Present Situation and Future Planning of Ultra High Voltage

Ultra high voltage refers to the voltage level of ±800 kV and above direct current and 1000 kV and above alternating current. The abbreviation of UHV is UHV; the symbol of voltage is U (in some places, it is represented by V); the unit of voltage is volts, and the unit symbol is also V; there are kV (kilovolts) larger than volts, and mV (millivolts) smaller than volts ), uV (microvolts), between them are thousands.

1. Current status of ultra high voltage

1.1 Significance to my country's electric power construction

Ultra high voltage can greatly enhance the transmission capacity of my country's power grid. According to the data provided by the State Grid Corporation of China, the ultra high voltage DC power grid of the primary circuit can transmit 6 million kilowatts of electricity, which is equivalent to 5 to 6 times that of the existing 500 kV DC power grid, and the power transmission distance is also 2 to 3 times that of the latter. Therefore, the efficiency is greatly improved. In addition, according to the calculations of the State Grid Corporation of China, if the power transmission of the same power is carried out, the use of ultra high voltage lines can save 60% of land resources compared with the use of 500 kV high-voltage lines.

1.2 Current situation of ultra high voltage in my country

The ultra-high voltage that China has built is the 750 kV AC experimental project of the Northwest Power Grid. The first ultra high voltage AC demonstration project with the highest voltage level in China is a 1,000 kV AC power transmission and transformation project independently developed, designed and constructed by my country with independent intellectual property rights—Jindongnan-Nanyang-Jingmen ultra high voltage AC test demonstration project. It is 640 kilometers long and spans the three provinces of Shanxi, Henan and Hubei, including two large spans of the Yellow River and the Han River. The line starts from Shanxi 1000kV Jindongnan Substation, passes through Henan 1000kV Nanyang Switching Station, and ends at Hubei 1000kV Jingmen Substation. At 22:00 on December 30, 2008, the project was put into trial operation, and at 22:00 on January 6, 2009, it successfully passed the 168-hour trial operation.

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

The State Grid Corporation of China announced for the first time on August 12, 2010 that by 2015, North China, East China, and Central China ("Sanhua") ultra high voltage power grids will be built, forming a "three vertical, three horizontal and one ring network".

On the same day, State Grid announced that the Jindongnan-Nanyang-Jingmen ultra high voltage AC test and demonstration project with the highest operating voltage of 1,000 kV in the world has passed the national acceptance, which indicates that ultra high voltage is no longer in the "test" and "demonstration" stage. The project approval and construction process is expected to be expedited.

On July 24, 2015, in Dongtai City, Jiangsu Province, the 1000 kV Huainan-Nanjing-Shanghai line ultra high voltage AC project started construction. The 1,000 kV Huainan-Nanjing-Shanghai ultra high voltage AC project is one of the 12 key transmission channels of the National Air Pollution Prevention and Control Action Plan, with a transformation capacity of 12 million kVA, a total length of 759.4 kilometers, and newly-built transmission lines of 2×780 kilometers , The project investment is 26.8 billion yuan. The project is by far the largest and most difficult ultra high voltage AC project in construction. After completion, it will enhance the interconnection and mutual support capabilities of the power grid in the Yangtze River Delta region.

China's ultra high voltage 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 ultra high voltage AC power transmission project put into commercial operation; the Xiangjiaba-Shanghai ultra high voltage DC power transmission project has the largest capacity and the longest distance among similar projects in the world. The most technologically advanced. China's achievements are called "an important milestone in the history of the world's electric power industry" by the International Power Grid Organization. In the future, China will build a national smart grid based on the ultra high voltage backbone network, and its investment in this aspect has surpassed that of the United States.

As the first ultra high voltage project among the 12 key transmission channels of the National Air Pollution Prevention and Control Action Plan to be approved and started construction, the Huainan-Nanjing-Shanghai 1000 kV ultra high voltage AC project across the Huaihe River and Yangtze River was completed in September 2014. Monthly construction started. 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 it is expected to be put into operation ahead of schedule.

1.3 Ultra high voltage Future Development Blueprint

Analysts said that in the next five years, the amount of investment in ultra high voltage is expected to reach 270 billion yuan, which is a full increase of 13 times compared with the 20 billion investment during the "Eleventh Five-Year Plan" period.

