UHV

Improving the voltage level of the power grid's transmission capacity

UHV is the abbreviation of Ultra High Voltage. In China, UHV refers to voltage levels of ±800 kV and above for direct current and 1000 kV and above for alternating current .

status quo

Significance to power construction

UHV can greatly improve the transmission capacity of China's power grid. According to data provided by the State Grid Corporation of China , a single-circuit UHV DC grid can transmit 6 million kilowatts of electricity, which is 5 to 6 times the current 500 kV DC grid, and the transmission distance is also 2 to 3 times that of the latter, so the efficiency is greatly improved. In addition, according to calculations by the State Grid Corporation of China, if the same power is transmitted through UHV lines, 60% of land resources can be saved compared to using 500 kV high-voltage lines.

Current UHV situation

The UHV that China has built is the 750 kV AC experimental project of the Northwest Power Grid . The first UHV AC demonstration project with the highest voltage level in China is a 1000 kV AC transmission and transformation project independently developed, designed and constructed by China with independent intellectual property rights. The Southeast Shanxi - Nanyang - Jingmen UHV AC experimental demonstration project is 640 kilometers long. At 22:00 on December 30, 2008, the project was put into trial operation. At 22:00 on January 6, 2009, it successfully passed the 168-hour trial operation.

In terms of DC, the Sichuan Xiangjiaba-Shanghai ±800 kV UHV DC transmission demonstration project has been successfully put into operation. This is the world's highest voltage level, longest transmission distance, and largest capacity DC transmission project planned and constructed; Jinping- South Jiangsu ±800 kV UHV DC line project also successfully passed the completion acceptance on May 13, 2012.

State Grid Corporation of China first announced on August 12, 2010 that it would build the North China , East China , and Central China ("Three Chinas") UHV power grids by 2015 , forming a "three vertical, three horizontal, and one ring network".

On the same day, State Grid Corporation of China announced that the world's highest operating voltage 1000 kV Jindongnan-Nanyang-Jingmen UHV AC test and demonstration project has passed the national acceptance, which means that UHV is no longer in the "test" and "demonstration" stage, and the approval and construction process of subsequent projects is expected to be accelerated.

On July 24, 2015, in Dongtai City, Jiangsu Province, the 1000 kV Huainan -Nanjing-Shanghai UHV AC project was started. The 1000 kV Huainan-Nanjing-Shanghai UHV AC project is one of the 12 key transmission channels in the National Air Pollution Prevention and Control Action Plan. The transformer capacity is 12 million kVA, the total length of the line is 759.4 kilometers, and the new transmission line is 2×780 kilometers. The project investment is 26.8 billion yuan. This project is the largest and most difficult UHV AC project to date. After completion, it will enhance the interconnection and mutual support capabilities of the power grids in the Yangtze River Delta region.

China's UHV transmission network has reached the world's highest level in less than 10 years, setting a number of world records. The Jindongnan-Nanyang-Jingmen line is the world's first UHV AC transmission and transformation project put into commercial operation; the Xiangjiaba -Shanghai UHV DC transmission project is the largest, longest-distance and most technologically advanced project of its kind in the world. China's achievements have been called "an important milestone in the history of the development of the world's power industry" by the International Large Grid Organization. In the future, China will build a national smart grid based on the UHV backbone network, and its investment in this area has surpassed that of the United States.

As the first UHV project approved and started construction among the 12 key transmission channels of the National Air Pollution Prevention and Control Action Plan, the Anhui Power Eastward Transmission Huainan-Nanjing-Shanghai 1000 kV UHV AC Project across the Huaihe River and the Yangtze River started construction in September 2014. It was originally planned to be put into operation in March 2016, and now the project is about to enter the acceptance stage in October 2015, and is expected to be put into operation ahead of schedule.

Future development blueprint

Analysts said that in the next five years, the investment in UHV is expected to reach 270 billion yuan, which is more than 13 times the 20 billion yuan investment during the 11th Five-Year Plan period.

