Advanced Topics in Science and Technology in China Hao Zhou et al Editors Ultra-high Voltage AC/DC Power Transmission 123 Advanced Topics in Science and Technology in China Zhejiang University is one of the leading universities in China In Advanced Topics in Science and Technology in China, Zhejiang University Press and Springer jointly publish monographs by Chinese scholars and professors, as well as invited authors and editors from abroad who are outstanding experts and scholars in their fields This series will be of interest to researchers, lecturers, and graduate students alike Advanced Topics in Science and Technology in China aims to present the latest and most cutting-edge theories, techniques, and methodologies in various research areas in China It covers all disciplines in the fields of natural science and technology, including but not limited to, computer science, materials science, life sciences, engineering, environmental sciences, mathematics, and physics More information about this series at http://www.springer.com/series/7887 Hao Zhou Wenqian Qiu Ke Sun Jiamiao Chen Xu Deng Feng Qian Dongju Wang Bincai Zhao Jiyuan Li Sha Li Yuting Qiu Jingzhe Yu • • • • • • • Editors Ultra-high Voltage AC/DC Power Transmission 123 Editors Hao Zhou College of Electrical Engineering Zhejiang University Hangzhou China Dongju Wang College of Electrical Engineering Zhejiang University Hangzhou China Wenqian Qiu China Energy Engineering Group Zhejiang Electric Power Design Institute Co., Ltd Hangzhou China Bincai Zhao State Grid Weifang Power Supply Company Weifang China Ke Sun Zhejiang Electric Power Company Hangzhou China Jiyuan Li College of Electrical Engineering Zhejiang University Hangzhou China Jiamiao Chen China Energy Engineering Group Zhejiang Electric Power Design Institute Co., Ltd Hangzhou China Sha Li College of Electrical Engineering Zhejiang University Hangzhou China Xu Deng Guangzhou Municipal Commission of Commerce Guangzhou China Yuting Qiu College of Electrical Engineering Zhejiang University Hangzhou China Feng Qian China Energy Engineering Group Zhejiang Electric Power Design Institute Co., Ltd Hangzhou China Jingzhe Yu College of Electrical Engineering Zhejiang University Hangzhou China ISSN 1995-6819 ISSN 1995-6827 (electronic) Advanced Topics in Science and Technology in China ISBN 978-3-662-54573-7 ISBN 978-3-662-54575-1 (eBook) https://doi.org/10.1007/978-3-662-54575-1 Jointly published with Zhejiang University Press, Beijing, China The print edition is not for sale in China Mainland Customers from China Mainland please order the print book from: Zhejiang University Press Library of Congress Control Number: 2017935388 © Zhejiang University Press, Hangzhou and Springer-Verlag GmbH Germany 2018 This work is subject to copyright All rights are reserved by the Publishers, 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 publishers, 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 publishers nor the authors or the editors give a warranty, express or implied, with respect to the material contained herein or for any errors or omissions that may have been made The publishers remains neutral with regard to jurisdictional claims in published maps and institutional affiliations Printed on acid-free paper This Springer imprint is published by Springer Nature The registered company is Springer-Verlag GmbH Germany The registered company address is: Heidelberger Platz 3, 14197 Berlin, Germany Preface Viewed from the distribution of the energy resources throughout China, though the total reserves are abundant, the resource distribution and productivity distribution are rather unbalanced The coal resource is mostly located in the North and Northwest China, the hydropower resource is mainly located in the Southwest China, the onshore wind energy and solar energy resources are mainly located in the Northwest China, while the energy demands are mainly concentrated in the Central China and China’s east coastal areas The distance between the energy base and the load center is up to 1000 km The energy resources used for power generation are mainly coal and water, and the energy resources and productivity development are reversely distributed, which is the basic national condition of China Since the reform and opening-up, the electricity demand of China has been continuously and rapidly increased, and the scale and capacity of the newly built power sources have been increased Subject to the energy transmission capacity and environmental protection requirements, China will inevitably develop the long-distance and large-capacity power transmission technology to improve the development and utilization rate of the resources, alleviate the pressure in the energy transmission, and meet the requirements of the environmental protection The UHV power transmission technology is the power transmission technology with the highest voltage level in the world presently, and the most prominent characteristic thereof is the large-capacity, long-distance, and low-loss power transmission The transmission capacity of the 1000 kV UHVAC system is about 4–5 times that of the 500 kV extra-high voltage (EHV) AC system The development of the UHVAC and UHVDC power transmission can effectively solve the issue of large-scale power transmission In addition, compared with the EHV power transmission line, the UHV line occupies less land resource and achieves quite prominent economic and social benefits under the same power transmission capacity The building of the