The national "Twelfth Five-Year Plan" announced on March 16, 2011 mentioned that "adapt to the requirements of large-scale cross-regional power transmission and new energy power generation grid integration, speed up the construction of a modern power grid system, and further expand the scale of west-to-east power transmission, Improve regional backbone power grids, develop ultra high voltage and other large-capacity, high-efficiency, long-distance advanced transmission technologies, rely on advanced technologies such as information, control and energy storage, promote the construction of smart grids, effectively strengthen the construction and transformation of urban and rural power grids, and enhance the optimal allocation of power grids capacity and reliability of power supply.” This will mean that the ultra high voltage power transmission project has been officially included in the national “Twelfth Five-Year Plan”.

Zhang Ke, an expert from the State Grid Development Planning Department, told China Business News that the future development of nuclear power, wind power, including hydropower as clean energy will depend on the construction of ultra high voltage power grids. Taking wind power as an example, the national planning of wind power will reach an installed capacity of more than 150 million kilowatts in 2020, but the installed capacity of the eight major wind power bases has already accounted for 80% of the total installed capacity, and five of them are located in the three north regions (North China, Northwest China, Northeast China), only Xinjiang, Gansu, Inner Mongolia, Jilin and other provinces and autonomous regions have installed wind power capacity of 80 million kilowatts, so there is a big problem in wind power consumption. Only with the help of the ultra high voltage power grid can such concentrated and unstable power be transmitted to the middle load centers in North China and Central China. He said that after the completion of ultra high voltage, wind power can be developed on a large scale and consumed with high efficiency, so that the once serious wind curtailment phenomenon can be controlled to 1%.

In an interview with Reuters, Zhang Zhengling, Director of the Development and Planning Department of the State Grid Corporation, said that ultra high voltage smart grids are necessary for the development of power in the Mainland. In the next five years, the State Grid will invest 620 billion yuan to build 20 ultra high voltage lines. To transmit hydropower from the southwest and wind power from the northwest to eastern China. The development of ultra high voltage power grid 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 current severe environmental pressure faced by the central and eastern regions.

The so-called ultra high voltage power grid refers to the transmission network with a voltage level of 1000 kV AC, plus or minus 800 kV DC and above. Its biggest feature is that it can transmit power over long distances, 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 regions. The transmission distance of ordinary power grids is only about 500 kilometers, which cannot meet the transmission requirements.

The ultra high voltage power grid in the Mainland has completed one ultra high voltage AC line and two ultra high voltage DC lines, totaling 4,633 kilometers, and two AC and two DC lines are under construction, totaling 6,412 kilometers.

On October 7, 2015, the 6000 MVA 1000 kV ultra high voltage main transformer system with the highest substation capacity in China was installed in Suzhou. This marks the completion of the core project of the 1000 kV ultra high voltage substation with the largest scale, the highest transformation capacity and the strongest single power supply capacity under construction in the world.

The 1000 kV ultra high voltage Suzhou Substation is the first ultra high voltage 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. In the future, the substation will build six sets of 3000 MVA main transformer systems, with a total substation capacity of 18000 MVA.

As an important node of the "Huainan-Nanjing-Shanghai 1000 kV ultra high voltage AC Power Transmission and Transformation Project", after the Suzhou substation is completed and put into operation, it will improve the capacity of clean energy and power grid load acceptance in East China, enhance the ability of the Yangtze River Delta power grid to resist major faults and the Anhui power grid. The reliability of the east transmission is of great significance.

2. Definition of ultra high voltage terms

2.1 What is the "Electrostatic Dust Collection Effect" of DC

Under the DC voltage, the charged particles in the air will be attracted to the surface of the insulator by the electric field force of a constant direction, which is the "electrostatic dust suction effect" of DC. Due to its function, under the same environmental conditions, the amount of pollution on the surface of DC insulators can be more than double that of 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 the technical characteristics of DC transmission lines, compared with AC transmission lines, its external insulation characteristics are more complicated.