The National 12th Five-Year Plan outline released on March 16, 2011 mentioned that "to meet the requirements of large-scale cross-regional power transmission and grid connection of new energy power generation ,

Accelerate the construction of a modern power grid system, further expand the scale of West-to-East Power Transmission, improve regional backbone power grids, develop large-capacity, high-efficiency, long-distance advanced power transmission technologies such as UHV, promote the construction of smart grids based on advanced technologies such as information, control and energy storage, effectively strengthen the construction and transformation of urban and rural power grids, and enhance the power grid's ability to optimize power allocation and power supply reliability . "This will mean that the UHV transmission project has been officially included in the country's "12th Five-Year Plan".

Zhang Ke, an expert from the Development Planning Department of the State Grid Corporation of China , told China Business News that the future development of nuclear power, wind power and hydropower, which are clean energy sources, will all depend on the construction of UHV power grids. Taking wind power as an example, the country plans to reach an installed capacity of more than 150 million kilowatts of wind power by 2020, but the installed capacity of the eight major wind power bases already accounts for 80% of the total installed capacity, of which five major wind power bases are in the Three Norths (North China, Northwest China , and Northeast China ). The wind power installed capacity in Xinjiang, Gansu, Inner Mongolia, Jilin and other provinces and autonomous regions alone is 80 million kilowatts, so there are great problems in wind power consumption. Only with the help of UHV power grids can such concentrated and unstable electricity be transmitted to load centers such as North China and Central China. He said that after the construction of UHV, wind power can be developed on a large scale and consumed with high efficiency, thus controlling the once serious wind abandonment phenomenon to 1%.

Zhang Zhengling, director of the Development Planning Department of State Grid Corporation of China, said in an interview with Reuters that ultra-high voltage (UHV) smart grids are essential for the development of mainland power. In the next five years, State Grid will invest 620 billion yuan to build 20 UHV lines to transmit hydropower in the southwest and wind power in the northwest to eastern China. The development of UHV grids is not only a technological innovation, but also realizes long-distance transportation, solves the large-scale development and utilization of renewable energy in China, and can improve the severe environmental pressure currently faced by central and eastern China.

The so-called ultra-high voltage power grid refers to a power transmission network with a voltage level of 1000 kV AC and 800 kV DC and above. Its biggest feature is that it can transmit electricity over long distances, with large capacity and low loss. 76% of the mainland's coal resources are in the north and northwest, 80% of the hydropower resources are in the southwest, and more than 70% of the energy demand is in the central and eastern parts. The transmission distance of ordinary power grids is only about 500 kilometers, which cannot meet the transmission requirements.

The mainland's ultra-high voltage power grid has completed one ultra-high voltage AC line and two ultra-high voltage DC lines, totaling 4,633 kilometers, and has two AC and two DC lines under construction, totaling 6,412 kilometers.

On October 7, 2015, the 6000MVA 1000kV UHV main transformer system with the highest transformer capacity in China was installed in Suzhou . This marks the completion of the core project of the world's largest, highest transformer capacity, and strongest single-unit power supply 1000kV UHV substation under construction .

The 1000 kV UHV Suzhou Substation is the first UHV substation in China to be built on the same site with two stations. The power systems installed in the first phase of the project serve Shanghai and Jiangsu respectively. The substation will build six sets of 3000 MVA main transformer systems in the future, with a total substation capacity of 18000 MVA.

As an important node of the "Huainan-Nanjing-Shanghai 1000 kV UHV AC Transmission and Transformation Project", Suzhou Substation, after its completion and operation, will be of great significance for improving the clean energy and power grid load acceptance capacity in East China , enhancing the Yangtze River Delta power grid's ability to resist major faults and the reliability of Anhui power transmission to the east.

DC transmission

Definition

What is the "electrostatic dust effect" of DC?

