Fault Analysis and Protection System Design for DC Grids (Power Systems) 9811529760, 9789811529764

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Power Systems

Abhisek Ukil Yew Ming Yeap Kuntal Satpathi

Fault Analysis and Protection System Design for DC Grids

Power Systems

Electrical power has been the technological foundation of industrial societies for many years. Although the systems designed to provide and apply electrical energy have reached a high degree of maturity, unforeseen problems are constantly encountered, necessitating the design of more efficient and reliable systems based on novel technologies. The book series Power Systems is aimed at providing detailed, accurate and sound technical information about these new developments in electrical power engineering. It includes topics on power generation, storage and transmission as well as electrical machines. The monographs and advanced textbooks in this series address researchers, lecturers, industrial engineers and senior students in electrical engineering. **Power Systems is indexed in Scopus**

More information about this series at http://www.springer.com/series/4622

Abhisek Ukil Yew Ming Yeap Kuntal Satpathi •

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Fault Analysis and Protection System Design for DC Grids

123

Abhisek Ukil Department of Electrical, Computer, and Software Engineering The University of Auckland Auckland, New Zealand

Yew Ming Yeap Institute for Infocomm Research Agency for Science, Technology and Research (A*STAR) Singapore, Singapore

Kuntal Satpathi School of Electrical and Electronic Engineering Nanyang Technological University Singapore, Singapore

ISSN 1612-1287 ISSN 1860-4676 (electronic) Power Systems ISBN 978-981-15-2976-4 ISBN 978-981-15-2977-1 (eBook) https://doi.org/10.1007/978-981-15-2977-1 MATLAB® is a registered trademark of The MathWorks Inc. PSCAD™ is a registered trademark of Manitoba Hydro International Ltd. © Springer Nature Singapore Pte Ltd. 2020 This work is subject to copyright. All rights are reserved by the Publisher, whether the whole or part of the material is concerned, specifically the rights of translation, reprinting, reuse of illustrations, recitation, broadcasting, reproduction on microfilms or in any other physical way, and transmission or information storage and retrieval, electronic adaptation, computer software, or by similar or dissimilar methodology now known or hereafter developed. The use of general descriptive names, registered names, trademarks, service marks, etc. in this publication does not imply, even in the absence of a specific statement, that such names are exempt from the relevant protective laws and regulations and therefore free for general use. The publisher, the authors and the editors are safe to assume that the advice and information in this book are believed to be true and accurate at the date of publication. Neither the publisher nor the authors or the editors give a warranty, expressed or implied, with respect to the material contained herein or for any errors or omissions that may have been made. The publisher remains neutral with regard to jurisdictional claims in published maps and institutional affiliations. This Springer imprint is published by the registered company Springer Nature Singapore Pte Ltd. The registered company address is: 152 Beach Road, #21-01/04 Gateway East, Singapore 189721, Singapore

Dedicated to our family.

Preface

Electric power in the twenty-first century is no longer what it used to be. Apart from meeting the growing demand, a strong push to produce electricity in an economically sound way and at the same time environmentally friendly manner without adding carbon footprint has taken the conventional power system in a new direction. Recent years have seen tremendous interest in transmitting bulk amounts of power using the dc system. High voltage direct current system, or commonly known as HVDC, is one of the key technologies in this trajectory. Implementation of the HVDC system is made possible by the advancement of power electronics, namely power converters which convert the ac to dc and vice versa. The converter technologies employed in the present HVDC system can be broken into two types: current source converter (CSC) and voltage source converter (VSC). The inception of the CSC dates back to the 1970s. Then, the VSC came along in the 1990s. DC system also offers numerous benefits, for example, being economical for long-distance transmission, flexibility in control and enabling integration of renewable energies, particularly wind and solar. Besides the HVDC-based long-distance bulk power transmission, dc systems are also being considered for microgrids, datacentre, more-electric aircraft, shipboard power systems, etc. in the medium- and low-voltage dc (MVDC and LVDC) power distribution domains. One of the most important and critical challenges is the fault analysis and design of the protection system to safeguard such a dc grid against network faults. For the ac grid, the circuit-breakers (CB), protective relays, and the protection standards are well established, which are not quite mature for the dc grid. This is mainly because over the last century, we have mainly relied on the ac power systems. Protection systems for the ac power systems do not always conform to the dc systems, as the nature of the dc fault transient is fundamentally different from that of the ac system. This book is a comprehensive reference guide on the important topic of dc grid protection design. It bridges a much-needed research gap to enable wide-scale implementation of energy-efficient dc grids in the near future. This book looks into dc grid architecture, operation, control along with rigorous fault analysis and design