national-level power grid in which the UHV grid acts as the backbone and the grids of all levels develop in a coordinated manner, meeting the basic national condition of China that the energy resources and economic development are reversely distributed and according with the China’s overall arrangement for energy-saving and emission reduction, is the effective way to v vi Preface realize the coordinated development of grids and power sources and the urgent demand for the construction of the resource-saving and environment-friendly society In the world, a few countries such as the former Soviet Union, Japan, America, Italy, and Canada have ever conducted tests and researches on the UHVAC power transmission technology During 1981–1994, the former Soviet Union successfully built a total of 2364 km 1150 kV power transmission lines, among which, the Ekibastuz–Kokshetau line (495 km in length) put into operation at 1150 kV in 1985 was the first UHV power transmission line put into actual operation in the world Japan built the 1000 kV UHVAC double-circuit power transmission line in the 1990s , which, however, was under the 500 kV reduced voltage operation all the time The overseas DC power transmission project with the highest voltage level that has been built and put into operation is the Itapúa Power Transmission Project in Brazil, which includes double-circuit DC line with voltage level of ±600 kV and rated transmission power of 3600 MW The Soviet Union ever planned to build a ±750 kV UHVDC power transmission line project from Ekibastuz to Tambovskaya Oblast, the first engineering practice of the UHVDC power transmission technology in the world, and commenced the construction in 1980, but finally ceased the construction due to the political and economic reasons, after the completion of the construction of 1090 km-long line The research on the UHV power transmission was started relatively late in China Since 1986, the research on the UHV power transmission has been successively included in the key science and technology research programs during China’s “Seventh Five-Year Plan,” “Eighth Five-Year Plan,” and “Tenth Five-Year Plan” During 1990–1995, the Significant Project Office of the State Council organized the “Demonstration of Long-distance Transmission Modes and Voltage Levels”; and during 1990–1999, the State Scientific and Technological Commission organized the monographic researches such as the “Preliminary Demonstration of UHV Power Transmission” and “Feasibility of Application of AC Megavolt Ultra-high Voltage for Power Transmission” State Grid Corporation of China put forward for the strategic concept of “establishment of the UHV-based robust state grid” in 2004 the first time to focus on the construction of a network system in which the UHV grid acts as the backbone and the grids of all levels develop in a coordinated manner China Southern Power Grid Co., Ltd also began to study the feasibility in the construction of ±800 kV DC power transmission project in 2003 In 2006, the National Development and Reform Commission formally approved the 1000 kV UHVAC Demonstration Project from Southeast Shanxi through Nanyang to Jingmen connecting the North China grid and Central China grid In 2007 and 2010, China respectively completed and put into operation the 1000 kV Southeast Shanxi–Nanyang–Jingmen UHVAC Power Transmission Demonstration Project and ±800 kV Yunnan–Guangdong and Xiangjiaba– Shanghai UHVDC Power Transmission Projects Since then, the UHV power transmission has accomplished a rapid development in China Up to August 2017, six 1000 kV UHVAC power transmission lines and nine ±800 kV UHVDC power transmission lines have been built and put into operation There is still another 1000 Preface vii kV UHVAC power transmission line and the other four ±800 kV UHVDC power transmission lines will be put into operation at the end of 2017 Moreover, one ±1100 kV UHVDC power transmission line is being built and will be put into operation in 2018 The UHV power transmission is the engineering technology leading the world’s power transmission technology Its rapid and successful development in China has fully proven the tremendous achievement accomplished by China in the technological aspect of the electric power system Meanwhile, the complexity of the UHV power transmission technology and the urgency of its development in China require that the professional personnel engaging in the work related to the electric power system have a deeper understanding and mastery of it Based on the significant research results obtained by Zhejiang University High Voltage Laboratory in the field of UHVAC and UHVDC power transmission in the last decade and the abundant practical experience accumulated by Zhejiang Electric Power Design Institute in the field of UHV power transmission engineering over the years, and in combination with the relevant research results in the aspect of UHVAC and UHVDC power transmission technology and the actual operation experience in China and abroad, this book systematically introduces the key technical issues existing in the UHVAC and UHVDC power transmission This book consists of four sections containing a total of 28 chapters, and focuses on the study of the overvoltage, insulation coordination and design of the UHV power grid Section I, consisting of three chapters provides an overview of the development of the UHV power transmission and the system characteristics and economy thereof Section II, consisting of ten chapters discusses