Under DC voltage , charged particles in the air are attracted to the surface of the insulator by the electric field force in a constant direction . This is the "electrostatic dust absorption effect" of DC. Due to its effect, under the same environmental conditions, the amount of pollution on the surface of DC insulators can be more than twice that under AC voltage. As the amount of pollution continues to increase, the insulation level decreases, and pollution flashover of insulators is prone to occur under certain weather conditions . Therefore, due to this technical characteristic of DC transmission lines, their external insulation characteristics are more complicated than those of AC transmission lines.

Equipment Technology

Since the high-voltage direct current transmission was put into operation in the 1950s , after more than 50 years of development, high-voltage and ultra-high-voltage direct current transmission technologies have been gradually improved. Among them, Brazil's two ±600 kV ultra-high-voltage direct current transmission projects have been in operation for more than 20 years, and China's ±500 kV ultra-high-voltage direct current transmission projects have also been built and operated for nearly 20 years. Through the construction and operation of ultra-high-voltage direct current transmission projects, we have a more mature understanding of direct current transmission technology , and have also laid a solid technical foundation for the equipment manufacturing of ±800 kV ultra-high-voltage direct current transmission projects.

In the 1970s and 1980s, the former Soviet Union carried out ±750 kV UHV DC transmission engineering practice, and its main equipment has passed factory tests and more than 1,000 kilometers of transmission lines have been built. The international industry and academia have never stopped researching UHV DC transmission technology above ±600 kV, and the main work is concentrated on the voltage level of ±800 kV. The research and development of 1,000 kV AC transmission technology, especially the construction and operation of AC UHV projects in the former Soviet Union and Japan, and the accumulation of more than 30 years of operating experience of 750 kV AC transmission, the design and manufacturing technology of key equipment such as AC transformers, lightning arresters , switches, etc. have matured. Although the relevant knowledge and experience cannot be directly copied, they can be fully referenced in the research and development of ±800 kV UHV DC equipment. Various studies and experiments have shown that the conditions for the engineering application of ±800 kV UHV DC transmission technology are already in place, and all the equipment required for ±800 kV UHV DC can be manufactured. It is completely feasible to use UHV DC transmission technology in actual projects.

Features and functions

The converter station is a system for mutual energy conversion between DC and AC in DC power transmission projects. In addition to the AC field and other equipment that are the same as AC substations, the DC converter station also has the following unique equipment: converter , converter transformer , AC/DC filter and reactive power compensation equipment, smoothing reactor . The main function of the converter is to convert AC/DC. From the initial mercury arc valve to the electronically controlled and light-controlled thyristor valve, the unit capacity of the converter is constantly increasing.

The converter transformer is a key device for AC/DC conversion in a DC converter station. Its grid side is connected to the AC field, and its valve side is connected to the converter. Therefore, its valve side winding needs to withstand AC and DC composite stress. Since the operation of the converter transformer is closely related to the nonlinearity caused by the commutation of the converter , it has different characteristics from ordinary power transformers in terms of leakage reactance, insulation, harmonics , DC bias, on-load voltage regulation and testing .

AC/DC filters provide an underground channel for characteristic harmonics generated during the operation of the converter. A large number of harmonics are generated during the operation of the converter, consuming 40% to 60% of the reactive power of the converter capacity. The AC filter also provides reactive power while filtering . When the reactive power provided by the AC filter is not enough, special reactive power compensation equipment is required.

Smoothing reactors can prevent DC side lightning and steep waves from entering the valve hall, thereby protecting the converter valve from the stress of these overvoltages; they can smooth the ripples in the DC current . In addition, in the event of a DC short circuit, smoothing reactors can also reduce the probability of commutation failure by limiting rapid current changes.

Main features of the technology

(1) The UHV DC transmission system does not have a drop point in the middle, and can directly transmit electricity to the load center point-to-point, high-power, and long-distance. When the transmission and reception relationship is clear, UHV DC transmission is used to achieve AC/DC parallel transmission or asynchronous networking, and the grid structure is relatively loose and clear.

(2) UHV DC transmission can reduce or avoid a large amount of power flow through the grid, and change the power flow according to the operation mode of the transmitting and receiving ends. The power flow direction and size of the UHV DC transmission system can be easily controlled.

