping and matching and power regulation but they are unable to prevent pole-shifting during pole-to-ground dc fault. To date hybrid dc circuit breakers target dc fault isolation times ranging from 3ms to 5ms; while the resonance-based dc circuit breakers with forced current zeros target dc fault clearance times from 8ms to 12.5ms

Review of dc-dc converters for multi-terminal HVDC transmission networks

This study presents a comprehensive review of high-power dc-dc converters for high-voltage direct current (HVDC) transmission systems, with emphasis on the most promising topologies from established and emerging dc-dc converters. In addition, it highlights the key challenges of dc-dc converter scalability to HVDC applications, and narrows down the desired features for high-voltage dc-dc converters, considering both device and system perspectives. Attributes and limitations of each dc-dc converter considered in this study are explained in detail and supported by time-domain simulations. It is found that the front-to-front quasi-two-level operated modular multilevel converter, transition arm modular converter and controlled transition bridge converter offer the best solutions for high-voltage dc-dc converters that do not compromise galvanic isolation and prevention of dc fault propagation within the dc network. Apart from dc fault response, the MMC dc auto transformer and the transformerless hybrid cascaded two-level converter offer the most efficient solutions for tapping and dc voltage matching of multi-terminal HVDC networks

High Power Density and High Efficiency Converter Topologies for Renewable Energy Conversion and EV Applications

This dissertation work presents two novel converter topologies (a three-level ANPC inverter utilizing hybrid Si/SiC switches and an Asymmetric Alternate Arm Converter (AAAC) topology) that are suitable for high efficiency and high-power density energy conversion systems. The operation principle, modulation, and control strategy of these newly introduced converter topologies are presented in detail supported by simulation and experimental results. A thorough design optimization of these converter topologies (Si/SiC current rating ratio optimization and gate control strategies for the three-level ANPC inverter topology and component sizing for the asymmetric alternate arm converter topology) are also presented. Performance comparison of the proposed converter topologies with other similar converter topologies is also presented. The performance of the proposed ANPC inverter topology is compared with other ANPC inverter topologies such as an all SiC MOSFET ANPC inverter topology, an all Si IGBT ANPC inverter topology and mixed Si IGBT and SiC MOSFET based ANPC inverter topologies in terms of efficiency and cost. The efficiency and cost comparison results show that the proposed hybrid Si/SiC switch based ANPC inverter has higher efficiency and lower cost compared to the other ANPC inverter topologies considered for the comparison. The performance of the asymmetric alternate arm converter topology is also compared with other similar voltage source converter topologies such as the modular multilevel converter topology, the alternate arm converter topology, and the improved alternate arm converter topology in terms of total device count, number of switches per current conduction path, output voltage levels, dc-fault blocking capability and overmodulation capability. The proposed multilevel converter topology has lower total number of devices and lower number of devices per current conduction path hence it has lower cost and lower conduction power loss. However, it has lower number of output voltage levels (requiring larger ac interface inductors) and lacks dc-fault blocking and overmodulation operation capabilities. A converter figure-of-merit accounting for the hybrid Si/SiC switch and converter topology properties is also proposed to help perform quick performance comparison between different hybrid Si/SiC switch based converter topologies. It eliminates the need for developing full electro-thermal power loss model for different converter topologies that would otherwise be needed to carry out power loss comparison between different converter topologies. Hence it saves time and effort