the UHVAC system Section III, consisting of ten chapters discusses the UHVDC system Section IV, consisting of four chapters discusses the design of the UHVAC substation and UHVDC converter station as well as UHVAC and DC power transmission lines Hao Zhou is responsible for the final compilation and editing of the whole book, and Wenqian Qiu, Xu Deng, Jiyuan Li and Jingzhe Yu act as the chief reviewers We sincerely hope that this book can better help the readers understand the UHVAC and UHVDC power transmission technology and can provide reference for the research work carried out by the technicians engaging in the work related to the electric power system This book is jointly edited by the relevant researchers from Zhejiang University, Zhejiang Electric Power Design Institute, State Grid Zhejiang Electric Power Company, China Electric Power Research Institute, China Southern Power Grid Corporation, East China Grid Company Limited, Southwest Electric Power Design Institute, and North China Electric Power University The editing of this book has received the guidance and help from numerous experts Gratitude is hereby expressed to Academician Han Zhenxiang, Academician Chen Weijiang, Professor Zhao Zhida, Professorate Senior Engineer Zhou Peihong, Professorate Senior Engineer Zhang Cuixia, Professorate Senior Engineer Li Yongwei, Professorate Senior Engineer Gu Dingxie, Professorate Senior Engineer Nie Dingzhen, Professorate Senior Engineer Tian Jie, Professor Kang Chongqing, Professor Cui Xiang, Professor Li Chengrong, Professor Wen Fushuan, Professor viii Preface Xu Zheng, Professorate Senior Engineer Su Zhiyi, Professorate Senior Engineer Wu Xiong, Professorate Senior Engineer Wan Baoquan, Professorate Senior Engineer Sun Zhaoying, Professorate Senior Engineer Chen Jiahong, Professorate Senior Engineer Dai Min, Professorate Senior Engineer Li Zhibing, Professorate Senior Engineer Wang Xinbao, Senior Engineer Huang Ying, Senior Engineer Shen Haibin, etc., for their support and help The editing of this book had been in progress for nearly years and agglomerates the research results of the authors Nevertheless, due to the authors’ limited theoretical level and practical experience, inappropriateness and errors are unavoidable Any comment will be highly appreciated Hangzhou, China August 2017 Hao Zhou Contents Part I Overview Development of UHV Power Transmission Ke Sun, Dongju Wang, Sha Li and Haifeng Qiu 1.1 UHV Power Transmission 1.1.1 Development of Power Transmission Voltage Level 1.1.2 Voltage Level Sequence in Power Grid 1.1.3 Selection of UHV Transmission Voltage Levels 1.2 Development of UHV Power Transmission Technology 1.2.1 The Former Soviet Union (Russia) 1.2.2 Japan 1.2.3 United States 1.2.4 Canada 1.2.5 Italy References 11 16 16 17 19 20 20 21 Development of UHV Power Transmission in China Ke Sun, Shichao Yuan and Yuting Qiu 2.1 Necessity in the Development of UHV Power Transmission in China 2.1.1 Objectively Required by the Sustained and Rapid Growth in Electricity Demands 2.1.2 Objectively Required by the Long-Distance and Large-Capacity Power Transmission 2.1.3 Objectively Required by the Basic Law of Power Grid Development 2.1.4 Required to Ensure Safe and Reliable Energy Transmission 2.2 Development Process of UHV Power Transmission in China 23 24 24 24 26 26 27 ix 1448 J Chen et al Table 28.8 Calculated specific creepage distance required for DC transmission line AC equivalent salt deposit density (mg/cm2) DC equivalent salt deposit density (mg/cm2) Reckoned specific creepage distance required for DC transmission line (cm/kV) 0.03 0.05 0.08 0.15 0.06 0.10 0.16 0.30 3.76 4.21 4.63 5.19 L ẳ 6:2606 ỵ 0:8891  lnðESDDÞ; ð28:5Þ where L the required specific creepage distance, cm/kV; ESDD the DC equivalent salt deposit density, mg/cm2 Test shows, under the same environmental condition in inland area, that the contaminant deposit ratio (equivalent slat deposit density) of insulator string of the DC transmission line is about 2.0 times that of the AC transmission line The specific creepage distance of the DC transmission line calculated by using the Eq (28.5) will meet the requirements as listed in Table 28.8 Number of insulators selected for the built DC projects For ±800 kV UHVDC transmission line, the suspension insulator strings in light and medium icing areas all use composite insulators, and the suspension strings in heavy icing area use disc-shaped insulators No matter in light, medium, or heavy icing areas, the tension insulator strings usually use disc-shaped insulators For selecting the number of insulators, the smaller one of the insulator number recommended by DC pollution withstand voltage method and the maximum insulator number recommended by specific DC creepage distance shall be taken With the areas at altitude 1000 m as an example, the insulator number of suspension insulator string selected for several ±800 kV DC lines that have been constructed or under construction is shown in Table 28.9 28.2.3.3 Selection of the Number of Insulators for Tension Insulator Strings Operation experience shows that, because the tension insulator string’s stress is higher than the suspension insulator string’s stress, and this can easily produce zero resistance insulators, the number of insulators of tension insulator strings is generally 1–2 pieces more than the number of insulators of suspension insulator strings with the same class For horizontal arrangement of insulator strings, the pollution withstand voltage value is significantly