(3) UHV DC transmission has high voltage, large transmission capacity and narrow line corridor, and is suitable for high-power and long-distance transmission.

(4) In the case of AC/DC parallel transmission, the use of DC active power modulation can effectively suppress the power oscillation of the parallel AC line, including regional low-frequency oscillation, and significantly improve the transient and dynamic stability performance of the AC.

(5) In high-power DC transmission, when the DC system is locked, the AC systems at both ends will be subjected to a large power shock.

Wire selection

In UHV DC transmission projects, the selection of line conductor types must not only meet the requirements of long-distance safe transmission of electric energy, but also meet the requirements of environmental protection. Among them, the requirements of line electromagnetic environment limits have become the most important factor in conductor selection. At the same time, from an economic point of view, the selection of line conductor types is also directly related to the construction investment and operating costs of the project. Therefore, in addition to meeting the requirements of economic current density and long-term allowable current carrying capacity, the research on the cross-section and splitting type of UHV DC conductors must also comprehensively consider the electromagnetic environmental limits, construction investment , and operating losses. Through the calculation and research of the conductor surface field strength and corona inception voltage under different structural methods and different altitudes , as well as the analysis of the electric field strength , ion current density, audible noise and radio interference, the final conductor splitting type and sub-conductor cross-section are determined. For ±800 kV UHV DC projects, in order to meet the requirements of environmental impact limits, especially the requirements of audible noise, a conductor structure of 6×720 square millimeters or above should be used.

How to determine the corridor width of UHV DC lines and the scope of house demolition when adjacent to residential houses?

The corridor width of UHV DC transmission lines is mainly determined by two factors:

1. Ensure the electrical clearance requirements when the conductor is at maximum wind deflection;

2. Meet the requirements of the limit values ​​of electromagnetic environment indicators (including electric field strength, ion flow density, radio interference and audible noise). According to the characteristics of line installation, the impact is most serious in the middle of the span . Studies have shown that for UHV DC projects, when the line is close to residential buildings, demolition measures are taken to ensure that the electrical clearance and environmental impact after the project is completed meet the requirements of national regulations. Usually, when conducting feasibility studies at the beginning of project construction, the indicators of electric field strength, ion flow density, radio interference and audible noise are calculated. Only when these indicators meet the relevant national regulations can the project meet the approval conditions.

Economic advantages of technology

The unit transmission capacity of the 800 kV DC transmission scheme is about 72% of the investment of the ±500 kV DC transmission scheme. Compared with the ±620 kV DC, the transmission projects of Xiluodu , Xiangjiaba, Wudongde and Baihetan hydropower stations can reduce the number of transmission lines from 10 to 6 and save about 15 billion yuan in comprehensive investment by using ±800 kV DC.

Technological innovation

Through the rolling research and comprehensive demonstration of the transmission schemes of the downstream hydropower of the Jinsha River and Jinping Hydropower, it is recommended that the first phase of the Jinsha River transmission project adopt a 3-circuit ±800 kV, 6.4 million kilowatt ultra-high voltage direct current transmission scheme. The feasibility study report of the direct current transmission project and the 500 kV supporting project at the sending end has been completed and passed the review. The six special projects of the direct current transmission project, namely environmental impact assessment, soil and water conservation plan , geological disaster risk assessment, buried mineral assessment, earthquake safety assessment and cultural relics exploration, have also been successfully completed recently.

In technical research, we have achieved leapfrog development based on scientific and technological innovation and made breakthrough progress:

1. A single-circuit ±800 kV, 6.4 million kW DC solution is proposed. This solution gives full play to the scale advantage of UHV DC. Through engineering practice, its standardized design has a very broad market prospect.

2. Research and develop 6-inch thyristor components and build the world's only 6-inch component production line in China. The research and development of 6-inch components (converter valves) will greatly enhance the manufacturing level of China's power electronics industry.