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

AN INVESTIGATION OF AN ENERGY DIVERTING CONVERTER FOR HVDC APPLICATIONS

Wind power generation in Europe has experienced an unprecedented expansion fuelled by a very favourable regulatory framework promoted to fight climate change. It is currently the second largest power generation source accounting for 17% of the total energy mix and in 2016 it covered an impressive 10.4% of the total energy demand. With faster wind speeds and better availability, offshore wind farm developments have also experienced a surge in recent years. There are 12.7 GW of cumulative installed capacity with the hot spot located in the North Sea. The grid integration of offshore wind farms has evolved to meet the requirements of recent projects, much larger in power capacity and located farther offshore. High voltage direct current (HVDC) connections using state of the art multilevel voltage source converters are now the industry standard for distant wind farms, with transmission capacities of up to 1 GW. The scale of the projects and frequent grid weakness at the onshore locations challenge transmission system operators which need to ensure the entire grid stability. Grid codes have evolved to regulate such interconnections, with a set of well specified requirements which need to be fulfilled. One such requirement is the fault ride-through capability, which defines the need for the HVDC interconnector to remain connected during onshore grid faults. A Dynamic Braking System (DBS) is a power electronics device that provides fault ride-through capability to the HVDC interconnector by absorbing the excess energy injected to the link for the duration of the fault. This energy is commonly dissipated in a resistive element. In this way the DC over-voltage is avoided and the operation of the connected wind farms is kept undisturbed. There is a lack of knowledge in the design and implementation of such devices. Therefore four concepts put forward by industry and other researchers are studied in this work. The rating of the different components in each circuit is investigated as the basis for the comparison. Taking into account the modular structure of AC/DC converters in HVDC stations it makes commercial sense to reuse the same modules as building blocks for the DBS. With modular structures, a good balancing of the total energy stored in the converter and its distribution among the different modules is one of the key elements. Modular DBS circuits can synthesize multilevel voltage waveforms, allowing for advanced power modulation strategies. Two novel strategies are developed in the thesis and an accurate mathematical modelling is performed to ensure that the energy balance conditions are met for all points of operation. An overall control strategy for each of the four circuits is also developed and presented in the thesis. A good coordination of the protective actions of the DBS and the main HVDC converters is important to ensure that no negative interactions occur. An operation strategy based on over-voltage thresholds is developed in the thesis. Accurate simulation models of the HVDC link integrating the DBS and controls are also implemented to give the required degree of confidence in the overall system behaviour. These are finally validated by a laboratory scaled-down test platform, where the control actions and the different converters are implemented in real hardware, and the correct coordination of all the elements during a fault event is experimentally tested. The main drawback of the DBS solution usually highlighted in literature is its cost. The option of adding some extra functionality to better justify the economic investment is explored in this thesis, resulting on a multifunctional circuit named Energy Diverting Converter (EDC). Two proposals including active filtering and HVDC tapping are developed in this thesis, for which two patent applications have been filed