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of protection schemes for the dc grid for different applications at various voltage levels. The book starts with the introductory description on the comparative analysis of the ac and the dc grid, along with the applications, benefits, and challenges of the dc grid. This is followed by a detailed description of the dc grid architecture, including the devices, operation, and control mechanism. Analytical methods for dc fault analysis are presented in detail for different types of faults. This is followed by extensive analysis of the various dc fault identification methods, using time- and frequency-domain analysis of the dc current and voltage signals. The unit and non-unit protection strategies are discussed in detail, along with the chapter on dc fault isolation devices. Besides the fault detection and analysis, the book also provides step-by-step guidelines to build hardware-based experimental test setup and methods to validate the different algorithms, which is a major challenge. This book also includes the application-driven case-studies showing the applicability of the different protection strategies and pointers to the research problems in each chapter. Fault Analysis and Protection System Design for DC Grids is helpful for engineers, researchers and students, working in the field of dc grid, HVDC, MVDC, LVDC, microgrid for designing robust and reliable future generation dc grid. Auckland Singapore Singapore April 2020

Abhisek Ukil Yew Ming Yeap Kuntal Satpathi

Contents

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Introduction to DC Grid . . . . . . . . . . . . . . . . . . . . . . . . . . 1.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1.2 DC Grid Applications . . . . . . . . . . . . . . . . . . . . . . . . 1.2.1 Transmission Systems . . . . . . . . . . . . . . . . . . 1.2.2 Utilities and Microgrid . . . . . . . . . . . . . . . . . 1.2.3 Datacenters . . . . . . . . . . . . . . . . . . . . . . . . . 1.2.4 Transportation Systems . . . . . . . . . . . . . . . . . 1.3 Relevant Standards and Voltage Levels . . . . . . . . . . . 1.3.1 Standards . . . . . . . . . . . . . . . . . . . . . . . . . . . 1.3.2 Voltage Levels . . . . . . . . . . . . . . . . . . . . . . . 1.4 Power Quality Issues . . . . . . . . . . . . . . . . . . . . . . . . . 1.5 Challenges in DC Grids: Design of Protection System 1.5.1 Repercussions of Faults in Existing Power Systems . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1.5.2 Challenges with Fault Detection in DC Grids . 1.5.3 Challenges with Fault Isolation in Grids . . . . 1.5.4 Some Practical Challenges . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

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Components and Architectures of DC Grid for Various Applications . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.2 Components in DC Grids . . . . . . . . . . . . . . . . . . . . 2.2.1 Diode Bridge Converters . . . . . . . . . . . . . . 2.2.2 Thyristor Based Current Source Converters . 2.2.3 IGBT Based Voltage Source Converters . . . 2.2.4 Emerging Converter Topologies . . . . . . . . . 2.2.5 DC/DC Converters . . . . . . . . . . . . . . . . . . . 2.2.6 Energy Storage Technologies . . . . . . . . . . .

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DC Grid Architectures and Applications . . . . . . . . . . 2.3.1 Transmission Applications: HVDC Systems . 2.3.2 Utilities Applications: Microgrids . . . . . . . . 2.3.3 Datacenter Applications . . . . . . . . . . . . . . . 2.3.4 Transportation Applications . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

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Modeling and Control of Generation System for DC Grid Applications . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.2 Generation Systems for HVDC and Microgrid Applications . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.2.1 CSC-Based Generation System . . . . . . . . . . . . . . . 3.2.2 VSC-Based Generation System . . . . . . . . . . . . . . . 3.3 Generation Systems for Marine and Aerospace Applications 3.3.1 AVR Based Generation System . . . . . . . . . . . . . . . 3.3.2 AFR Based Generation System . . . . . . . . . . . . . . . 3.3.3 Comparison . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

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Faults in DC Networks . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.1.1 Types of Faults in DC Networks . . . . . . . . . . . 4.1.2 Statistics of Faults in DC Networks . . . . . . . . . 4.1.3 Effect of Topology on Faults in DC Networks . 4.2 Fault Current Calculations: CSC-Based DC System . . . 4.3 Fault Current Calculations: VSC-Based DC System . . . 4.3.1 Pole-to-Pole Fault . . . . . . . . . . . . . . . . . . . . . . 4.3.2 Pole-to-Ground Fault . . . . . . . . . . . . . . . . . . . 4.4 Fault Current Calculations: MMC-Based DC System . . 4.4.1 Pole-to-Pole Fault . . . . . . . . . . . . . . . . . . . . . . 4.4.2 Pole-to-Ground Fault . . . . . . . . . . . . . . . . . . . 4.5 Fault Current Calculation: Travelling Wave Approach . 4.6 Example . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Open Problems . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