2.2 Equipment Technology

Since the high-voltage direct current transmission was put into operation in the 1950s, after more than 50 years of development, the high-voltage and ultra-high-voltage direct-current transmission technology has been gradually improved. Among them, two ±600 kV ultra-high-voltage direct-current transmission projects in Brazil have been in operation for more than 20 years. my country's ± The 500 kV EHV DC transmission project has also been constructed and operated for nearly 20 years. Through the construction and operation of the EHV DC transmission project, a more mature understanding of DC transmission technology has been obtained, and it is also the basis for the ±800 kV ultra high voltage DC transmission project. Equipment manufacturing has laid a solid technical foundation.

In the 1970s and 1980s, the former Soviet Union carried out the practice of ±750 kV ultra high voltage DC transmission projects. Its main equipment has passed the factory test and more than 1,000 kilometers of transmission lines have been built. The international industrial and academic circles have not interrupted the research on ultra high voltage DC transmission technology exceeding ±600 kV, and the main work is concentrated on the voltage level of ±800 kV. Research and development of 1000 kV AC transmission technology, especially the construction and operation of AC ultra high voltage projects in the former Soviet Union and Japan, as well as the accumulation of more than 30 years of experience in 750 kV AC transmission, AC transformers, arresters, switches and other key equipment The design and manufacturing technology of the company has matured. Although the relevant knowledge and experience cannot be copied directly, they can be fully used for reference in the research and development of ±800 kV ultra high voltage DC equipment. Various studies and tests have shown that the conditions for the engineering application of ±800 kV ultra high voltage DC transmission technology have been met, and all the equipment required for ±800 kV ultra high voltage DC transmission technology can be manufactured. Ultra high voltage DC transmission technology is completely suitable for practical projects. feasible.

2.3 Features and functions of converter station equipment

The converter station is a system for mutual energy conversion between DC and AC in a DC transmission project. In addition to the same equipment as the AC substation, such as the AC field, the DC converter station also has the following unique equipment: converters, converter transformers, AC DC filter and reactive power compensation equipment, smoothing reactor.

The main function of the converter is to convert AC to DC. From the initial mercury arc valve to the electric control and light control thyristor valve, the unit capacity of the converter is constantly increasing.

The converter transformer is the key equipment for AC-DC conversion in the DC converter station. Its grid side is connected to the AC field, and its valve side is connected to the converter. Therefore, the windings on the valve side must bear the combined stress of AC and DC. 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.

The AC-DC filter provides a path to the ground for the characteristic harmonics generated during the operation of the converter. A large number of harmonics are generated during the operation of the converter, which consumes 40% to 60% of the reactive power of the converter capacity. AC filters also provide reactive power while filtering. When the reactive power provided by the AC filter is not enough, special reactive power compensation equipment is also required.

The smoothing reactor can prevent the DC side lightning and steep waves from entering the valve hall, so that the converter valve is not subject to the stress of these overvoltages; it can smooth the ripple in the DC current. In addition, the smoothing reactor can also reduce the probability of commutation failure by limiting the rapid change of current when the DC is short-circuited.

2.4 Main features of the technology

2.4.1 The ultra high voltage DC transmission system does not have a point in the middle, and can directly send power to the load center point-to-point, high-power, and long-distance. In the case of a clear relationship between sending and receiving, ultra high voltage DC transmission is adopted to realize AC-DC parallel transmission or asynchronous networking, and the grid structure is relatively loose and clear.

2.4.2 UHV DC transmission can reduce or avoid a large number of power flows through the grid, and the power flow can be changed according to the change of the operation mode of the sending and receiving ends. The flow direction and magnitude of the ultra high voltage DC transmission system can be easily controlled.

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

2.4.4 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 of AC.

2.4.5 For high-power DC transmission, when the DC system is blocked, the AC system at both ends will bear a large power impact.

2.5 Selection of wires

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

How to determine the corridor width of the UHV DC line and the scope of house demolition when it is adjacent to private houses?