Dc-dc converters for HVDC heterogeneous interconnections

(English) High voltage direct current (HVDC) technologies have been used for bulk power transmission over long distances since the 1950s. These technologies have proven to be the most cost-efficient compared to the high voltage alternate current (HVAC) for some applications such as offshore power transmission, connecting remote loads or generation, and the interconnection of non-synchronized grids. In recent years, the study of HVDC grids has been of interest in some research projects but, its development is still uncertain. The de grid can be planned beforehand or it can use the installed lines. But, from the installed HVDC projects, it can be identified different operating voltages, used technologies, and line topologies. There are two HVDC technologies: the line commutated converter (LCC), and the voltage source converter (\/SC). Four different line topologies are identified: asymmetric monopole, symmetric monopole, bipole, and rigid bipole. Developing a de grid interconnecting isolated lines with different characteristics cannot be possible without an intermediary device: the de-de converter. This thesis studies the de-de converters interconnecting HVDC lines with different characteristics. These converters can be seen as the equivalent of ac transformers in de applications because they are capable to adapt the voltage difference between two de systems. These converters are also capable to adapt the line topology and differenttechnologies. The power electronics required for these de-de converters provide increased control flexibility used to supply additional ancillary services that the classical transformers cannot furnish. Three de-de converter topologies are modeled and simulated for the interconnection between a bipole and a symmetric monopole. The front-to-front modular multi-level converter (F2F-MMC) is chosen as the reference because it represents state­ of-the-art technology. The second converter is the de-de MMC (de-MMC) because of the topological similarityto the MMC. Then, a third converter is proposed and studied as a result of this thesis, the asymmetric de-de converter (ADCC). A set of simulations are performed for multiple operating points and faults scenarios. Then, the converters are compared quantitatively and qualitatively. The results and analysis are used to conclude and bring some perspectives for future works.(Español) as transmision en corriente continua a alta tension (HVDC-por sus siglas en ingles) se han utilizado para la transmision de grandes cantidades de energfa a largas distancias desde la decada de 1950. Estas tecnologfas han demostrado ser las mas rentables en comparacion con la corriente alterna de alta tension (HVAC) para algunas aplicaciones como la transmision de energfa en alta mar, la conexion de cargas o generacion remotas y la interconexion de redes no sincronizadas. En los ultimos aiios, el estudio de las redes HVDC ha sido de interes en algunos proyectos de investigacion, pero su desarrollo aun es incierto. La red de de se puede planificar de antemano o puede utilizar las lfneas instaladas. Pero, a partir de los proyectos HVDC instalados, se pueden identificar diferentes voltajes de operacion, tecnologfas utilizadas ytopologfas de lfnea. Hay dos tecnologfas HVDC: el convertidor de lfnea conmutada (LCC) y el convertidor de fuente de voltaje (VSC). Se identifican cuatro topologfas de lfnea diferentes: monopolo asimetrico, monopolo simetrico, bipolo y bipolo rfgido. El desarrollo de una red de de que interconecte lfneas aisladas de diferentes caracterfsticas no es posible sin un dispositivo intermediario: el convertidor de-de. Esta tesis estudia los convertidores de-de interconectando lfneas HVDC de diferentes caracteristicas. Estos convertidores pueden verse como el equivalente de los transform adores de ac en aplicaciones de de porque son capaces de adaptar la diferencia de tension entre dos sistemas de de. Pero, estos convertidores tambien son capaces de adaptar la topologia de linea ydiferentes tecnologias. La electronica de potencia necesaria para estos convertidores de-de proporciona una mayor flexibilidad de control que se utiliza para proporcionar servicios auxiliares adicionales que los transform adores clasicos no pueden proporcionar. Se modelan y simulan tres topologias de convertidores de-de para la interconexion entre un bipolo y un monopolo simetrico. El convertidor multinivel modular de frente a frente (F2F-MMC) se elige como referencia porque representa tecnologia de punta. El segundo convertidor es el MMC de-de (de-MMC) debido a la similitud topologica con el MMC. Luego, se propone y estudia un tercer convertidor como resultado de esta tesis, el convertidor asimetrico de-de (ADCC). Se realiza un conjunto de simulaciones para multiples puntos de operacion yescenarios de fallas. Luego, los convertidores se comparan cuantitativa y cualitativamente. Los resultados y el analisis se utilizan para concluir y aportar algunas perspectivas para trabajos futuros.Enginyeria elèctric