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Time-Domain Based Fault Detection in DC Grids 5.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 5.2 Overcurrent Based Protection . . . . . . . . . . . . 5.3 Rate of Change-Based Protection . . . . . . . . . . 5.3.1 Current . . . . . . . . . . . . . . . . . . . . . . 5.3.2 Voltage . . . . . . . . . . . . . . . . . . . . . . 5.3.3 Practical Application . . . . . . . . . . . . .

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Capacitive Discharge Method . 5.4.1 Background . . . . . . . . 5.4.2 Principle of Operation 5.4.3 Example . . . . . . . . . . 5.5 Conclusion . . . . . . . . . . . . . . . Open Problems . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . .

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Frequency-Domain Based Fault Detection: Application of Short-Time Fourier Transform . . . . . . . . . . . . . . . . . . . . . . 6.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6.2 Operation of STFT . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6.3 Application of STFT to Constant and Step Change in DC Current . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6.3.1 STFT Application on Constant DC Current . . . . . 6.3.2 STFT Application on Step Change in DC Current 6.4 Fault Detection by STFT . . . . . . . . . . . . . . . . . . . . . . . . . 6.4.1 Fault Detection Criteria . . . . . . . . . . . . . . . . . . . . 6.4.2 Selection of Window Length . . . . . . . . . . . . . . . . 6.4.3 Effect of Window Function . . . . . . . . . . . . . . . . . 6.4.4 Determining Tripping Threshold . . . . . . . . . . . . . 6.4.5 Implementing STFT Based Fault Detection . . . . . 6.5 Test System to Evaluate STFT Based Fault Detection Algorithm . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6.5.1 Point-to-Point DC System . . . . . . . . . . . . . . . . . . 6.5.2 Multi-terminal DC System . . . . . . . . . . . . . . . . . 6.6 Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Open Problems . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

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Time-Frequency Domain Analysis: Wavelet-Transform Based Fault Detection . . . . . . . . . . . . . . . . . . . . . . . . . . 7.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7.2 Selection of Mother Wavelet . . . . . . . . . . . . . . . . . 7.3 Detection Algorithm . . . . . . . . . . . . . . . . . . . . . . . 7.4 Example . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7.4.1 Two-Terminal HVDC System . . . . . . . . . . 7.4.2 Multi-terminal HVDC System . . . . . . . . . . 7.5 Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Open Problems . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

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Non-unit Protection Strategies for DC Power Systems . . . . . . . 8.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8.2 Non-unit Protection Strategies in AC System and Implementation Challenges in DC System . . . . . . . . . . 8.3 Fault Current Computation: Current Derivatives and Associated Parameters . . . . . . . . . . . . . . . . . . . . . . . . 8.3.1 Computing Peak Fault Current and Time to Reach Peak Fault Current . . . . . . . . . . . . . . . . . 8.3.2 Computing Derivative Using Difference Equations . 8.3.3 Comparison of Approximation of Derivative . . . . . 8.4 System Description for Non-unit Protection Studies . . . . . . 8.5 Definite Time Based Protection Coordination . . . . . . . . . . . 8.5.1 Using Current Magnitude . . . . . . . . . . . . . . . . . . . 8.5.2 Using Current Derivatives . . . . . . . . . . . . . . . . . . . 8.6 Definite Time Based Protection Coordination Using Estimated Inductance . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8.7 Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Open Problems . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Introduction to Directional Protection and Communication Assisted Protection Systems . . . . . . . . . . . . . . . . . . . . . . . . 9.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9.2 Need for Directional Protection . . . . . . . . . . . . . . . . . . 9.3 Analysis of Directional Fault Currents . . . . . . . . . . . . . 9.3.1 System Description . . . . . . . . . . . . . . . . . . . . . 9.3.2 Fault Analysis Using Superimposed Quantities . 9.4 Directional Protection Design . . . . . . . . . . . . . . . . . . . 9.4.1 Directional Element Design . . . . . . . . . . . . . . . 9.4.2 Fault Detection . . . . . . . . . . . . . . . . . . . . . . . . 9.5 Performance Comparison of Various Directional Protection Strategies . . . . . . . . . . . . . . . . . . . . . . . . . . 9.6 Communication Assisted Protection Strategies . . . . . . . 9.7 Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Open Problems . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