higher than that of suspension insulator strings in the same polluted area due to relatively good self-cleaning capability 28 Design of UHVDC Transmission Lines 1449 Table 28.9 Number of insulators of suspension insulator strings (Bell V-type string) Insulator type Pollution degree Moderately Lightly polluted area polluted area (0.8 mg/cm2) (0.5 mg/cm2) Severely polluted area (0.15 mg/cm2) Project CA-745EZ (210 kN) CA-756EZ (300 kN) CA-765EZ (400 kN) CA-765EZ (400 kN) 63 77 88 56 70 81 61 74 81 65 79 86 71 86 98 Jinping–South Jiangsu Line Jinping–South Jiangsu Line Jinping–South Jiangsu Line Ningdong– Zhejiang Line in Southern Region Ningdong– Zhejiang Line in Northern Region CA-765EZ (400 kN) In terms of economy, the number of insulators of tension insulator strings shall not be increased on the basis of that of suspension insulator strings and it is not necessary to use large creepage distance anti-pollution insulators The tension insulator strings of EHVAC/EHVDC or UHVAC/DC transmission lines in China and other countries all use disc-shaped ceramic or glass insulators Under the identical pollution condition and same altitudes, it is recommended that the same number of insulators be used for both tension insulator strings and suspension insulator strings and margin be reserved for insulation coordination Table 28.10 shows the configuration of the number of tension insulators on the ±800 kV Lingzhou–Shaoxing DC Line The number of insulators is designed based on the differentiated design principle with the climate characteristics being taken into consideration Table 28.11 shows the configuration of the number of tension insulators on the ±1100 kV Zhundong–East China DC Line 28.2.3.4 Selection of Composite Insulators The results of the pollution flashover tests carried out in China and other countries demonstrate that, under the same pollution condition and even under the hydrophilic condition, the pollution flashover voltage of the composite insulator is still above 50% higher than that of the porcelain and glass insulators Therefore, under the same operating voltage, the creepage distance of the composite insulator only requires 2/3 of that of the porcelain and glass insulators In the engineering design, the creepage distance of the composite insulator is normally taken as above 3/4 of that of the disc insulator Using the pollution withstand voltage method to design the configuration solution of composite insulators also requires pollutant component correction and 1450 J Chen et al Table 28.10 Basic number of insulators of tension insulator strings on ±800 kV line at different altitudes Pollution degree Type of insulator Number of insulators per string Altitude of Altitude of Altitude of 1000 m 2000 m 2500 m 550 kN (Bell) 60/56 64/59 66/61 550 kN 44/41 45/43 46/44 (Tri-umbrella) 550 kN (Bell) 74/67 79/72 81/74 Moderately polluted area (0.08 mg/cm2) 550 kN 55/51 57/53 59/54 (Tri-umbrella) 550 kN (Bell) 85/74 90/79 93/82 Severely polluted area (0.15 mg/cm2) 550 kN 73/64 76/67 78/68 (Tri-umbrella) Note The values on the left of “/” are the numbers with the northern climate characteristics considered, and those on the right are the numbers with the southern climate characteristics considered Lightly polluted area (0.05 mg/cm2) Table 28.11 Basic number of insulators of tension insulator strings on ±1100 kV line at different altitudes Pollution degree Type of insulator Number of insulators of each string Lightly polluted area (0.05 mg/cm2) 550 kN (Bell) 87/81 550 kN 80/72 (Tri-umbrella) 840 kN (Bell) 77/70 550 kN (Bell) 103/92 Moderately polluted area (0.08 mg/cm2) 550 kN 98/79 (Tri-umbrella) 840 kN (Bell) 85/78 Severely polluted area 550 kN (Bell) 113/102 (0.15 mg/cm2) 550 kN 103/95 (Tri-umbrella) 840 kN (Bell) 92/90 Note The values on the left of “/” are the numbers with the northern climate characteristics considered, and those on the right are the numbers with the southern climate characteristics considered pollution non-uniform distribution correction just like the ceramic insulators In addition, taking the conclusion of the report “Research on the pollution discharge characteristics and high-altitude discharge coefficient of ±800 kV DC insulators” presented by CEPRI as reference, the altitude correction coefficient of transmission line insulator is as follows: the composite insulators will be compensated by 6.4% with every 1000 m increase of altitude 28 Design of UHVDC Transmission Lines 1451 Table 28.12 Basic insulation configuration of ±800 kV DC composite insulators Altitude (m) Polluted area Length of composite insulator string (m)/creepage distance (m) Moderately polluted area Severely polluted area Lightly polluted area (0.08 mg/cm2) (0.15 mg/cm2) (0.05 mg/cm2) 1000 1500 2000 9.6/36.9 9.6/36.9 10.6/40.8 9.6/36.9 10.6/40.8 10.6/40.8 10.6/40.8 11.0/42.3 11.8/45.4 Table 28.13 Basic insulation configuration of ±1100 kV DC composite insulators Altitude (m) Polluted area Length of composite insulator string (m)/creepage distance (m) Moderately polluted area Severely polluted area Lightly polluted area (0.08 mg/cm2) (0.15 mg/cm2) (0.05 mg/cm2) 1000 2000 3000 12.3/50.4 12.3/50.4 13.9/56.9 12.3/50.4 13.9/56.9 15.4/63.1 13.9/56.9 15.4/63.1 16.6/68.0 The length and creepage distance of composite insulator strings used for ±800 kV DC line projects are shown in Table 28.12 The length and creepage distance of composite insulator strings used for ±1100 kV DC Zhundong–East China DC transmission line project are shown in Table 28.13 28.2.3.5 