Hybrid AC/DC hubs for network connection and integration of renewables

High-voltage direct current (HVDC) technology has been identified as a preferred choice for long-distance power transmission, especially offshore. With the rapid development of wind energy, many point-to-point HVDC systems with different voltage levels have been built. For increased operation flexibility and reliability, and better use of the existing assets, there is a need to interconnect different AC and DC networks as part of the future transmission network infrastructure development. To address the demands of connecting wind farm converter stations with other AC/DC systems, different hybrid HVDC converters for network connection and integration of renewables are proposed and evaluated in this thesis with the consideration of converter power rating, cost, efficiency and operation flexibility including response during faults. A hybrid LCC-MMC AC/DC hub (LCC-MMC Hub) is proposed in this research, where a modular multilevel converter (MMC) and a line-commutated converter (LCC) are paralleled at the AC side to integrate onshore wind power, and connected in series at the DC sides to interconnect two DC networks with different voltages. To investigate the design requirement and performance of the hybrid AC/DC hub, power flow analysis is assessed to evaluate the converter power rating requirement. Compared to the “conventional” DC network interconnection based on a DC/DC converter, the proposed hybrid LCC-MMC Hub requires the lower power rating of a MMC with large part of the power handled by a LCC, potentially leading to higher overall efficiency and lower cost. Coordinated controls of the LCC and MMC are developed to ensure stable system operation and system safety. To ride through DC faults at either side of the interconnected DC networks, a coordinated DC fault protection method for the hybrid AC/DC hub is proposed and studied. This hybrid hub uses large AC side filters, which might be the disadvantage for certain applications. Considering the future development of offshore production platforms (e.g. oil/gas and hydrogen production plants), a diode rectifier-modular multilevel converter AC/DC hub (DR-MMC Hub) is proposed to integrate offshore wind power to onshore DC network and offshore production platforms with different DC voltage levels. In this design, the DR and MMCs are connected in parallel at the offshore AC collection network to integrate offshore wind power, and in series at the DC terminals of the offshore production platform and the onshore DC network. Compared to the parallel operation of the DR-MMC HVDC system, the required MMC power rating in the proposed DR-MMC Hub can be reduced due to the series connection, potentially leading to lower investment cost and power loss. System control of the DR-MMC AC/DC hub is designed for different operating scenarios. System behaviours and requirements during AC and DC faults are investigated. The hybrid MMCs with halfbridge and full-bridge sub-modules (HBSMs and FBSMs) are used for safe operation and protection during DC faults. Power regulation of series-connected configuration might be problematic in certain applications. To address the needs for increased DC network interconnection and the high cost of the existing F2F DC/DC converter design, a hybrid F2F DC/DC converter, as a potential option, is proposed for unidirectional applications. In the proposed DC/DC converter, the internal AC grid is established by a small MMC based STATCOM, and the active power is transferred through the DR and LCC. Compared to the conventional F2F DC/DC converters in terms of topological features and operation efficiency, the proposed DC/DC converter could offer higher power capability, higher converter efficiency and lower investment cost than those of the MMC based F2F DC/DC converters. The operation and control of the LCC and MMC-STATCOM is designed, and the system start-up procedure is presented. Detailed analysis of the behaviours and protection methods during DC faults is demonstrated. It needs to acknowledge that the converter requires large amount of passive AC filters which may lead to large footprint. In addition, the proposed DC/DC converter only support unidirectional power flow.For the three proposed topologies, extensive time-domain simulation results based on PSCAD/EMTDC software have been provided to verify the feasibilities and effectiveness (including steady state and dynamic performance) in normal operation and various fault scenarios.High-voltage direct current (HVDC) technology has been identified as a preferred choice for long-distance power transmission, especially offshore. With the rapid development of wind energy, many point-to-point HVDC systems with different voltage levels have been built. For increased operation flexibility and reliability, and better use of the existing assets, there is a need to interconnect different AC and DC networks as part of the future transmission network infrastructure development. To address the demands of connecting wind farm converter stations with other AC/DC systems, different hybrid HVDC converters for network connection and integration of renewables are proposed and evaluated in this thesis with the consideration of converter power rating, cost, efficiency and operation flexibility including response during faults. A hybrid LCC-MMC AC/DC hub (LCC-MMC Hub) is proposed in this research, where a modular multilevel converter (MMC) and a line-commutated converter (LCC) are paralleled at the AC side to integrate onshore wind power, and connected in series at the DC sides to interconnect two DC networks with different voltages. To investigate the design requirement and performance of the hybrid AC/DC hub, power flow analysis is assessed to evaluate the converter power rating requirement. Compared to the “conventional” DC network interconnection based on a DC/DC converter, the proposed hybrid LCC-MMC Hub requires the lower power rating of a MMC with large part of the power handled by a LCC, potentially leading to higher overall efficiency and lower cost. Coordinated controls of the LCC and MMC are developed to ensure stable system operation and system safety. To ride through DC faults at either side of the interconnected DC networks, a coordinated DC fault protection method for the hybrid AC/DC hub is proposed and studied. This hybrid hub uses large AC side filters, which might be the disadvantage for certain applications. Considering the future development of offshore production platforms (e.g. oil/gas and hydrogen production plants), a diode rectifier-modular multilevel converter AC/DC hub (DR-MMC Hub) is proposed to integrate offshore wind power to onshore DC network and offshore production platforms with different DC voltage levels. In this design, the DR and MMCs are connected in parallel at the offshore AC collection network to integrate offshore wind power, and in series at the DC terminals of the offshore production platform and the onshore DC network. Compared to the parallel operation of the DR-MMC HVDC system, the required MMC power rating in the proposed DR-MMC Hub can be reduced due to the series connection, potentially leading to lower investment cost and power loss. System control of the DR-MMC AC/DC hub is designed for different operating scenarios. System behaviours and requirements during AC and DC faults are investigated. The hybrid MMCs with halfbridge and full-bridge sub-modules (HBSMs and FBSMs) are used for safe operation and protection during DC faults. Power regulation of series-connected configuration might be problematic in certain applications. To address the needs for increased DC network interconnection and the high cost of the existing F2F DC/DC converter design, a hybrid F2F DC/DC converter, as a potential option, is proposed for unidirectional applications. In the proposed DC/DC converter, the internal AC grid is established by a small MMC based STATCOM, and the active power is transferred through the DR and LCC. Compared to the conventional F2F DC/DC converters in terms of topological features and operation efficiency, the proposed DC/DC converter could offer higher power capability, higher converter efficiency and lower investment cost than those of the MMC based F2F DC/DC converters. The operation and control of the LCC and MMC-STATCOM is designed, and the system start-up procedure is presented. Detailed analysis of the behaviours and protection methods during DC faults is demonstrated. It needs to acknowledge that the converter requires large amount of passive AC filters which may lead to large footprint. In addition, the proposed DC/DC converter only support unidirectional power flow.For the three proposed topologies, extensive time-domain simulation results based on PSCAD/EMTDC software have been provided to verify the feasibilities and effectiveness (including steady state and dynamic performance) in normal operation and various fault scenarios

