On line monitoring device for transmission line tower tilt, which reflects the tilt and deformation of transmission tower in operation

Tubular conductor power cable

Tubular conductor power cable is a kind of current carrying equipment whose conductor is copper or aluminum metal circular tube and is wrapped

with insulation, and the insulation is wrapped with grounding metal shielding layer. At present, the common voltage level is 6-35kV.

Compared with traditional power cables, due to its structural characteristics, it has the following technical advantages:

1) The conductor is tubular, with large sectional area, good heat dissipation, large current carrying capacity (the current carrying capacity of a single

conventional equipment can reach 7000A), and good mechanical performance.

2) Covered with solid insulation, with shielding and grounding, safe, space saving and small maintenance;

3) The outer layer can be equipped with armor and sheath, with good weather resistance.

Tubular conductor cables are suitable for the fixed installation lines with large capacity, compactness and short distance in modern power development.

Tubular conductor cable, with its outstanding technical advantages such as large carrying capacity, space saving, strong weather resistance, safety, easy

installation and maintenance, can replace conventional power cables, GIL, etc. in certain application scenarios and become a choice for heavy load

connection design.

In recent years, tubular conductor power cables have been widely used in domestic new smart substations, large-scale photovoltaic, wind power, nuclear

power engineering, petroleum, steel, chemical, electrified railway, urban rail transit and other fields, and the voltage level has also entered the high-voltage

field from the initial low voltage. The number of manufacturers has increased from a few European and American manufacturers to dozens, mainly in China.

The insulation of domestic tubular conductor power cables is divided into epoxy impregnated paper casting, silicone rubber extrusion, EPDM extrusion,

polyester film winding and other forms. From the current production and operation experience, the main problems encountered are insulation problems,

such as the long-term performance of solid materials and the selection of insulation thickness, the development mechanism and detection of solid insulation

defects, and the research on intermediate connection and terminal field strength control. These problems are similar to those of conventional extruded

insulated power cables.

Gas insulated cable (GIL)

Gas Insulated Transmission Lines (GIL) is a high voltage and large current power transmission equipment that uses SF6 gas or SF6 and N2 mixed gas

insulation, and the enclosure and conductor are arranged in the same axis. The conductor is made of aluminum alloy pipe, and the shell is closed by

aluminum alloy coil. GIL is similar to the coaxial pipeline bus in the gas insulated metal enclosed switchgear (GIS). Compared with GIS, GIL has no

breaking and arc extinguishing requirements, and its manufacturing is relatively simple. It can choose different wall thickness, diameter and insulation

gas, which can meet different requirements economically. Because SF6 is a very strong greenhouse gas, SF6-N2 and other mixed gases are gradually

used as substitutes internationally.

GIL has the advantages of convenient installation, operation and maintenance, low failure rate, less maintenance work, etc. It can simplify the wiring of

power stations and substations, with a design service life of more than 50 years. It has nearly 40 years of operation experience abroad, and the total global

installation length has exceeded 300 km. GIL has the following technical features:

1) Large capacity transmission is realized with high current carrying capacity up to 8000A. The capacitance is much smaller than that of conventional high-

voltage cables, and reactive power compensation is not required even for long-distance transmission. The line loss is lower than that of conventional high-

voltage cables and overhead lines.

2) High reliability of safe operation, metal enclosed rigid structure and pipe sealing insulation are adopted, which are generally not affected by harsh climate

and other environmental factors compared with overhead lines.

3) Get along with the surrounding environment in a friendly manner, with very low electromagnetic impact on the environment.

GIL costs more than overhead lines and conventional high-voltage cables. General service conditions: transmission circuit with voltage of 72.5kV and above;

For circuits with large transmission capacity, conventional high-voltage cables and overhead lines cannot meet the transmission requirements; Places with

high environmental requirements, such as high drop vertical shafts or inclined shafts.

Since the 1970s, European and American countries have put GIL into practice. In 1972, the first AC GIL transmission system in the world was built in Hudson

Power Plant in New Jersey (242kV, 1600A). In 1975, Wehr Pumped Storage Power Station in Germany completed the first GIL transmission project in Europe

(420kV, 2500A). In this century, China has launched a large number of large-scale hydropower projects, such as Xiaowan Hydropower Station, Xiluodu

Hydropower Station, Xiangjiaba Hydropower Station, Laxiwa Hydropower Station, etc. The unit capacity of these hydropower projects is huge, and most of

them adopt the underground powerhouse layout. GIL has become one of the main ways of incoming and outgoing lines, and the line voltage grade is 500kV

or even 800kV.