10 Fault 10.1 10.2 10.3 10.4

Isolation in DC Grids . . . . . . . . . . . . . Introduction . . . . . . . . . . . . . . . . . . . . . Time Line of Fault Isolation . . . . . . . . . DC Grid Protection Devices . . . . . . . . . DC Circuit Breakers . . . . . . . . . . . . . . . 10.4.1 Resonant Type DC Breaker . . . 10.4.2 Non-resonant Type DC Breaker

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10.5 Converter Based Isolation . . . . . . . . . . . . . . . . . . . . . . . 10.5.1 SSCB Based on VSC with Freewheeling Diode . 10.5.2 SSCB Based on H-Bridge Converter . . . . . . . . . 10.6 Commercial DC Breakers . . . . . . . . . . . . . . . . . . . . . . . 10.6.1 HVDC . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10.6.2 MVDC . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10.6.3 LVDC . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Open Problems . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11 Design of Experiment and Fault Studies . . . . . . . . . . . . . 11.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11.2 Experimental Setup Description . . . . . . . . . . . . . . . . 11.2.1 Converter . . . . . . . . . . . . . . . . . . . . . . . . . . 11.2.2 DC Line . . . . . . . . . . . . . . . . . . . . . . . . . . 11.2.3 Measurement and Control . . . . . . . . . . . . . . 11.2.4 Controller Tuning . . . . . . . . . . . . . . . . . . . . 11.2.5 Fault and Protection Measure . . . . . . . . . . . 11.3 Experimental Results . . . . . . . . . . . . . . . . . . . . . . . . 11.3.1 Steady State . . . . . . . . . . . . . . . . . . . . . . . . 11.3.2 Fault on DC Line . . . . . . . . . . . . . . . . . . . . 11.3.3 Load Change . . . . . . . . . . . . . . . . . . . . . . . 11.4 Validation of Fault Detection Methods on Real Fault Signal . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11.4.1 Wavelet Transform . . . . . . . . . . . . . . . . . . . 11.4.2 Capacitive Discharge . . . . . . . . . . . . . . . . . 11.4.3 Short-Time Fourier Transform . . . . . . . . . . . 11.4.4 Comparison . . . . . . . . . . . . . . . . . . . . . . . . 11.5 Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Open Problems . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12 Case Studies . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12.2 Protection System Design for Long-Distance HVDC Systems . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12.2.1 Fault Clearance and Recovery Strategy . . . . 12.2.2 Fault Clearance Method . . . . . . . . . . . . . . . 12.2.3 Recovery Strategy . . . . . . . . . . . . . . . . . . . 12.2.4 Results and Discussion . . . . . . . . . . . . . . . .

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356 356 358 363 366

xiv

Contents

12.3 Protection System Design for Compact DC Distribution Systems . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12.3.1 Transient Analysis and Protection Requirements 12.3.2 Fault Detection and Selectivity Methods . . . . . . 12.3.3 Protection Design . . . . . . . . . . . . . . . . . . . . . . . 12.4 Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Open Problems . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

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371 372 376 379 384 384 386

Index . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 389

Chapter 1

Introduction to DC Grid

1.1 Introduction The earliest development of the power systems was based on dc which was supported by Edison. However, three-phase ac power systems became popular with a number of inventions by Nikola Tesla. AC power was further accepted by the industrial units with the advent of three-phase ac machines, step-up/down transformers which resulted in easier power conversion along with the benefits of high voltage ac (HVAC) transmission systems. HVAC transmission could transfer the bulk amount of power over long distances with lesser power losses. The manufacturing industries accepted the induction machines which were rugged, easier to install and use. As a result, electrical grids in the 20th century was dominated by the ac power systems. The prime drawback of widespread adoption of the dc power systems was the absence of power electronic devices which could step up/down the voltage levels for effective transmission and distribution. However, with the rapid development of efficient power electronic devices and conversion systems, electric power in the 21st century is no longer what it used to be. Apart from meeting growing demand, a strong push to produce electricity in an economically sound way, at the same time being environmentally friendly without adding significant carbon footprint, has taken conventional ac power system in a new direction. Governmental policies and push towards increased integration of the renewable energy sources have changed the needs of existing and future electric grid. One of the prime drawbacks of using the renewable energy sources such as photovoltaic (PV) systems, wind power is its intermittent nature. However, with the development of power-electronic conversion systems with advanced control architectures, such intermittent renewable energy sources could be easily integrated with the existing grid while maintaining acceptable power quality. Efficiency of the system can further be improved by using dc grid. Apart from the efficiency factor, other advantages of dc power systems for different applications are discussed later in the subsequent sections.