Check Based on Overvoltage Condition The number of insulators and the length of insulator strings on the DC line are determined based on the pollution condition under the operating voltage and should also be checked based on the overvoltage condition Currently, the calculation results of the switching overvoltage level of the Chinese ±800 kV DC lines are within 1.6–1.8 p.u [1], while the calculated value of the switching overvoltage level of the ±1100 kV Zhundong–East China line is lower than 1.6 p.u The switching impulse flashover voltage of the polluted insulators on the DC line decreases with the increase of the pollution degree The test carried out by America EPRI verified that, under the same pollution condition, the DC switching withstand voltage of insulator is 2.2–2.3 times the DC withstand voltage of the insulator of the same model Furthermore, a large number of tests and researches demonstrate that, when the DC voltage is pre-applied, its 50% switching impulse voltage is 1.7–2.3 times the 50% pollution flashover operating voltage Therefore, the switching overvoltage is not a control factor for the selection of the number of insulators 1452 J Chen et al Due to the pollution, the DC line has more insulators and longer insulator strings compared to the AC 1000 kV line; besides, it has a large insulation margin under the lightning impulse voltage and back flashover lightning current of over 200 kA Therefore, the lightning overvoltage is not a control factor for the selection of the number of insulators 28.2.4 Air Clearance of Tower Head For the power transmission line, the determination of various air clearances of tower head is the basis for the determination of tower head dimension and the design of tower head structure After the deflection by wind, the air clearance between conductor and tower member shall meet the requirements of operating voltage, switching overvoltage, and lightning overvoltage [1] 28.2.4.1 Influence of Various Overvoltages on Air Clearance The maximum overvoltage of the UHVDC line often occurs at the midpoint of the non-faulty pole line when the ground fault occurs at the midpoint of the line, and the line’s overvoltage level presents a decrease trend from the line’s midpoint to the converter stations at both sides with a slight up and down The overvoltage distribution curve along the line is obtained through the electromagnetic transient calculation The air clearances corresponding to different switching overvoltage multiples are listed according to the analysis of the overvoltage level It is found that the air clearance increases as the switching overvoltage multiple increases and it is also related to the altitude at which the line is located With regard to the operation of the UHVDC line, since the DC converter valve acts fast, the time for restart is extremely short, basically without influence on the continuous operation of the line Therefore, the influence of the lightning overvoltage is not considered in the clearance design of tower head, and the clearance controlling the tower head dimension is the switching overvoltage clearance 28.2.4.2 Air Clearance of Tower Head All the tangent towers along the UHVDC line adopt the V-type insulator strings The operating voltage and lightning overvoltage not play control effect on the tower head air clearance, while the switching overvoltage will directly affect the design of tower head In the engineering design, it is necessary to carry out analysis and calculation in combination with the practical conditions of specific project, and the switching overvoltage multiple and air clearance, in particular, shall be determined based on the results of the scientific research and test organizations 28 Design of UHVDC Transmission Lines 1453 Table 28.14 Tower head air clearance of ±800 kV DC line (m) Altitude (m) 500 1000 2000 3000 Remarks Operating voltage 2.10 2.30 2.50 – 2.10 2.30 2.50 2.85 2.10 2.30 2.50 – 2.55 4.90 2.70 5.30 3.05 5.90 – – 5.30 5.70 6.40 7.10 Code for Designing of DC Transmission Line Xiangjiaba–Shanghai Line, Jinping– South Jiangsu Line Xiluodu–Zhejiang Line, Hami– Zhengzhou Line Yunnan–Guangdong Line Code for Designing of DC Transmission Line Jinping–South Jiangsu Line 4.90 5.30 5.90 – 5.50 6.30 6.90 – 5.50 5.55 6.20 Switching overvoltage 1.6 p.u 1.6 p.u 1.6 p.u 1.7 p.u 1.7 p.u Xiluodu–Zhejiang Line, Hami– Zhengzhou Line Xiangjiaba–Shanghai Line Yunnan–Guangdong Line Table 28.15 Recommended values for tower head air clearance of ±1100 kV DC line (m) Altitude (m) 1000 2000 3000 Working overvoltage clearance (m) Clearance S (m) when switching overvoltage is 1.5 p.u Clearance S (m) when switching overvoltage is 1.58 p.u 3.2 8.1 8.9 3.7 8.7 9.5 4.2 9.2 9.9 Table 28.14 shows the values of the air clearance of several ±800 kV DC lines built in China and the comparison with the air clearances specified in the Code for Designing of DC Transmission Line Table 28.15 lists the recommended values for the air clearance of the ±1100 kV Zhundong–East China DC line 28.3 Design of Insulator Strings and Fittings of DC Line 28.3.1 Insulator String of Conductor 28.3.1.1 Suspension Insulator String of Conductor The I-type strings and V-type strings are the suspension string types widely used on the UHV lines When the V-type strings are used, the interpolar distance of the conductors on the DC line will be reduced, which can effectively reduce the corridor width and steel consumption of steel towers [4] 1454 J Chen et al Fig 28.5 Comparison of I-type string and V-type