In September 2019, Sutong GIL comprehensive pipe gallery project was officially put into operation, marking the formal formation of East China ultra-high

voltage AC double loop network. The single phase length of the double circuit 1000kV GIL pipeline in the tunnel is about 5.8km, and the total length of the

double circuit six phase pipeline is about 35km. The voltage level and total length are the highest in the world.

Thermoplastic polypropylene insulated cable (PP)

Nowadays, medium and high voltage AC power cables are basically insulated with cross-linked polyethylene (XLPE), which has high long-term working

temperature due to its excellent thermodynamic properties. However, XLPE material also brings negative effects. In addition to being difficult to recycle,

the cross-linking process and degassing process also result in long cable production time and high cost, and the cross-linked polar by-products such as

cumyl alcohol and acetophenone will increase the dielectric constant, which will increase the capacitance of AC cables, thus increasing the transmission

loss. If used in DC cables, cross-linking by-products will become an important source of space charge generation and accumulation under DC voltage,

seriously affecting the life of DC cables.

Thermoplastic polypropylene (PP) has the characteristics of excellent insulation, high temperature resistance, plasticizing and recycling. The modified

thermoplastic polypropylene overcomes the defects of high crystallinity, low temperature resistance and poor flexibility, and has advantages in optimizing

cable processing technology, reducing cost, increasing production rate, and increasing cable extrusion length. The cross-linking and degassing links are

omitted, and the production time is only about 20% of that of XLPE insulated cables. As the content of polar components decreases, it will become a

potential choice for high-voltage DC cable insulation.

In this century, European cable manufacturers and material manufacturers began to develop and commercialize thermoplastic PP materials and gradually

applied them to medium and high voltage power cable lines. At present, the medium voltage PP cable has been put into operation for tens of thousands of

kilometers in Europe. In recent years, the process of using modified PP as high-voltage DC cables in Europe has been significantly accelerated, and 320kV,

525kV and 600kV modified polypropylene insulated DC cables have passed type tests. China has also developed a modified PP insulated medium voltage

AC cable and put it into project demonstration application through type test to explore products with higher voltage levels. Standardization and engineering

practice are also in progress.

High temperature superconducting cable

For large metropolitan areas or large current connection occasions, the transmission density and safety requirements are extremely high. At the same time,

the transmission corridor and space are limited. The technical progress of superconducting materials makes superconducting transmission technology a

possible option for projects. By using the existing cable channel and replacing the existing power cable with high-temperature superconducting cable, the

transmission capacity can be doubled, and the contradiction between load growth and limited transmission space can be well solved.

The transmission conductor of the superconducting cable is superconducting material, and the transmission density of the superconducting cable is large

and the impedance is extremely low under normal working conditions; When the short circuit fault occurs in the power grid and the transmission current is

greater than the critical current of the superconducting material, the superconducting material will lose its superconducting ability, and the impedance of

the superconducting cable will be far greater than that of the conventional copper conductor; When the fault is eliminated, the superconducting cable will

resume its superconducting ability under normal working conditions. If the high temperature superconducting cable with certain structure and technology

is used to replace the traditional cable, the fault current level of the power grid can be effectively reduced. The ability of the superconducting cable to limit

the fault current is proportional to the cable length. Therefore, the large-scale use of the superconducting power transmission network composed of

superconducting cables can not only improve the transmission capacity of the power grid, reduce the transmission loss of the power grid, but also improve

its inherent fault current limiting ability, Improve the safety and reliability of the whole power grid.

In terms of line loss, superconducting cable loss mainly includes conductor AC loss, heat leakage loss of insulation pipe, cable terminal, refrigeration system,

and loss of liquid nitrogen overcoming circulating resistance. Under the condition of comprehensive refrigeration system efficiency, the operation loss of HTS

cable is about 50%~60% of that of conventional cable when transmitting the same capacity. Low temperature insulated superconducting cable has good

electromagnetic shielding function, theoretically it can completely shield the electromagnetic field generated by the cable conductor, so as not to cause

electromagnetic pollution to the environment. Superconducting cables can be laid in dense ways such as underground pipes, which will not affect the operation

of surrounding power equipment, and because it uses nonflammable liquid nitrogen as the refrigerant, it also eliminates the risk of fire.