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

2.5.1 Requirements to ensure electrical clearance when the wire is at maximum windage;

2.5.2 Satisfy the limit value requirements of electromagnetic environment indicators (including electric field strength, ion current density, radio interference and audible noise). According to the characteristics of line erection, the impact is most serious in the center of the span. Studies have shown that for ultra high voltage DC projects, when the line is adjacent to private houses, demolition measures are taken to ensure that the electrical clearance and environmental impact of the project after completion meet the requirements of the state. Usually, the indicators of electric field strength, ion current density, radio interference and audible noise must be calculated during the feasibility study at the initial stage of project construction. Only when these indicators meet the relevant national regulations can the project meet the approval conditions.

2.6 Economic advantages of technology

The unit transmission capacity investment of the 800 kV DC transmission scheme is about 72% of that of the ±500 kV DC transmission scheme. The transmission projects of Xiluodu, Xiangjiaba, Wudongde, and Baihetan hydropower stations use ±800 kV DC compared with ±620 kV DC, the transmission lines can be reduced from 10 to 6, and the comprehensive investment can be saved by about 150 billion.

2.7 Technological innovation

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

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

2.7.1 A single-circuit ±800 kV, 6.4 million kW DC scheme is proposed. This scheme gives full play to the scale advantages of ultra high voltage DC. Through engineering practice, its standardized design has a very broad market prospect.

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

2.7.3 To study the ice melting of lines in heavy ice areas, by appropriately changing the wiring mode of the ultra high voltage DC system, increasing the current through the line in a short time, and melting ice on the line during the period of severe icing, the investment of the line body can be reduced on a large scale.

2.7.4 Carry out pollution measurement, adopt the DC pollution measurement system designed and developed completely independently, and carry out the DC pollution test of the UHV project site, and the overall technology is at the international advanced level.

2.7.5 Carry out digitalization of corridors and overall flight, take the outgoing line planning of Xiluodu and Xiangjiaba hydropower stations as a systematic project, and carry out overall air flight, which improves the accuracy of outgoing line planning and saves engineering costs.

2.7.6 Propose and study the ultra high voltage DC electromagnetic environment indicators, and propose to change the nominal field strength required by the original "Guidelines for the Design of High-Voltage DC Overhead Transmission Lines" to a synthetic field strength index that can actually affect the environment and can be directly measured , which is used to measure the electric field of the DC line, optimizes the original guidelines and has been adopted by the State Environmental Protection Administration.

2.8 Application prospects in my country

UHV DC transmission has point-to-point, ultra-long-distance, and large-capacity power transmission capabilities, and is mainly positioned for ultra-long-distance, ultra-large-capacity outbound transmission of large hydropower bases in southwest my country and large coal power bases in northwest China.

UHV DC has broad application prospects in my country. Taking the State Grid as an example, the first phase of the Jinsha River Xiluodu and Xiangjiaba transmission projects will use 3 circuits of ±800 kV, 6.4 million kilowatts DC UHV transmission, and Sichuan Jinping Hydropower Station will use 1 circuit of ±800 kV, 6.4 million kilowatts DC ultra high voltage transmission, the above projects are planned to be completed and put into operation successively from the end of 2011 to 2016. The transmission project of the Wudongde and Baihetan hydropower stations in the second phase of the Jinsha River will also use 3 rounds of ±800 kV, 6.4 million kW DC UHV transmission. The development of UHV DC transmission also provides an economical power transmission method for the development of hydropower in Tibet and coal power in Xinjiang, my country's backup energy base, and provides technical support for strengthening power cooperation with Russia, Mongolia, Kazakhstan and other countries.

2.9 Differences from AC transmission

From a technical point of view, with the use of ±800 kV UHV DC transmission, there is no need for a landing point in the middle of the line, and a large amount of power can be directly sent to a large load center; in the case of parallel AC and DC transmission, bilateral frequency modulation can be used to effectively suppress regional low frequency oscillations. Improve the temporary (dynamic) stability limit of the section; solve the problem of excessive short-circuit current of the large receiving end power grid. Adopt 1000 kV AC transmission, which can be placed in the middle, and has the function of power grid; strengthen the power grid to support large-scale DC power transmission; fundamentally solve the problems of excessive short-circuit current of 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 performance, with ±800 kV UHVDC transmission, the transmission stability depends on the effective short-circuit ratio (ESCR) and effective inertia constant (Hdc) of the receiving end grid and the structure of the sending end grid. Using 1000 kV AC transmission, 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 the landing points of two adjacent substations); the stability of power transmission (synchronization ability) depends on the power angle of the operating point Size (power angle difference at both ends of the line).