© Springer Nature Singapore Pte Ltd. 2020 A. Ukil et al., Fault Analysis and Protection System Design for DC Grids, Power Systems, https://doi.org/10.1007/978-981-15-2977-1_1

1

2

1 Introduction to DC Grid

In modern power systems, dc grid is mostly operational for the transmission applications. The high-voltage direct current, commonly known as HVDC, is one of the key technologies in this trajectory. Historically, the development of dc grid was not very straightforward. Developing the dc conversion systems were the significant advances in the development of dc grids. Some of the major breakthroughs in dc power systems are detailed as following [1]: • 1901/02: Mercury-vapour rectifier invented by Peter Cooper Hewit. • 1940: Ongoing experimental efforts on thyratrons in America and mercury arc valves in Europe. • 1954: World’s first commercial HVDC transmission, “Gotland 1” in Sweden. • 1970: Demonstration of first solid state semiconductor valves. • 1979: Development of earliest prototype of microcomputer based controller for HVDC applications. • 1984: Achievement of dc transmission voltage, ±300 kV for HVDC applications in Itaipu, Brazil. • 1994: Demonstration of dc filters for filtering applications. • 1997: Development of voltage source converter (VSC), namely IGBT-based HVDC transmission in Sweden. • 1998: Capacitor commutated converter (CCC) to interconnect Argentina-Brazil. • 2013: Rio-Madeira HVDC link is commissioned in Brazil, for a transmission line length 2375 km. • 2018–19: Changji-Guquan, China UHVDC link is set to be commissioned for record ±1100 kV, 12 GW, for a transmission line length 3284 km. Table 1.1 highlights the major HVDC systems installed worldwide in chronological order. In recent years, application of dc power systems for transportation, microgrids, data centers are becoming widely popular which will be described later in this chapter.

1.2 DC Grid Applications 1.2.1 Transmission Systems HVAC and ultra-high voltage ac (UHVAC) systems have been traditionally considered for transferring bulk power over long distances. A generalized schematic of the ac transmission is shown in Fig. 1.1a. Power flow in such circuits is supported by the phase-shifting transformers and Flexible Alternating Current Transmission System (FACTS) devices [2]. Power flow control using phase-shifting transformer is generally enabled by the mechanical tap changers thus restricting the speed of power flow control. The FACTS devices however, are used to improve the voltage stability, while controlling the power flow within a prescribed range. On the other hand, basic architecture of the point-to-point (P2P) HVDC systems is shown in Fig. 1.1b.

Type

Mercury-arc Mercury-arc Mercury-arc Mercury-arc Mercury-arc Thyristor valve Thyristor valve Thyristor valve Thyristor valve Thyristor valve Thyristor valve Thyristor valve Thyristor valve Thyristor valve Thyristor valve Thyristor valve Thyristor valve IGBT (trial) Thyristor valve Thyristor valve IGBT Thyristor valve IGBT

HVDC system, country

Gotland, Sweden Volgograd-Donbass, USSR Benmore-Haywards, New Zealand Pacific Intertie Stage I, USA Nelson River Pole 1, Canada Skagerrak, Norway-Denmark Nelson River Pole 2, Canada Caborra Bassa, South Africa Hokkaido-Honshu, Japan Itaipu, Brazil Pacific Intertie, USA Cross Channel, England-France Rihand-Delhi, India Liberty-Mead, USA Quebec-New England, Canada Benmore-Haywards, New Zealand Nelson River Pole 3, Canada Hellsjön, Sweden Leyte-Luzon, Philippines Chandrapur, India Terranora, Australia Tian-Guang, China Murraylink, Australia

1954 1962 1965 1970 1973 1976 1978 1979 1979 1984 1985 1986 1987 1989 1991 1992 1992 1997 1998 1999 2000 2001 2002

30 720 600 1600 1850 500 900 1920 300 1575 2000 2000 1500 1600 2250 1200 2000 3 440 1500 180 1800 200

Commissioning year Rated power (MW)