3. Research on ice melting of lines in heavy ice areas. By appropriately changing the wiring method of the UHV DC system and temporarily increasing the current passing through the line, the line can be melted during periods of severe icing, which can greatly reduce the investment in the line itself.

4. Carry out pollution measurement, adopt the DC pollution measurement system designed and developed completely independently , and carry out DC pollution accumulation test at UHV project sites. The overall technology is at the international advanced level.

5. Carry out corridor digitization and overall navigation, treat the outgoing line planning of Xiluodu and Xiangjiaba hydropower stations as a systematic project, and conduct overall navigation, which improves the accuracy of outgoing line planning and saves engineering costs .

6. Propose and study the electromagnetic environment index of UHV DC, and propose to change the nominal field strength required by the original "Guidelines for Design of High-voltage DC Overhead Transmission Lines" to a synthetic field strength index that has an actual impact on the environment and can be directly measured, to measure the electric field of the DC line. This revision optimizes the original guidelines and has been adopted by the State Environmental Protection Administration .

Application prospects in my country

UHV DC transmission has point-to-point, ultra-long-distance, large-capacity power transmission capabilities, and is mainly positioned for ultra-long-distance, ultra-large-capacity power transmission from my country's large hydropower bases in the southwest and large coal-fired power bases in the northwest.

UHV DC has a broad application prospect in China. Taking the State Grid as an example, the Jinsha River Phase I Xiluodu and Xiangjiaba transmission projects will use 3 ±800 kV, 6.4 million kilowatts of UHV DC transmission, and the Sichuan Jinping Hydropower Station will use 1 ±800 kV, 6.4 million kilowatts of UHV DC transmission. The above projects are planned to be completed and put into operation from the end of 2011 to 2016. The Jinsha River Phase II Wudongde and Baihetan Hydropower Station Transmission Project will also use 3 ±800 kV, 6.4 million kilowatts of UHV DC transmission. The development of UHV DC transmission also provides an economical transmission method for the development of Tibet hydropower and Xinjiang coal-fired power, China's reserve energy base, and provides technical support for strengthening power cooperation with Russia, Mongolia, Kazakhstan and other countries .

Difference from AC transmission

From a technical point of view, ±800 kV UHV DC transmission is adopted, and there is no need for a drop point in the middle of the line, which can directly send a large amount of electricity to the large load center; in the case of AC and DC parallel transmission, double-sided frequency modulation can be used to effectively suppress regional low-frequency oscillations and improve the temporary (dynamic) stability limit of the section; solve the problem of excessive short-circuit current in the large receiving-end power grid . 1000 kV AC transmission is adopted, and a drop point can be made in the middle, which has the function of a power grid; strengthen the power grid to support large-scale DC power transmission; fundamentally solve the problem of excessive short-circuit current in the large receiving-end power grid and low transmission capacity of 500 kV lines, and optimize the power grid structure.

From the perspective of transmission capacity and stability, when ±800 kV UHV DC transmission is adopted, the transmission stability depends on the effective short-circuit ratio (ESCR) and effective inertia constant (Hdc) of the receiving-end power grid and the structure of the sending-end power grid. When 1000 kV AC transmission is adopted, the transmission capacity depends on the short -circuit capacity of each support point of the line and the distance of the transmission line (the distance between two adjacent substations); the transmission stability (synchronization capability) depends on the power angle of the operating point (the power angle difference between the two ends of the line).

From the perspective of key technical issues that require attention, when using ±800 kV UHV DC transmission, attention should be paid to the static reactive power balance and dynamic reactive power reserve and voltage stability of the receiving-end power grid, and attention should be paid to the system voltage safety issues caused by simultaneous phase switching failures in multiple DC feed-in systems. When using 1000 kV AC transmission, attention should be paid to the phase and voltage regulation of the AC system when the operating mode changes; attention should be paid to the high power transfer of relatively weak sections under severe fault conditions; attention should be paid to the hidden dangers of large-scale power outages and their prevention measures.