Since the 1990s, the progress in the preparation technology of high temperature superconducting tapes has promoted the research and development of

superconducting power transmission technology worldwide. The United States, Europe, Japan, China, South Korea and other countries and regions have

carried out research and application of high-temperature superconducting cables. Since 2000, the research on HTS cables has focused on AC transmission

cables, and the main insulation of cables is mainly cold insulation. At present, the high temperature superconducting cable has basically completed the

laboratory verification stage and gradually entered into practical application.

Internationally, the research and development of high temperature superconducting cables can be divided into three stages. First, it went through the

preliminary exploration stage for the high temperature superconducting cable technology. Second, it is for the research and development of the low

temperature (CD) insulated high temperature superconducting cable that can truly realize commercial application in the future. Now, it has entered the

application research stage of the CD insulated high temperature superconducting cable demonstration project. In the past decade, the United States,

Japan, South Korea, China, Germany and other countries have carried out a number of CD insulated high-temperature superconducting cable

demonstration application projects. At present, there are mainly three types of CD insulated HTS cable structures: single core, three core and three-

phase coaxial.

In China, the Institute of Electrical Engineering of the Chinese Academy of Sciences, Yundian Inna, Shanghai Cable Research Institute, China Electric Power

Research Institute and other institutions have successively carried out the research and development of superconducting cables and made great achievements.

Among them, Shanghai Cable Research Institute completed the type test of the first 30m, 35kV/2000A CD insulated single core superconducting cable in

China in 2010, and completed the installation, test and operation of 35kV/2kA 50m superconducting cable system of Baosteel’s superconducting cable

demonstration project in December 2012. This line is the first low temperature insulated high temperature superconducting cable that runs on the grid in China,

and it is also the CD insulated high temperature superconducting cable line with the largest load current in the same voltage level in the world.

In October 2019, Shanghai Cable Research Institute passed the type test of the first 35kV/2.2kA CD insulated three core superconducting cable system in

China, laying a solid foundation for the subsequent demonstration project construction. The superconducting cable system demonstration project in Shanghai

urban area, led by Shanghai Cable Research Institute, is under construction and is expected to be completed and put into power transmission operation by

the end of 2020. However, there is still a long way to go for the promotion and application of superconducting cables in the future. More research will be

carried out in the future, including superconducting cable system development and experimental research, system engineering application technology

research, system operation reliability research, system life-cycle cost, etc.

Overall evaluation and development suggestions

The technical level, product quality and engineering application of power cables, especially high-voltage and ultra-high voltage power cables, represent

the overall level and industrial capacity of a country’s cable industry to a certain extent. During the “13th Five Year Plan” period, with the rapid development

of power engineering construction and the strong promotion of industrial technology innovation, remarkable technical progress and impressive engineering

achievements have been made in the field of power cables. Evaluated from the aspects of manufacturing technology, manufacturing capacity and engineering

application, it has reached the international advanced level, some of which are in the international leading level.

Ultra-high voltage power cable for urban power grid and its engineering application

The AC 500kV XLPE insulated power cable and its accessories (the cable is manufactured by Qingdao Hanjiang Cable Co., Ltd., and the accessories are

partially provided by Jiangsu Anzhao Cable Accessories Co., Ltd.), which are manufactured by China for the first time, are used in the construction of

500kV cable projects in Beijing and Shanghai, and are the highest voltage grade urban cable lines in the world. It has been put into operation normally

and has made important contributions to the regional social and economic development.

Ultra-high voltage ac submarine cable and its engineering application

The Zhoushan 500kV interconnected power transmission and transformation project, completed and put into operation in 2019, is a cross sea interconnection

project of cross-linked polyethylene insulated power cables with the highest voltage level manufactured and applied internationally. Large length cables and

accessories are completely manufactured by domestic enterprises (among which, large length submarine cables are manufactured and provided by Jiangsu

Zhongtian Cable Co., Ltd., Hengtong High Voltage Cable Co., Ltd. and Ningbo Dongfang Cable Co., Ltd. respectively, and cable terminals are manufactured

and provided by TBEA), which reflects the technical level and manufacturing capacity of China’s ultra-high voltage submarine cables and accessories.

Ultra-high voltage dc cable and its engineering application

Three Gorges Group will build an offshore wind power generation project in Rudong, Jiangsu Province, with a total transmission capacity of 1100MW.