Table 1.1 Major HVDC systems worldwide in chronological manner

150 ±400 ±200 ±400 ±463 ±250 ±250 ±533 ±250 ±300 ±500 ±300×2 ±500 ±500 ±450 ±350 ±500 180 ±350 ±500 80 ±500 150

Rated voltage (kV) 96 473 610 1360 890 240 890 1400 193 785 1362 72 810 400 1105 610 890 10 450 750 60 960 176

(continued)

Transmission distance (km)

1.2 DC Grid Applications 3

Type

Thyristor valve Thyristor valve Thyristor valve IGBT Thyristor valve IGBT Thyristor valve IGBT Thyristor valve Thyristor valve Thyristor valve IGBT IGBT IGBT IGBT Thyristor valve Thyristor valve IGBT IGBT IGBT

HVDC system, country

Talcher-Kolar, India Three Gorges-Guangdong, China Three Gorges-Shanghai, China Estlink, Finland Guizhou-Guangdong, China Caprivi, Namibia Yunnan-Guangdong, China BorWin1, Germany Rio-Madeira, Brazil Jinping-Sunan, China Hami-Zhengzhou, China Zhoushan MTDC, China BorWin2, Germany NordBalt, Sweden DolWin1, Germany Champa-Kurukshetra, India Agra, India DolWin2, Germany DolWin3, Germany Maritime, Canada

Table 1.1 (continued)

2003 2004 2006 2006 2007 2010 2011 2012 2013 2013 2014 2014 2015 2015 2015 2016 2016 2016 2017 2017

2500 3000 3000 350 3000 300 5000 400 7100 7200 8000 400 800 700 800 3000×2 6000 900 900 500

Commissioning year Rated power (MW) 1450 940 1060 105 1200 950 2071 200 2375 2090 2192 135 200 450 165 1365 1728 135 160 360

±500 ±500 ±500 ±150 ±500 350 ±800 ±150 ±600 ±800 ±800 ±200 ±300 300 ±320 ±800 ±800 ±320 ±320 200

(continued)

Transmission distance (km)

Rated voltage (kV)

4 1 Introduction to DC Grid

Type

Thyristor valve Thyristor valve IGBT IGBT IGBT Thyristor valve Thyristor valve

HVDC system, country

Labrador-Island, Canada Dianxibei-Guangdong, China Caithness Moray, UK BorWin3, Germany Nemo, Belgium-UK Raigarh-Pugalur, India Changji-Guquan, China

Table 1.1 (continued)

2017 2017 2018 2019 2019 2019 2019

900 5000 1200 900 1000 6000 12000

Commissioning year Rated power (MW)

Transmission distance (km) 1135 1928 160 200 140 1830 3284

Rated voltage (kV) ±350 ±800 320 ±300 400 ±800 ±1100

1.2 DC Grid Applications 5

6

1 Introduction to DC Grid

(a)

(b)

Fig. 1.1 a Schematic of the simplified ac transmission. b Basic architecture of point-to-point HVDC transmission system

In such dc grid, power flow can be easily controlled by the interfaced converters which would be described in detail in Chaps. 2 and 3. Further, the dc transmission provides superior performance regarding the power transfer capability limits, lower insulation requirements, lower cost and advantages in the transmission tower installations. These advantages are substantiated by the comparative studies of the dc and ac systems for transmission applications. For such purpose, 2-wire central-point earthed dc system and 3-phase ac system are considered, as shown in Fig. 1.2a and b respectively. The following parameters are considered for the study, • • • • • • • •

Vd : dc voltage between the 2-wires in dc system Id : dc current Pd : power transmitted by the dc system Va : rms ac voltage (line-to-neutral for each wire) Ia : rms ac current Pa : power transmitted by ac system R : resistance of each wire cos φ : power factor of the ac system.

1.2 DC Grid Applications

7

(a)

(b)

Fig. 1.2 Schematics of a 2-wire, central-point earthed dc system, b 3-phase ac system

1.2.1.1

Comparison of Insulation Requirements

To compare the insulation requirements for the dc and the ac systems, it is considered that both the 3-phase ac and the dc system would carry equal amount of power, incurring equal loss. As shown in Fig. 1.2a, for the dc system, the voltage of one circuit will be +Vd /2, and the other one −Vd /2, with respect to the earth. Thus, dc insulation will be dependent on the maximum voltage level which is given by, Vins_dc =

Vd . 2

(1.1)

On the otherhand, ac insulation level is determined by the peak value of ac voltage, given by, √ Vins_ac = 2Va . (1.2) Power transmitted by the 2-wire dc system is given as, Pd = 2

Vd Id = Vd Id . 2

(1.3)

Assuming unity power factor (only theoretically), the power transmitted by the 3-phase ac system is given as, Pa = 3Va Ia cos φ = 3Va Ia .