Technical and economic advantages

Compared with ±600 kV and below UHV DC, the main technical and economic advantages of UHV DC transmission can be summarized into the following six aspects:

1. Large transmission capacity. Using 4000A thyristor valves , the ±800kV DC UHV transmission capacity can reach 6.4 million kilowatts, which is 2.1 times that of the ±500kV, 3 million kilowatt HVDC method and 1.7 times that of the ±600kV, 3.8 million kilowatt HVDC method, which can give full play to the advantages of large-scale transmission.

Second, long transmission distance. The use of ±800 kV DC transmission technology makes it possible to transmit electricity over ultra-long distances. The economic transmission distance can reach 2,500 kilometers or even farther, providing transmission guarantee for the development of large hydropower bases in the southwest.

3. Line loss Low. When the total conductor cross-section and transmission capacity are the same, the resistance loss of the ±800 kV DC line is 39% of that of the ±500 kV DC line and 60% of that of the ±600 kV DC line, which improves transmission efficiency and saves operating costs.

4. Low engineering investment. According to calculations by relevant design departments, for ultra-long distance and ultra-large capacity transmission needs, the comprehensive cost per unit transmission capacity of the ±800 kV DC transmission scheme is about 72% of that of the ±500 kV DC transmission scheme, which has significant benefits in saving engineering investment.

5. High corridor utilization rate. The line corridor of the ±800 kV, 6.4 million kW DC transmission scheme is 76 meters, and the transmission capacity per unit corridor width is 84,000 kW/m, which is about 1.3 times that of the ±500 kV, 3 million kW scheme and the ±620 kV, 3.8 million kW scheme, which improves the utilization efficiency of the transmission corridor and saves precious land resources; due to the large transmission capacity of a single-circuit line, it significantly saves limited resources at valleys and river crossing points.

6. Flexible operation mode. The State Grid Corporation of China plans to adopt a 400+400 kV dual twelve-pulse converter series wiring scheme for UHV DC transmission, which has a flexible operation mode and greatly improves system reliability. If any converter valve module fails, the system can still guarantee the delivery of 75% of the rated power.

Number of insulators

Due to the electrostatic adsorption of DC lines, the pollution level of DC lines is higher than that of AC lines under the same conditions, and the number of insulators required is also more than that of AC lines. Its insulation level is mainly determined by the pollution discharge characteristics of the insulator string . Therefore, there are two main methods for selecting the number of insulators:

1. According to the artificial pollution test of insulators, the pollution flashover voltage of insulators under different salt densities is measured by using the insulator pollution tolerance method to determine the number of insulators.

2. According to the operating experience , the creepage distance method is adopted. In general areas, the creepage distance of DC lines is twice that of AC lines. Of the two methods, the former is intuitive, but requires a large amount of test and detection data, and the test results are highly dispersed. The latter is simple and easy to use, but has poor accuracy. In practical applications, the two are usually combined.

Problems faced by the equipment

The key issues facing UHV DC converter station equipment are as follows:

1. As the voltage level increases, the internal insulation problem of the valve-side winding, outgoing line structure and bushing of the converter transformer will be one of the main problems that need to be solved. The valve-side winding is subjected to a high AC/DC mixed field strength, and a large number of insulating materials such as insulating molded parts are required. The development and testing of the valve-side lead insulation molded parts of the ±800 kV converter transformer, and the field strength design and testing of the main insulation and turn insulation of the valve winding are the key problems that need to be solved in the equipment development.