From the point of view of the key technical issues that need to be paid attention to, when adopting ±800 kV UHV DC transmission, it is necessary to pay attention to the static reactive power balance and dynamic reactive power backup and voltage stability of the receiving end grid, and to pay attention to the multi-circuit DC feed-in system due to simultaneous System voltage safety problems caused by commutation failure. When using 1000 kV AC transmission, it is necessary to pay attention to the phase modulation and voltage regulation of the AC system when the operation mode changes; it is necessary to pay attention to the problems of high power transfer in relatively weak sections under serious fault conditions; it is necessary to pay attention to the hidden dangers of large-scale power outages and their preventive measures.

2.10 Technical and Economic Advantages

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

2.10.1 Large conveying capacity. Using 4000 ampere thyristor valves, the ±800 kV DC ultra high voltage power transmission capacity can reach 6.4 million kilowatts, which is 2.1 times that of the ±500 kV and 3 million kW high-voltage direct current mode, and is the ±600 kV class and 3.8 million kilowatt high-voltage direct current mode 1.7 times of that, able to give full play to the advantages of large-scale power transmission.

2.10.2 The power transmission distance is long. The adoption of ±800 kV DC transmission technology makes ultra-long-distance power transmission possible, and the economical power transmission distance can reach 2,500 kilometers or even further, providing power transmission guarantee for the development of the Southwest Hydropower Base.

2.10.3 Line loss is low. Under the condition that the total cross-section and transmission capacity of the wires are the same, the resistance loss of the ±800 kV DC line is 39% of the ±500 kV DC line and 60% of the ±600 kV DC line, which improves the transmission efficiency and saves running costs.

2.10.4 Project investment saving. According to the calculations of relevant design departments, for the 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 saves significant project investment benefits.

2.10.5 The corridor utilization rate is high. 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 the ±500 kV, 3 million kW scheme and the ±620 kV, 3.8 million kw scheme It improves the utilization efficiency of the transmission corridor and saves precious land resources; due to the large transmission capacity of the single-circuit line, it significantly saves the limited resources of valleys and river crossing points.

2.10.6 Flexible operation mode. The ultra high voltage DC transmission of the State Grid Corporation of China plans to adopt the connection scheme of 400+400 kV double twelve-pulse converters in series, which has a flexible operation mode and greatly improves system reliability. If any converter valve module fails, the system can still guarantee 75% of rated power delivery.

2.11 Number of insulator pieces

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. The insulation level is mainly determined by the pollution discharge characteristics of the insulator strings. Therefore, there are two main methods when selecting the number of insulators:

2.11.1 According to the artificial pollution test of insulators, the insulator pollution tolerance method is used to measure the pollution flashover voltage of insulators under different salt densities, so as to determine the number of insulators.

2.11.2 According to the operating experience, the creepage distance method is adopted, and the creepage distance of DC lines in general areas is twice that of AC lines. Among the two methods, the former is intuitive, but requires a large amount of testing and testing data, and the results of testing and testing are highly dispersed. The latter is simple and easy to implement, but less accurate. In practice, the two are usually combined.

2.12 Problems faced by converter station equipment

The key problems faced by ultra high voltage DC converter station equipment are as follows:

2.12.1 Due to the rising voltage level, the internal insulation of the valve side winding, outlet structure and bushing of the converter transformer will be one of the main problems to be solved. The winding on the valve side is subjected to high AC and DC mixed field strength, and a large number of insulating materials such as insulating moldings are required. The development and test of the lead insulation molded part of the valve side of the ±800 kV converter transformer, the design and test of the field strength of the main insulation and turn insulation of the valve winding are the key problems to be solved in the development of the equipment.