(1.4)

Power loss in the dc system is given as, Ploss_dc = 2Id2 R.

(1.5)

Power loss in the ac system is given as, Ploss_ac = 3Ia2 R.

(1.6)

8

1 Introduction to DC Grid

As the power losses are assumed to be equal, from (1.5) and (1.6), we get, 2Id2 R = 3Ia2 R, Ia = Id



2 . 3

(1.7)

(1.8)

As the transmitted power are assumed to be equal, from (1.3), (1.5) and (1.8), 3Va Ia = Vd Id ,  2 √ Vd 3Ia = 6. = =3 Va Id 3

(1.9)

(1.10)

Using (1.1), (1.2) and (1.10), √ 6 Vd /2 Vins_dc =√ = √ = 0.867. Vins_ac 2Va 2 2

(1.11)

Therefore, to transmit equal amount of power while incurring similar power losses, insulation requirement of dc system is 86.7% of the ac system.

1.2.1.2

Comparison of Power Transfer Capability

For comparing the power transfer capability, it is considered that the dc and the ac systems have same number of conductor and insulation, carrying same amount of current. This means, (1.12) I d = Ia . Furthermore, as the insulation levels are same for both the systems, the peak voltages would be same. Thus, from (1.1) and (1.2), we get √ Vd = 2Va , 2

(1.13)

√ Vd = 2 2. Va

(1.14)

As per Fig. 1.2, it is considered that the transmitted power per conductor in the dc and the ac systems, with 2 conductors for the dc system and 3 conductors for the ac system. Thus, by using (1.3), (1.4) and (1.14),

1.2 DC Grid Applications

(a)

Erection 8%

Other Equipment 10%

9

(b) Engineering 10% Freight / Insurance 5%

Control 7%

Converter Transformers 16%

AC Filters 10%

Civil Works 14%

Valves 20%

Fig. 1.3 a Cost breakdown of the HVDC substation. b Comparison of cost for dc and ac transmission systems

√ 2 2 √ Pd Vd Id /2 = = 2 = 1.414. = Pa 3Va Ia /3 2

(1.15)

Therefore, converting an existing ac line into dc line would increase the power transfer capacity roughly by 41.4%.

1.2.1.3

Comparison of Costs

Cost breakdown of HVDC system depends on many factors such as installed power capacity, converter requirements, transmission system (cables/overhead lines), environmental conditions, safety/regulatory requirements and so on. A typical cost structure of the HVDC converter station is shown in Fig. 1.3a. Broadly, the capital cost of the HVDC power transmission system has two prime components namely the capital cost for the substations and the lines. The dc system has less number of conductor than the ac system with lower transmission tower and footing area. The dc lines and cables are cheaper than the ac counterpart. Also, the power transfer capability of the dc systems is higher than the ac system. Therefore, the line cost would be lower for the dc systems. On the other hand, the dc system would require converter stations (discussed in detail in Chap. 2) for converting ac to dc (at sending end) and dc to ac (at receiving end). This is higher than the ac systems (transformer and substation). Combining the two costs, the total costs versus the line length are plotted for the dc and the ac systems in Fig. 1.3b. The abscissa of the point of intersection of the two cost curves is the breakeven distance. If the length of the line is more than the break-even distance, then the dc transmission system would be more economical than the ac transmission system. Typically the break-even distance for overhead lines is in the range of 400–700 km, and for underground cables 30–50 km. However, the break-even distance varies for each individual project with different requirements. Hence, it is usually checked per project.

10

1 Introduction to DC Grid

Fig. 1.4 Tower configuration for HVDC and HVAC systems transmitting 2 GW power [3]

1.2.1.4

Tower Installations

As discussed earlier, the number of conductors required for dc transmission is 2 and ac transmission is 3. Thus, the tower configuration changes for same amount of power transmission capacity. Also, due to higher insulation requirements in ac transmission, the clearance of the lines above earth is higher than HVDC. An illustration of the tower configuration for ac and dc transmission, transmitting 2 GW power is shown in Fig. 1.4 [3]. Moreover, the dc system has zero frequency which results in minimum charging currents in the dc transmission cables. As a result, the dc power systems do not typically require reactive power compensation techniques thus saving cost and additional infrastructure needs.

