Multi Terminal Direct Current Grids

A generic DC grid model that is compatible with the standard AC system stability model is presented and used to analyse the interaction between the DC grid and the host AC systems. A multi-terminal DC (MTDC) grid interconnecting multiple AC systems and offshore energy sources (e.g. wind farms) across the nations and continents would allow effective sharing of intermittent renewable resources and open market operation for secure and cost-effective supply of electricity. However, such DC grids are unprecedented with no operational experience. Despite lots of discussions and specific visions for setting up such MTDC grids particularly in Europe, none has yet been realized in practice due to two major technical barriers: Lack of proper understanding about the interaction between a MTDC grid and the surrounding AC systems. Commercial unavailability of efficient DC side fault current interruption technology for conventional voltage sourced converter systems This book addresses the first issue in details by presenting a comprehensive modeling, analysis and control design framework. Possible methodologies for autonomous power sharing and exchange of frequency support across a MTDC grid and their impact on overall stability is covered. An overview of the state-of-the-art, challenges and on-going research and development initiatives for DC side fault current interruption is also presented.

A generic DC grid model that is compatible with the standard AC system stability model is presented and used to analyse the interaction between the DC grid and the host AC systems. A multi-terminal DC (MTDC) grid interconnecting multiple AC systems and offshore energy sources (e.g. wind farms) across the nations and continents would allow effective sharing of intermittent renewable resources and open market operation for secure and cost-effective supply of electricity. However, such DC grids are unprecedented with no operational experience. Despite lots of discussions and specific visions for setting up such MTDC grids particularly in Europe, none has yet been realized in practice due to two major technical barriers: Lack of proper understanding about the interaction between a MTDC grid and the surrounding AC systems. Commercial unavailability of efficient DC side fault current interruption technology for conventional voltage sourced converter systems This book addresses the first issue in details by presenting a comprehensive modeling, analysis and control design framework. Possible methodologies for autonomous power sharing and exchange of frequency support across a MTDC grid and their impact on overall stability is covered. An overview of the state-of-the-art, challenges and on-going research and development initiatives for DC side fault current interruption is also presented.

Presents the latest developments in switchgear and DC/DC converters for DC grids, and includes substantially expanded material on MMC HVDC This newly updated edition covers all HVDC transmission technologies including Line Commutated Converter (LCC) HVDC; Voltage Source Converter (VSC) HVDC, and the latest VSC HVDC based on Modular Multilevel Converters (MMC), as well as the principles of building DC transmission grids. Featuring new material throughout, High Voltage Direct Current Transmission: Converters, Systems and DC Grids, 2nd Edition offers several new chapters/sections including one on the newest MMC converters. It also provides extended coverage of switchgear, DC grid protection and DC/DC converters following the latest developments on the market and in research projects. All three HVDC technologies are studied in a wide range of topics, including: the basic converter operating principles; calculation of losses; system modelling, including dynamic modelling; system control; HVDC protection, including AC and DC fault studies; and integration with AC systems and fundamental frequency analysis. The text includes: A chapter dedicated to hybrid and mechanical DC circuit breakers Half bridge and full bridge MMC: modelling, control, start-up and fault management A chapter dedicated to unbalanced operation and control of MMC HVDC The advancement of protection methods for DC grids Wideband and high-order modeling of DC cables Novel treatment of topics not found in similar books, including SimPowerSystems models and examples for all HVDC topologies hosted by the 1st edition companion site. High Voltage Direct Current Transmission: Converters, Systems and DC Grids, 2nd Edition serves as an ideal textbook for a graduate-level course or a professional development course.

This book discusses HVDC grids based on multi-terminal voltage-source converters (VSC), which is suitable for the connection of offshore wind farms and a possible solution for a continent wide overlay grid. HVDC Grids: For Offshore and Supergrid of the Future begins by introducing and analyzing the motivations and energy policy drives for developing offshore grids and the European Supergrid. HVDC transmission technology and offshore equipment are described in the second part of the book. The third part of the book discusses how HVDC grids can be developed and integrated in the existing power system. The fourth part of the book focuses on HVDC grid integration, in studies, for different time domains of electric power systems. The book concludes by discussing developments of advanced control methods and control devices for enabling DC grids. Presents the technology of the future offshore and HVDC grid Explains how offshore and HVDC grids can be integrated in the existing power system Provides the required models to analyse the different time domains of power system studies: from steady-state to electromagnetic transients This book is intended for power system engineers and academics with an interest in HVDC or power systems, and policy makers. The book also provides a solid background for researchers working with VSC-HVDC technologies, power electronic devices, offshore wind farm integration, and DC grid protection.

This book discusses novel methods for planning and coordinating converters when an existing point-to-point (PtP) HVDC link is expanded into a multi-terminal HVDC (MTDC) system. It demonstrates that expanding an existing PtP HVDC link is the best way to build an MTDC system, and is especially a better option than the build-from-scratch approach in cases where several voltage-sourced converter (VSC) HVDC links are already in operation. The book reports in detail on the approaches used to estimate the new steady-state operation limits of the expanded system and examines the factors influencing them, revealing new operation limits in the process. Further, the book explains how to coordinate the converters to stay within the limits after there has been a disturbance in the system. In closing, it describes the current DC grid control concept, including how to implement it in an MTDC system, and introduces a new DC grid control layer, the primary control interface (IFC).