A ± 400kV submarine DC cable system will be used. The length of a single cable will reach 100km. The cable will be manufactured and provided by

Jiangsu Zhongtian Technology Submarine Cable Company. The project is planned to be completed in 2021 for power transmission. Up to now, the first

± 400kV submarine DC cable system in China, composed of cables manufactured by Jiangsu Zhongtian Technology Submarine Cable Co., Ltd. and cable

accessories manufactured by Changsha Electrical Technology Co., Ltd., has passed type tests in the National Wire and Cable Quality Supervision and

Testing Center/Shanghai National Cable Testing Center Co., Ltd. (hereinafter referred to as “National Cable Testing”), and has entered the production stage.

In order to cooperate with the 2022 International Winter Olympic Games in Beijing Zhangjiakou, the Zhangbei ± 500kV flexible DC transmission project

built by the State Grid Corporation of China is planned to build a ± 500kV flexible DC cable demonstration project with a length of about 500m. The cables

and accessories are planned to be completely manufactured by domestic enterprises, including insulation and shielding materials for cables. The work

is in progress.

Superconducting cable and its engineering application

The demonstration project of superconducting cable system in Shanghai urban area, which is mainly manufactured and constructed by Shanghai Cable

Research Institute, is under way, and is expected to be completed and put into power transmission operation by the end of 2020. The 1200m three core

superconducting cable (currently the longest in the world) required by the project construction, with the voltage level of 35kV/2200A and rated current,

has reached the international advanced level in general, and its core indicators are at the international leading level.

Ultra High Voltage Gas Insulated Cable (GIL) and Its Engineering Application

The East China UHV AC double loop network transmission project was officially put into operation in September 2019 in Jiangsu Province, where the Sutong

GIL comprehensive pipe gallery project crosses the Yangtze River. The single phase length of the two 1000kV GIL pipelines in the tunnel is 5.8km, and the

total length of the double circuit six phase transmission project is nearly 35km. The project voltage level and total length are the highest in the world. The

ultra-high voltage gas insulated cable (GIL) system is jointly completed by domestic manufacturing enterprises and engineering construction parties.

Performance testing and evaluation technology of ultra-high voltage cable

In recent years, the type test, performance test and evaluation of many domestic ultra-high voltage XLPE insulated cables and accessories, including AC and

DC cables, land cables and submarine cables, have been mostly completed in the “National Cable Inspection”. The system’s detection technology and perfect

test conditions are at the world’s advanced level, and have also made outstanding contributions to China’s cable manufacturing industry and power engineering

construction. The “National Cable Inspection” has the technical ability and conditions to detect, test and evaluate 500kV grade ultra-high voltage XLPE

insulated cables (including AC and DC cables, land cables and submarine cables) according to advanced standards and specifications at home and abroad, and

has completed dozens of detection and test tasks for many users at home and abroad, with the maximum voltage of ± 550kV.

The above representative ultra-high voltage cables and accessories and their engineering applications fully reflect that China’s cable industry is at the international

advanced level in terms of technical innovation, technical level, manufacturing capacity, testing and evaluation in this field.

Industry “Soft Ribs” and “Shortcomings”

Although the cable industry has made great progress and outstanding achievements in this field in recent years, there are also outstanding “weaknesses”

or “soft ribs” in this field. These “weaknesses” require us to make great efforts to make up for and innovate, which is also the direction and goal of

continuous efforts and development. A brief analysis is as follows.

(1) EHV XLPE insulated cables (including AC and DC cables, land cables and submarine cables)

Its outstanding “soft rib” is that the super clean insulation materials and super smooth shielding materials are completely imported, including the insulation

and shielding materials for the above major projects. This is an important “bottleneck” that must be broken through.

(2) Key production equipment used in the manufacture of ultra-high voltage cross-linked polyethylene insulated cables

At present, all of them are imported from abroad, which is another “soft rib” of the industry. At present, the major progress we have made in the field of

ultra-high voltage cables is mainly “processing” rather than “creative”, because the main materials and key equipment still rely on foreign countries.

(3) Ultra-high voltage cable and its engineering application

The above ultra-high voltage cables and their engineering applications represent the best level in China’s high-voltage cable field, but not our overall level.

The overall level of the power cable field is not high, which is also one of the main “short boards” of the industry. There are also many other “short boards” and

weak links, such as: basic research on high-voltage and ultra-high voltage cables and their systems, synthesis technology and process equipment of super clean

resin, performance stability of domestic medium and high voltage cable materials, industrial supporting capacity including basic devices, components and

auxiliary materials, long-term service reliability of cables, etc.

These “soft ribs” and “weaknesses” are obstacles and obstacles for China to become a strong cable country, but they are also the direction of our efforts to

overcome obstacles and continue to innovate.