1.2.1.5

Practical Advantages of HVDC Systems

The dc grid is an economical and efficient alternative to the traditional ac transmission which is supported by the aforementioned discussions. Some of the advantages and motivation to opt for the HVDC grids are described below [1]. 1. Itaipu, Brazil Project: This HVDC project in Brazil was chosen to interconnect 50 Hz and 60 Hz system while economically transmitting large amount of hydro power over large distance (800 km). 2. Leyte-Luzon Project: This HVDC project in Philippines was crucial to interconnect geothermal power and enhance stability across the ac network in Manila. 3. Rihand-Delhi Project: This HVDC project in India was selected to enable bulk power transmission (1500 MW) while incurring minimum losses and better stability and control.

1.2 DC Grid Applications

11

4. Garabi Project: This is an independent transmission project (ITP), transferring power from Argentina to Brazil. Back-to-back HVDC system was chosen to ensure supply of 50 Hz bulk (1000 MW) power to a 60 Hz system. 5. Gotland, Sweden: HVDC was chosen to integrate a newly developed wind power site and also to improve the power quality. 6. Queensland, Australia: HVDC was chosen to connect two independent grids of New South Wales and Queensland. This enabled electricity trading between the two systems including change in power direction, low environmental impact and reduced construction time. With regard to the aforementioned discussions, advantages of the dc power systems for the transmission applications i.e. HVDC systems can be summarized as follows: 1. HVDC transmission is economical for long-distance bulk power transmission, with reduced losses. Moreover, HVDC system has greater power transfer capability per conductor than the HVAC transmission. 2. For dc lines, no reactive power compensation is required, which is an essential requirement in ac transmission. 3. For dc lines, as only resistive losses occur, voltage regulation is much less. 4. Due to the absence of frequency factor, there will be no skin effect for HVDC transmission. 5. Due to less loss, minimal stability issue and charging current, HVDC transmission is popular for long-distance power transmission, e.g., large hydro-electric plants, which are often located thousands of kms away from load centres. 6. With no frequency issue, HVDC system is particularly helpful for renewable energy integration. 7. The HVDC system forms an asynchronous link between two ac systems, i.e., the sending- and receiving-end may operate at different frequencies. Therefore, HVDC is used for interconnecting major regional ac grids. This improves the flexibility and overall grid stability. 8. The power flow through the HVDC lines can be controlled faster than ac system in steady-state as well as transient conditions. 9. Fast control of the HVDC lines, helps to mitigate ac system oscillations. 10. As discussed in previous section, there is significant amount of insulation savings in the HVDC transmission. 11. Due to higher insulation level, for HVAC transmission, the clearance of the lines above the earth is higher than HVDC. 12. For HVDC system, with less number of conductors, and narrower towers, reduced amount of Right of Way (ROW) is required. For example, for 400 kV HVAC transmission, typically 20 m spacing is required for any two outside conductors, while that is only 10 m for HVDC. 13. HVDC has significantly less radio interference than ac transmission, especially when negative polarity voltage is used.

12 Fig. 1.5 World energy consumption and share of renewable energy [6]

1 Introduction to DC Grid Renewable Energy 25.3% Coal 38.3%

Nuclear 10.2%

Natural Gas 22.9%

Oil 3.3%

1.2.2 Utilities and Microgrid With the depletion of fossil energy, renewable energy technologies like the photovoltaic (PV) and wind are growing day by day. Besides, oftentimes it is expensive to provide power to remote area, which are not connected to the main grid. Therefore, standalone energy system, e.g., a microgrid, is an ideal source for the remote area. Wind, PV, ocean wave, etc. are the universal renewable energy sources. According to [4], the concept of microgrids is that ‘microgrid is a solution for the reliable integration of distributed generations, including energy storage system and controllable load’. Microgrid can work in islanded or grid connected mode [5]. Figure 1.5 shows the world energy consumption and the share of renewable energy, as per international energy agency (IEA) statistics [6]. The major drawback of naturedependent renewable energy sources is intermittent nature. For example, generation of PV is affected by temperature, irradiance and shadings effects. These will hinder the grid integration of the renewable energy sources. To address this problem, microgrids connect distributed generations (DGs) along with hybrid energy storage systems (HESS) [7, 8] to the utility grid. Wind power is typically connected to grid at high (>65 kV) and medium (