2.12.2 The DC field equipment insulation problem caused by the high pollution level of the converter station. Pollution flashover of DC equipment accounts for a large proportion of accidents in DC field, and it 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, and the pollution of ultra high voltage DC converter stations has reached level II or even level III. According to this requirement, the creepage distance must reach 70mm/kV or higher. Under UHV voltage, according to the creepage ratio design required by the standard, the equipment has exceeded the height that the existing manufacturing or operation can withstand. In heavily polluted areas, indoor field or equipment synthesis are two feasible ways to solve the problem of pollution resistance. The State Grid Corporation of China has taken this issue as a key research project, conducted pollution measurements under the DC field strength at the converter station site, determined a reasonable and objective DC pollution level, and conducted in-depth research on actual size tests to ensure that the equipment is safe and reasonable. External insulation level to ensure the safe and stable operation of ultra high voltage DC.

The State Grid Corporation of China proposed for the first time the ±800 kV, 4000 ampere, 6.4 million kW series ultra high voltage DC project plan. Xiluodu, Xiangjiaba and Jinping are transmitted by 4 rounds of ±800 kV UHV DC transmission projects, each with a transmission capacity of 6.4 million kilowatts, which is the DC transmission project with the highest voltage level and the largest capacity in planning.

Due to the large transmission capacity and high voltage, the high-end converter transformer is bulky 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 per set. Due to the limited transportation conditions in the sending end area, after technical and economic analysis, the State Grid Corporation of China concentrated all the three converter stations at the sending end in Yibin City, Sichuan Province in the site planning of the UHV DC transmission project outside the Jinsha River, which not only solves the problem of large The problem of equipment transportation saves the project cost and is beneficial to the operation and maintenance of the ultra high voltage converter station.

2.13 Development prospect

As an important goal and task of power grid construction, it 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 began to study ultra high voltage power transmission technology in the 1970s, and it has lasted for 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 30 to 40 years is that the higher the AC power supply load and the larger the coverage area, the greater the potential safety hazard. The power grid provides relatively abundant reserve capacity. Power grids are prone to natural and man-made disasters such as typhoons, rainstorms, lightning strikes, icy snow, pollution flashovers, military sabotage, etc. If there is not enough reserve capacity, the accidents will spread rapidly and expand. Therefore, in order to ensure the safe operation of the power grid, it is necessary to study the structure of the power grid during planning, design, construction, and operation, and to implement three protections according to the principle of "layered and partitioned". State Grid and China Southern Power Grid proposed that the goal of building "UHV National Grid" is to optimize the allocation of energy resources.

3. Famous ultra high voltage projects

The United States, the former Soviet Union, Japan and Italy have all built AC ultra high voltage test lines and conducted a large number of AC ultra high voltage transmission technology research and tests. In the end, only the former Soviet Union and Japan built AC ultra high voltage lines.

3.1 1150kV project in the former Soviet Union

The rated voltage (nominal voltage) of the 1000kV AC system in the former Soviet Union 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 2350km 1150kV transmission lines and four 1150kV substations (one of which is a step-up station) since August 1985. Among them, there are 907km lines and three 150kV substations (one of which is a step-up station) which have been operated for 5 years from 1985 to 1990 according to the system rated voltage of 1150kV. Later, due to the economic disintegration of the former Soviet Union and political reasons, Kazakhstan’s Central Dispatch Bureau reduced the entire line to 500kV voltage level for operation. During the entire operation period, the design of the overvoltage protection system did not need to be modified, and the operation was in good condition.

3.2 Japan 1000kV project

Japan's 1000kV power system is concentrated in Tokyo Electric Power Company. In 1988, the construction of 1000kV power transmission and transformation projects began. In 1999, two 1000kV transmission lines with a total length of 430km and one 1000kV substation were completed. The first line was from the northern coast of the Sea of Japan The 1000kV transmission line from the atomic power plant to the southern Tokyo area is called the South-North Line (190km in length), the South Niigata Main Line, and the West Gunma Main Line; the second is a 1000kV transmission line connecting power plants on the Pacific coast, called the East-West Line (Length 240 km), East Gunma main line, Nanqing city main line, in addition, Japan has also built a new 1100kV substation, all 1000kV lines and substations have been reduced to 500kV voltage level operation since the completion, consider cooperation The construction of nuclear power plants in the Pacific coast and Northeast regions is planned to be boosted to a rated voltage of 1000kV. However, due to the stagnation of load growth, the construction of power sources and the 1000kV boost plan have also been greatly delayed. run.