A new modular multilevel converter for HVDC applications

In the coming years, due to an increasing shift towards electric mobility and further industrialisation, a rapid growth in the demand for electricity is expected. At the same time, this energy demand must be met in a clean and sustainable manner, to reduce climate change as well as to ensure security of supply. It is predicted that the High Voltage Direct Current (HVDC) transmission technology will play a key role in the future power systems which are expected to feature higher levels of interconnection and more renewable-based generation. HVDC transmission is preferred over AC transmission in applications such as power transmission over long distances and from offshore wind sources, and interconnection of asynchronous systems. The main elements of an HVDC system are the AC/DC converters that take up the majority of the initial set up cost, and therefore, there has been a huge focus lately on improving these converters in terms of functionality, cost and efficiency. Today, the state-of-the-art converter topology for Voltage Source Converters (VSC) based HVDC transmission is the Modular Multilevel Converter (MMC), which replaced the earlier two- and three-level VSC topologies. Recently, a new breed of VSC converters, known as the `hybrid VSCs' are introduced, that combine the aspects of two- and three-level VSCs with the modular multilevel structure of the MMC. In this work, a new hybrid VSC, the Switched Mid-Point Converter (SMPC), has been proposed. While maintaining the same efficiency as the MMC, the energy storage requirement of the SMPC is shown to be less than half of that of the MMC. The operating principle and the particular voltage waveshaping of the chainlinks of the submodules is investigated. For effective operation of the SMPC, suitable control strategies are proposed. The converter concept and the developed control schemes are verified both using computer simulations and a lab-scaled experimental prototype