string tower heads of ±800 kV UHVDC line (mm) The tower dimension of ±800 kV DC transmission line is large, occupies wide line corridor, and requires large amount of house demolition, so it is necessary to use tower type suitable for small corridor width For the straight line towers using V-type insulator string, the interpolar distance of conductors can be reduced by about m compared with that using I-type insulator strings, as shown in Fig 28.5 The V-type insulator strings have restriction to the offset of conductor Although the cross arm of the conductor is longer than that of I-type string tower, the conductor point-to-tower body horizontal distance of V-type string tower is shorter than the I-type string tower Therefore, its conductor load-to-tower body torque is smaller than the I-type string tower For this reason, the use of V-type insulator string can reduce line corridor width, house demolition, tree felling, and project construction cost, and the consumption of steel by a single V-type string tower can reduce by 7–9% as compared with I-type string tower With identical insulation configuration, the suspension length of V-type string of ±800 kV DC transmission line will be m shorter than I-type string In other words, if identical span is used, the straight line tower with V-type string can lower the tower positioning height by m as compared with I-type string On the average, the straight line tower can further reduce the tower weight by about tons On the other hand, under the condition of same string length, the V-type string arrangement can effectively reduce the pollution flashover voltage of insulator strings This provides good technical support to the safe operation of transmission line Therefore, in the aspect of enhancing insulation, the use of V-type insulator string arrangement has significant superiority In view of this, the straight line towers of UHVDC transmission lines that have been built or under construction in China all use V-type suspension insulator strings The value of included angle between two V-type suspension insulator strings has major influence on the tower head design According to large amount research and design data collected in China and other countries, the included angle of V-type insulator strings is basically in the range of 70°–120° In engineering design, the angle is generally selected as a half of V-type string included angle, and minus 5°–10° of the maximum calculated wind swing angle of I-type insulator strings 28 Design of UHVDC Transmission Lines 1455 Fig 28.6 Assembly diagram of single V-type suspension insulator strings for ±800 kV DC transmission lines (mm) Composite insulators are mostly used for the conductor suspension insulator strings of DC transmission lines (except for transmission lines in heavy icing area) in China The commonly used connection of composite insulator ends is the ball– bowl structure Through operation investigation of the built EHV transmission lines, so far, several fall-off accidents of ball head or bowl head have occurred in conductor side of composite insulator in China To effectively prevent the occurrence of similar problems, starting from Xiangjiaba–Shanghai DC transmission project, the ball head–bowl head connection structure has been reconstructed as ring–ring connection structure, thus the fall-off problems of ball head of the V-type composite insulator strings on the conductors have been solved well Composite insulator V-type string assembly using ring–ring connection is shown in Fig 28.6 28.3.1.2 Conductor Tension Insulator String Multiple bundle and large-section conductors are used for the UHV transmission lines, which have higher requirements on the mechanical strength of the conductor tension insulator strings UHVDC transmission line generally uses six-bundle or eight-bundle conductors with sub-conductor section from 630 to 1250 mm2 Therefore, insulators used for tension string are generally multiple columns of 550, 760, and 840 kN disc-type insulators For example, triple 550 kN or double 760 kN insulators can be used for 6ÂJL/G3A-900/40 conductors, quadruple 550 kN insulators can be used for 6ÂJL/G3A-1250/70 conductors, and sextuple 550 kN or quadruple 840 kN insulators can be used for 8ÂJL1/G3A-1250/70 conductors 1456 J Chen et al Fig 28.7 Schematic of assembled typical six-bundle conductor with triple tension insulator strings for ±800 kV UHVDC line A schematic of assembled typical six-bundle conductor with triple tension insulator strings is shown in Fig 28.7 28.3.1.3 Inter-string Spacing of Multiple Insulators The inter-string spacing of multiple insulator strings will be determined by the electric field intensity distribution, pollution flashover voltage, and mechanical strength of the insulator strings to guarantee the electrical strength of insulator strings, and the inter-string insulators will not contact and collide with each other under operating condition In terms of the electrical property, according to the analysis on the results of the pollution withstand voltage test on the DC double insulator strings in high-altitude areas carried out by the scientific research organization, the parallel connection of double strings will increase the probability in flashover of the whole string Under the heavy icing condition, the small inter-string distance will cause the ice and snow to fill the inter-string spacing, bridge the insulators, and reduce the icing withstand voltage of insulators In terms of the mechanical property, as the insulator