2. Insulation problems of DC field equipment caused by high pollution level of converter station. Pollution flashover of DC equipment accounts for a large proportion of DC field accidents and is a difficult problem that needs to be solved. According to previous engineering experience and experimental research, due to the pollution absorption characteristics of DC field, the creepage distance of DC equipment is about twice that of AC equipment under the same pollution conditions. With the development of urbanization and industrialization, the problem of air pollution is becoming more and more serious. The pollution of UHV DC converter station has reached level II or even level III. According to this requirement, the creepage distance needs to reach 70 mm/kV or higher. Under UHV voltage, the equipment has exceeded the height that can be tolerated by existing manufacturing or operation according to the creepage distance design required by the standard. In areas with heavy pollution, indoor field or equipment synthesis are two feasible ways to solve the pollution resistance problem. State Grid Corporation of China has made this issue a key research project, and carried out pollution measurement under DC field strength at the converter station site to determine a reasonable and objective DC pollution level. Through in-depth research such as actual size test, it is ensured that the equipment has a safe and reasonable external insulation level to ensure the safe and stable operation of UHV DC.

State Grid Corporation of China proposed for the first time a series of ±800 kV, 4,000 ampere, 6.4 million kilowatt UHV DC project schemes. Xiluodu, Xiangjiaba and Jinping use a total of 4 ±800 kV UHV DC transmission projects, each with a transmission capacity of 6.4 million kilowatts, which is the highest voltage level and largest capacity DC transmission project in the plan.

Due to the large transmission capacity and high voltage, the high-end converter transformer is large in size and the transportation weight increases. According to the manufacturer's conceptual design estimate, the maximum weight of the high-end converter transformer at the sending end can reach 360 tons/unit. Due to the limited transportation conditions in the sending end area, after technical and economic analysis, the State Grid Corporation of China concentrated all three converter stations at the sending end in Yibin City, Sichuan Province in the site planning of the Jinsha River UHV DC transmission project, which not only solved the transportation problem of large equipment, but also saved the project cost and was conducive to the operation and maintenance of the UHV converter station.

Development prospects

Building "UHV national power grid and realizing optimal allocation of energy resources" as an important goal and task of power grid construction is of great significance to ensure the safe and stable operation of power supply and power grid. The United States, Canada, Russia, Japan, Italy, Spain and other countries have been studying UHV transmission technology since the 1970s, and have gone through more than 40 years. The lesson of more than a dozen large-scale power outages in Europe , North America and other places in the past three or four decades is that the higher the load supplied by AC power and the larger the coverage area, the greater the safety risks. Therefore, it is necessary to provide more abundant backup capacity through a closely connected UHV AC power grid. The power grid is prone to natural and man- made disasters such as typhoons , heavy rains, lightning strikes , ice , pollution flashover, and military destruction. If there is not enough backup capacity, the accident will spread rapidly. Therefore, to ensure the safe operation of the power grid, it is necessary to study the power grid structure in planning, design, construction and operation , and implement three-way protection according to the principle of "layering and zoning". The State Grid and the Southern Power Grid proposed that the goal of building a "UHV national power grid" is to implement the optimal allocation of energy resources.

Famous Projects

The United States, the former Soviet Union, Japan and Italy have all built UHV AC test lines and conducted extensive research and testing on UHV AC transmission technology. Ultimately, only the former Soviet Union and Japan built UHV AC lines.

1150kV Project

The rated voltage (nominal voltage) of the former Soviet Union's 1000kV AC system is 1150kV, and the maximum voltage is 1200kV, which is the highest among existing projects in the world.

The former Soviet Union built a total of 2,350 km of 1,150 kV transmission lines and four 1,150 kV substations (one of which was a booster station ) from August 1985. Among them, 907 km of lines and three 150 kV substations (one of which was a booster station) operated at the rated voltage of 1,150 kV for five years from 1985 to 1990. Later, due to the economic disintegration of the former Soviet Union and political reasons, the Central Dispatching Bureau of Kazakhstan reduced the voltage of the entire line to 500 kV. During the entire operation period, the design of the overvoltage protection system did not need to be modified and the operation was good.