3.3 Italy 1050kV test project

In the 1970s, Italy and France were entrusted by the Western European International Power Generation and Supply Federation to conduct demonstration work on the selection of AC 800kV and 1050kV transmission schemes in the European continent. After that, the Italian ultra high voltage AC transmission project carried out basic technology research and equipment manufacturing under the auspices of the state. After a series of work, the 1050kV test project was completed in October 1995. Until December 1997, it was carried out at the system rated voltage (nominal voltage) of 1050kV for more than 2 years, and certain operating experience was obtained.

The test project is located in the Suvereto 1000kV test station in Italy, including two parts:

3.3.1 1050/400kV substation;

3.3.2 2.8km1050kV transmission line.

3.4 Ultra high voltage transmission projects in China

China's research on UHV power transmission technology began in the 1980s. After more than 20 years of hard work, a number of important scientific research results have been obtained. Studies have shown that the development of UHV transmission is an inevitable choice for the development of China's electric power industry. The UHV AC power transmission and transformation projects under construction by State Grid: one is the middle line project of northern Shaanxi-Southeast Shanxi-Nanyang-Jingmen-Wuhan, and the other is the east line project of Huainan-South Anhui-Northern Zhejiang-Shanghai. In addition, China's third ultra high voltage transmission project - "Sichuan-Shanghai ±800 kV UHV DC Transmission Demonstration Project", also started construction in Yibin County, Sichuan Province on December 21, 2007. By 2020, my country's ultra high voltage power grid will be basically completed, and the transmission power will reach more than 200 million kWh, accounting for 25% of the country's total installed capacity.

On January 6, 2009, the 1000 kV AC power transmission and transformation project independently developed, designed and constructed by my country with independent intellectual property rights - Jindongnan-Nanyang-Jingmen ultra high voltage AC test demonstration project successfully passed the trial operation. This marks a major breakthrough in my country's long-distance, large-capacity, low-loss ultra-high voltage (UHV) core technology and localization of equipment, which is of great significance for optimizing the allocation of energy resources and ensuring national energy security and reliable power supply.

This UHV AC line, which is put into operation for the first time in the world, has a total length of 640 kilometers. Its voltage level is the highest in the world, reaching 1,000 kV. The electric energy transmitted is five times that of the existing 500 kV. The land area can be saved by more than half, and the investment of the whole project can be saved by one-third compared with the 500 kV line. Longitudinal across the three provinces of Shanxi, Henan and Hubei, it also includes two large spanning sections of the Yellow River and the Han River. The line starts from Shanxi 1000kV Jindongnan Substation, passes through Henan 1000kV Nanyang Switching Station, and ends at Hubei 1000kV Jingmen Substation.

The project obtained the project approval document issued by the National Development and Reform Commission in August 2006, and started construction at the end of the same year. It was fully completed in December 2008, and the system commissioning was completed on December 30 and put into trial operation. Completed 168 hours of trial operation and put into commercial operation, the operation is in good condition.

On December 15, 2015, the world's first 10 million-kilowatt UHV DC transmission project - Ximeng-Taizhou ±800 kV UHV DC transmission project started in Xinghua. The project is an important part of the national air pollution prevention and control action plan "four intersections and four directs" UHV project, and it is planned to be completed and put into operation in 2017.

4. The next two years will be the peak period of UHV construction in my country

Recently, the State Grid is in full swing of the preparatory work for multiple ultra high voltage constructions, and 2015-2016 will be the peak period for UHV line construction.

The mid-year work conference of the State Grid Corporation of China has clearly stated that the construction of the "three AC and DC" Huainan-Nanjing-Shanghai, Ximeng-Shandong, Mengxi-Tianjin AC and Ningdong-Zhejiang DC UHV projects will start before the end of the year. Mengxi-Tianjin AC ultra high voltage has not been approved. At the same time, strive to approve the Jiuquan-Hunan UHV DC transmission project before the end of the year.

Following the approval of the Huainan-Shanghai (North Ring) ultra high voltage AC line in the first half of the year, the Ximeng-Shandong UHV AC project was also approved by the National Development and Reform Commission in the third quarter. In addition, the two AC lines of Mengxi-Tianjin and Yuheng-Weifang are about to be approved, and the State Grid is in full swing for the landing of multiple UHV lines.