string of the UHV transmission line is long, the insulator string offset caused by wind load shall also be considered in addition to the diameter of insulator disc when determining the inter-string distance, in order to guarantee that the two strings in parallel connection will have no collision The basic inter-string distance of 650 mm can be used for the suspension insulator string and tension insulator string of conductors on DC transmission line When transmission line is heavy icing, the inter-string distance of multiple insulator strings should be properly increased For Jinping–South Jiangsu and 28 Design of UHVDC Transmission Lines 1457 Xiluodu–Zhejiang DC transmission line projects in the heavy icing area, 800 and 1000 mm inter-string distances are used for suspension insulator strings and tension insulator strings, respectively 28.3.2 Selection of Main Fittings 28.3.2.1 Tower-Connecting Fittings The tower-connecting fittings commonly used on the overhead lines include the UB-type clevises, U-type shackles, trunnion clevises, GD-type clevises In previous projects, wear and break accidents occurred to UB-type clevises and U-type shackles connected with tower Considering the importance of UHV transmission line, in order to avoid similar problems, it is recommended that UB-type clevises and U-type shackles no longer be used as tower connection fittings It is recommended to use trunnion clevises and GD-type clevises to guarantee flexible rotation in each direction, and the clevises are shown, respectively, in Figs 28.8 and 28.9 Wearing should be considered for the top part connecting with tower, so the strength of the tower connection end should be higher than the real service strength and should be higher than the strength of other fittings in the string Tower connection fittings are required to use overall forging method for manufacturing to provide sufficient guarantee in mechanical property Fig 28.8 Suspension string tower-connecting fittings-trunnion clevis 1458 J Chen et al Fig 28.9 Tension string tower-connecting fittings-GD clevis Fig 28.10 Six-bundle integral suspension yoke plate for DC line (mm) 28.3.2.2 Suspension Yoke Plates The yoke plates in the suspension string include two types, namely the integral yoke plate and combined yoke plate Most of China’s ±800 kV lines using six-bundled conductors basically use the integral yoke plates In view of the design, manufacturing, construction, and installation, the application on the lines in the mountainous areas has no problem The structure of the six-bundled integral yoke plate is shown in Fig 28.10 The ±1100 kV line adopts the eight-bundled conductors with large section, increasing in the number of sub-conductors and load, which puts forward higher requirements on the manufacturing of the eight-bundled suspension yoke plates For example, when the integral yoke plate structure design is adopted, 28 Design of UHVDC Transmission Lines 1459 the weight of single piece will be above 300 kg, and the conditions for installation and transportation in mountainous areas need be fully considered 28.4 Clearance of Conductor to Ground for DC Line 28.4.1 Minimum Clearance of Conductor to Ground The DC transmission line’s clearances of conductor to ground and conductor to the crossed facility are the key parameters for the design of line project Not only is the normal insulation level need to be considered to meet the basic requirements of the large-capacity transmission, but also the influences of factors such as the electrostatic field intensity, total electric field intensity, and ion current density should be considered The clearances of conductor to ground and conductor to the crossed facility used in the DC line design can be classified into three categories depending on the principles for value selection: Clearances determined by electric field intensity The ground electric field intensity and ion current density control values of UHVDC overhead transmission line are shown in Table 28.16 The corona inception electric field intensity of conductors will reduce with the increase of altitude Under the conditions of identical voltage, conductor and line structure dimension, the ground total electric field intensity, and ion current density will increase In order to control the ground total electric field intensity and ion current density, pole conductor height should be properly increased Under the condition of identical interpolar distance, the clearance of conductor to ground should be increased by 6% with every 1000 m increase of altitude Clearances determined by electrical insulation strength This kind of distance refers to the distance determined by the discharge clearance of operating overvoltage Table 28.16 Total electric field intensity and ion current density limits Place Total electric field intensity (kV/m) Fine Rainy day day Ion current density (nA/m2) Fine Rainy day day Residential area General non-residential area Sparsely populated non-agriculture cultivation area 25 30 35 80 100 150 30 36 42 100 150 180 1460 J Chen et al Table 28.17 Clearance values of conductor to ground (m) S/N Area ±800 kV ±1100 kV Calculation conditions Residential area Non-residential area Area with poor traffic condition 30.0 (South of China) 32.0 (North of China) 26.0 (South of China) 28.0 (North of China) 21 Max sag of conductor 21.0 (South of China) 23.0 (North of China) 18.0 (South of China) 20.0 (North of China) 15.5 Mountain slope accessible on foot 13.0 15.5 (Net clearance for the) mountain