This book is the first of its kind to provide a comprehensive framework for connecting wind farms to weak power grids using High Voltage DC technology. Most onshore wind energy potential is located in areas that are hardly inhabited and the majority of wind energy that is being harnessed by European countries is currently offshore, both sourced from locations that lack the presence of a strong power grid. This book focuses on the many challenges the wind farm industry faces integrating both onshore and offshore wind to ‘weak’ grids using HVDC technology. Through case studies and illustrative examples the author presents a framework for theoretical and mathematical analysis of HVDC technology, its application and successful integration of onshore and offshore wind farms. Presents a unified approach for integrating onshore and offshore wind energy to existing AC systems through MTDC grids; Includes an extensive treatment of onshore wind farms connected to LCC HVDC systems; Provides a comprehensive analysis of offshore wind farms connected to VSC HVDC systems.

Modeling, Operation, and Analysis of DC Grids presents a unified vision of direct current grids with their core analysis techniques, uniting power electronics, power systems, and multiple scales of applications. Part one presents high power applications such as HVDC transmission for wind energy, faults and protections in HVDC lines, stability analysis and inertia emulation. The second part addresses current applications in low voltage such as microgrids, power trains and aircraft applications. All chapters are self-contained with numerical and experimental analysis. Provides a unified, coherent presentation of DC grid analysis based on modern research in power systems, power electronics, microgrids and MT-HVDC transmission Covers multiple scales of applications in one location, addressing DC grids in electric vehicles, microgrids, DC distribution, multi-terminal HVDC transmission and supergrids Supported by a unified set of MATLAB and Simulink test systems designed for application scenarios

Presents the latest developments in switchgear and DC/DC converters for DC grids, and includes substantially expanded material on MMC HVDC This newly updated edition covers all HVDC transmission technologies including Line Commutated Converter (LCC) HVDC; Voltage Source Converter (VSC) HVDC, and the latest VSC HVDC based on Modular Multilevel Converters (MMC), as well as the principles of building DC transmission grids. Featuring new material throughout, High Voltage Direct Current Transmission: Converters, Systems and DC Grids, 2nd Edition offers several new chapters/sections including one on the newest MMC converters. It also provides extended coverage of switchgear, DC grid protection and DC/DC converters following the latest developments on the market and in research projects. All three HVDC technologies are studied in a wide range of topics, including: the basic converter operating principles; calculation of losses; system modelling, including dynamic modelling; system control; HVDC protection, including AC and DC fault studies; and integration with AC systems and fundamental frequency analysis. The text includes: A chapter dedicated to hybrid and mechanical DC circuit breakers Half bridge and full bridge MMC: modelling, control, start-up and fault management A chapter dedicated to unbalanced operation and control of MMC HVDC The advancement of protection methods for DC grids Wideband and high-order modeling of DC cables Novel treatment of topics not found in similar books, including SimPowerSystems models and examples for all HVDC topologies hosted by the 1st edition companion site. High Voltage Direct Current Transmission: Converters, Systems and DC Grids, 2nd Edition serves as an ideal textbook for a graduate-level course or a professional development course.

In its concluding section, the thesis also outlines a few possible avenues of further research in this area.

Recently, HVDC based on VSC technology has become an area of growing interest because of its suitability in forming a transmission link for transmitting large amounts of power. M-VSC-HVDC has the possibility of being an attractive alternative to AC transmission in city centres, where underground cable transmission is preferred for safety and environmental reasons. Multi-terminal DC grids based on VSC-HVDC could be a competitive and attractive option, for many applications such as the integration of renewable energy and oil/gas platforms into the onshore grid system for supplying power to large metropolitan areas. Therefore, this thesis focuses on the control of M-VSC-HVDC and DC grids based on VSC. Firstly, a detailed non-linear model on a Power System Computer Aided Design/ElectroMagnetic Transients including Direct Current (PSCAD/EMTDC) simulation software for a 2-terminal HVDC based on VSC is presented in chapter 3. In the context of what is a complicated controller analysis and design task, the detailed analytical linear small signal state-space VSC-HVDC test system is modelled in MATLAB and is presented in chapter 3. The model should have good accuracy within the frequency range for the main HVDC control loop i.e. below 100Hz. Secondly, an eigenvalue stability study for control gains optimization is presented in chapter 4, with the use of the root locus technique. Very good matching accuracy is established in chapter 4 for the linear analytical model when compared with the detailed non-linear PSCAD test system models. A detailed comparison of the outer-loop control performance at the receiving end is presented in chapter 5. PID control (inner-loop) with d-axis current control and the DC voltage droop control (outer-loop) is confirmed to be adequate for advanced control design for an M-VSC-HVDC system and DC grid network. A 121st order MIMO small signal linearized dynamic model of a 5-terminal DC network is presented in chapter 6. The model accuracy is verified using detailed non-linear PSCAD simulation. The model has been used to study the effects of the DC voltage droop control on the dynamic and transient behaviour of the DC network. The work presented in this thesis therefore seeks to make a novel contribution by; presenting a detailed non-linear and linearized dynamic model of a DC grid based on a VSC test system. This model has significantly increased our confidence in the feasibility of DC grid networks. A higher order MIMO small signal linearized dynamic model of a 5-terminal DC network and an M-VSC-HVDC has been developed. They are the most detailed analytical models currently available. These models can be used for larger DC grids of any complexity. This thesis applies modeling knowledge boundaries to the automated building of an analytical model of a DC system and could be adapted for a very complex DC system. Two main issues regarding the implementation of the droop scheme have been investigated systematically by using the developed small signal model. Namely, the impacts and the selections of the DC droop gain and the cutoff frequency of the DC voltage droop filter. A systematic design of DC droop gains for DC grids has been presented. This thesis resolves a number of issues with developing DC grids and increases our confidence in building future complex DC transmission systems.