On October 16, the construction of the first bid section of the Ningdong-Zhejiang ±800 kV UHV DC project started, and the casting of the first foundation began at Tower No. 232.

The first tender line undertaken by Shaanxi Power Transmission and Transformation Engineering Company starts from Tower 201 in Hongjingzi Township at the junction of Ningxia and Shaanxi, and ends at No. 401 West Naizhang Tower in Xiaowanqu, Wuqi County. The total length of the line is 87.955 kilometers, with a total of 160 towers base. Most of the foundations in this section are manually dug holes, the pole tower is a self-supporting iron tower, and the wire adopts the first steel-cored aluminum wire with the largest cross-section in the country.

On October 21, the first meeting of the State Grid Corporation Huainan-Nanjing-Shanghai 1000 kV ultra high voltage AC Project Hepingwei Phase III Expansion Project Site Construction Coordination Leading Group was held in Jiangsu. The State Grid requires all participating units to form a technical plan for Taizhou Station and the Yangtze River Great Span as soon as possible to ensure that the third phase of the Pingwei project will be put into operation in the first quarter and the overall safety and quality of the project will be stable. Make good arrangements for large-scale transportation, handover tests, construction and commissioning of supporting projects, and other related work.

The Huainan-Nanjing-Shanghai ultra high voltage AC project is by far the largest, most expensive, and most difficult AC UHV power transmission and transformation project. It has three major characteristics: technical difficulty, equipment challenges, and construction difficulties.

It is reported that the Huainan-Nanjing-Shanghai ultra high voltage AC project newly built 3 substations in Nanjing, Taizhou and Suzhou, expanded the Huainan and Huxi substations, added 12 million kVA of substation capacity, and built a new transmission line of 2×779.5 kilometers. Double-circuit erection (partial four-circuit erection on the same tower), passing through Anhui, Jiangsu and Shanghai. The Pingwei Phase III transmission project includes a 1,000 kV outgoing line interval at Huainan Station, and a new double-circuit line on the same tower (one-side hanging line in this phase) is about 5 kilometers long. Land acquisition, design, procurement, construction and other aspects of the project are progressing smoothly according to the milestone plan, and the overall safety and quality situation is stable.

On October 23, the Hunan Electric Power Maintenance Company broke the news that State Grid Hunan Electric Power plans to expand the Zhuzhou 500 kV Yuntian Substation to build it into the largest substation in Hunan to connect with the ultra high voltage landing and meet economic development and resources. Use needs.

A major breakthrough has been made in the approval of ultra high voltage lines, and 33 lines are to be built. It is a year when a major breakthrough has been made in line approval, and there is a high probability that there will be "three AC and two direct" UHV lines approved. In order to meet the delivery needs of large-scale energy bases in my country, it is estimated that around 2020, about 24 UHV DC transmission lines and 9 UHV AC lines will need to be newly approved. In addition to the 12 power transmission channels of the Energy Bureau, the State Grid is planning a new "five-way and five-way" plan, which will ensure the continuity of UHV construction.

In September 2015, the first foundation pouring ceremony was held in Suzhou for the Anhui 1 bid section of the Shanjiang ±800 kV UHV DC transmission line project, marking that the project has entered the full-scale construction stage.

As the first ultra high voltage project among the 12 key transmission channels of the National Air Pollution Prevention and Control Action Plan to be approved and started construction, the Huainan-Nanjing-Shanghai 1000 kV UHV AC project across the Huaihe River and the Yangtze River was completed in September 2014. The construction started in March, and it was originally planned to be put into operation in March 2016. The project is about to enter the acceptance stage and is expected to be put into operation ahead of schedule.

The Zhundong-South Anhui ultra high voltage project is scheduled to be completed and put into operation in 2018. Zhundong-South Anhui Project is the UHV transmission project with the highest voltage level, the largest transmission capacity, the longest transmission distance, and the most advanced technology in the world. It has opened a new era in the development of UHV power transmission technology and has a significant demonstration effect on the development of the global energy Internet.