slope, cliff and rock inaccessible on foot 11.0 13.5 Max sag of conductor Max sag of conductor Max wind deflection of conductor Max wind deflection of conductor Clearances determined by other factors This kind of distance is to avoid the influence of power transmission line on the facilities of other departments, and often use the values recognized by relevant organizations according to usual practice In the current line design, the values have no relationship with voltage level At the altitude of 1000 m, the clearances of conductor to ground of the ±800 and ±1100 kV UHVDC lines adopting the V-type suspension insulator strings can be referred to the values as listed in Table 28.17 28.4.2 Relation Between Clearance of Conductor to Ground and Environmental Climate The air quality and humidity in the north and the south of China has large difference, which can produce significant influence on the ground total electric field of the DC transmission line; however, it is impossible to predict correctly by calculation at present The research carried out by CEPRI based on Hami–Zhengzhou, Lingzhou–Shaoxing ±800 kV DC transmission line projects concludes that, for the transmission line project crossing the south and the north of China, because of large difference in climate, when the transmission line passes through residential area and 28 Design of UHVDC Transmission Lines 1461 non-residential area, the minimum clearance of pole conductor to ground should be corrected according to environmental climate [5] CEPRI carried out the full voltage test on electromagnetic environment of ±800 kV DC transmission line under different clearances of conductor to ground and different climate environments by using  720 and  900 mm2 conductors, proposing the recommended correction value of environmental climate for the minimum clearance of conductor to ground of ±800 kV DC transmission line constructed in the south and north of China: the minimum clearance of pole conductor to ground in general areas, non-residential areas, and residential areas in the north should be increased by 11.1 and 9.5%, respectively, than that in the general areas in the south For areas where dust absorption is serious, the minimum clearance of pole conductor to ground in residential areas and non-residential areas should be increased by 18.5 and 19.3%, respectively, as compared with that in general areas in the south 28.5 Tower Design of DC Line 28.5.1 Tower Types of DC Line Currently, all the UHVDC lines already built and under construction adopt the mode of bipolar single circuit However, with increasing construction of UHVDC transmission projects, the transmission line has entered the coastal area in east China where economy is developed, residential buildings are densely concentrated, and road resources are urgently short of The UHVDC project using the solution of double-circuit transmission line on the same tower shall also be implemented The ±500 kV Jinmen–Fengjing transmission line of Gezhouba–Shanghai DC integrated reconstruction project put into operation in 2011 uses the solution of double-circuit transmission line on the same tower (as shown in Fig 28.11) The construction of this transmission line has also provided a reference for UHVDC transmission line to use double-circuit transmission line on the same tower For viewing the tower types used for EHVDC and UHVDC transmission lines currently in China and other countries, iron towers are widely used Depending on the construction form and stress characteristics, iron towers can be divided into two basic types: guyed tower and self-supporting tower, as shown in Fig 28.12 A guyed tower is light in weight, for which the steel consumption and construction cost can be reduced This type of tower has mature experiences in mechanical analysis, processing and fabrication, tower erection, and wire installation However, guyed towers occupy large area of land and can easily be restricted by terrain A self-supporting tower has the advantages of small land area occupation, excellent rigidity, and suitable to various terrains The first ±500 kV Gezhouba–Shanghai DC transmission line in China used large amount of guyed towers At present, that line has been put out of service 1462 J Chen et al Fig 28.11 Double-circuit DC line of ±500 kV Jinmen–Fengjing Project Fig 28.12 ±800 kV DC tower types Most of ±500 kV and above DC transmission lines in China use self-supporting towers Depending on the functions, self-supporting towers can be divided into straight line tower, suspension angle tower, and tension angle tower The single-circuit DC transmission lines generally use T-type towers with bipolar conductor horizontally arranged The straight line towers are subdivided into full “I”-string type and “V”-string type depending on the type of conductor suspension insulators In economically developed area with crowded corridor and high corridor clearing cost, the tower type with bipolar conductor vertically ... low-loss power transmission The transmission capacity of the 1000 kV UHVAC system is about 4–5 times that of the 500 kV extra -high voltage (EHV) AC system The development of the UHVAC and UHVDC power. .. 1000 kV UHVAC power transmission lines and nine ±800 kV UHVDC power transmission lines have been built and put into operation There is still another 1000 Preface vii kV UHVAC power transmission. .. UHVAC/EHVAC Power Transmission 3.3.2 Comparison of Economy for UHVDC/EHVDC Power Transmission 3.4 Applicable Occasions of UHVAC/UHVDC Power Transmissions