A new modular multilevel converter for HVDC applications

In the coming years, due to an increasing shift towards electric mobility and further industrialisation, a rapid growth in the demand for electricity is expected. At the same time, this energy demand must be met in a clean and sustainable manner, to reduce climate change as well as to ensure security of supply. It is predicted that the High Voltage Direct Current (HVDC) transmission technology will play a key role in the future power systems which are expected to feature higher levels of interconnection and more renewable-based generation. HVDC transmission is preferred over AC transmission in applications such as power transmission over long distances and from offshore wind sources, and interconnection of asynchronous systems. The main elements of an HVDC system are the AC/DC converters that take up the majority of the initial set up cost, and therefore, there has been a huge focus lately on improving these converters in terms of functionality, cost and efficiency. Today, the state-of-the-art converter topology for Voltage Source Converters (VSC) based HVDC transmission is the Modular Multilevel Converter (MMC), which replaced the earlier two- and three-level VSC topologies. Recently, a new breed of VSC converters, known as the `hybrid VSCs' are introduced, that combine the aspects of two- and three-level VSCs with the modular multilevel structure of the MMC. In this work, a new hybrid VSC, the Switched Mid-Point Converter (SMPC), has been proposed. While maintaining the same efficiency as the MMC, the energy storage requirement of the SMPC is shown to be less than half of that of the MMC. The operating principle and the particular voltage waveshaping of the chainlinks of the submodules is investigated. For effective operation of the SMPC, suitable control strategies are proposed. The converter concept and the developed control schemes are verified both using computer simulations and a lab-scaled experimental prototype

DC/DC converters for high voltage direct current transmission

High Voltage Direct Current (HVDC) transmission has to date mostly been used for point-to-point projects, with only a few select projects being designed from the outset to incorporate multiple terminals. Any future HVDC network is therefore likely to evolve out of this pool of HVDC connections. As technology improves, the voltage rating, at the point of commission, of the these connections increases. Interconnection therefore requires the DC equivalent of the transformer, to bridge the voltage levels and create a multi-terminal network. This thesis investigates new potential DC/DC converter topologies, which may be used for a range of HVDC applications. Simple interconnections of new and legacy HVDC links is unlikely to require a large voltage-step, but will be required to transfer a large amount of power. As the HVDC network develops it may become feasible for wind-farms and load-centres to directly connect to the DC network, rather than requiring new and dedicated links. Such a connection is called an HVDC tap and is typically rated at only a small fraction of the link's peak capacity (around 10\%). Such taps would connect a distribution voltage level to the HVDC network. DC/DC converters suitable for large-step ratios (>5:1) may find their application here. In this work DC/DC converters for both small and large step-ratios are investigated. Two approaches are taken to design such converters: first, an approach utilising existing converter topologies is investigated. As each project comes with a huge price-tag, their reliability is paramount. Naturally, technology that has already proven itself in the field can be modified more readily and quickly for deployment. Using two modular multilevel converters in a front-to-front arrangement has been found to work efficiently for large power transfers and low step-ratios. Such a system can be operated at higher than 50 Hz frequencies to reduce the volume of a number of passive components, making the set-up suitable for compact off-shore applications. This does however incur a significant penalty in losses reducing the overall converter efficiency. In the second approach DC/DC converter designs are presented, that are more experimental and would require significantly more development work before deployment. Such designs do not look to adapt existing converter topologies but rather are designed from scratch, purely for DC/DC applications. An evolution of the front-to-front arrangement is investigated in further detail. This circuit utilises medium frequency (>50 Hz) square current and voltage waveforms. The DC/DC step-ratio is achieved through a combination of the stacks of cells and a transformer. This split approach allows for high-step ratios to be achieved at similar system efficiencies as for the front-to-front arrangement. The topology has been found to be much more suitable for higher than 50 Hz operation from a losses perspective, allowing for a compact and efficient design.Open Acces