1000kV Project

Japan's 1000kV power system is concentrated in Tokyo Electric Power Company . In 1988, it began to build a 1000kV transmission and transformation project. In 1999, it built two 1000kV transmission lines with a total length of 430km and a 1000kV substation. The first is a 1000kV transmission line from the nuclear power plant on the northern coast of the Sea of ​​Japan to the southern Tokyo area, called the North-South Line (length 190km), the South Niigata Line and the West Gunma Line; the second is a 1000kV transmission line connecting the power plants on the Pacific coast, called the East-West Line (length 240km). km), Higashi-Gunma Main Line, Minami-Keijo Main Line. In addition, Japan has built a new 1100kV substation. All 1000kV lines and substations have been stepped down to 500kV voltage level since their construction. Considering the construction of nuclear power plants on the Pacific coast and in the northeast region, it is planned to increase the voltage to the rated voltage of 1000kV. However, due to the stagnation of load growth, power supply construction and 1000kV boosting plan have been greatly delayed. It is expected that the voltage will be increased to 1000kV in the late 2010s.

Italy 1050kV

In the 1970s, Italy and France were commissioned by the Western European International Power Generation and Supply Association to conduct demonstration work on the selection of 800kV and 1050kV AC transmission schemes for the European continent. After that, the Italian UHV AC transmission project carried out a series of work such as basic technology research and equipment manufacturing under the auspices of the state, and built a 1050kV test project in October 1995. By December 1997, it had been operating at the rated voltage (nominal voltage) of 1050kV for more than two years and gained certain operating experience.

The test project is located in the Suvereto 1000kV test station in Italy and consists of two parts:

(1) 1050/400kV substation;

(2) 2.8km 1050kV transmission line. The single-line schematic diagram of the 1050kV test project is shown in the figure.

China UHV transmission

China's research on UHV transmission technology began in the 1980s. After more than 20 years of efforts, a number of important scientific research results have been achieved. Research shows that the development of UHV transmission is an inevitable choice for the development of China's power industry. The State Grid has already built and is building UHV AC transmission and transformation projects: one is the middle line project of northern Shaanxi -southern Shanxi-Nanyang-Jingmen-Wuhan, and the other is the east line project of Huainan- southern Anhui - northern Zhejiang -Shanghai. In addition, China's third UHV transmission project, the "Sichuan-Shanghai ±800 kV UHV DC Transmission Demonstration Project", was also started on December 21, 2007 in Yibin County, Sichuan Province. By 2020, China's UHV power grid will be basically completed, and the power transmission will reach more than 200 million kWh, accounting for 25% of the country's total installed capacity.

On January 6, 2009, the Southeast Shanxi-Nanyang-Jingmen UHV AC Experimental Demonstration Project, a 1000 kV AC power transmission and transformation project independently developed, designed and constructed by my country with independent intellectual property rights, successfully passed the trial operation. This marks a major breakthrough in the localization of my country's long-distance, large-capacity, low-loss UHV core technology and equipment, which is of great significance to optimizing energy resource allocation and ensuring national energy security and reliable power supply.

This world's first ultra-high voltage AC line is 640 kilometers long and has the highest voltage level in the world, reaching 1000 kV. The power it transmits is five times that of the existing 500 kV line. The power loss and floor space in the transmission process can be saved by more than half, and the investment in the entire project is one-third less than that of the 500 kV line. It spans three provinces, Shanxi, Henan, and Hubei, and also includes two large crossing sections, the Yellow River and the Han River . The line starts from the 1000kV Jindongnan Substation in Shanxi, passes through the 1000kV Nanyang Switch Station in Henan, and ends at the 1000kV Jingmen Substation in Hubei .

The project obtained the project approval document from the National Development and Reform Commission in August 2006, and construction started at the end of the same year. It was fully completed in December 2008, and the system debugging was completed and put into trial operation on December 30. At 22:00 on January 6, 2009, the 168-hour trial operation was completed and put into commercial operation, and the operation is in good condition.

On December 15, 2015, the world's first 10 million kilowatt UHV DC transmission project, the Ximeng- Taizhou ±800 kV UHV DC transmission project, started construction in Xinghua . The project is an important part of the "four AC and four DC" UHV project of the National Air Pollution Prevention and Control Action Plan and is scheduled to be completed and put into operation in 2017.