AC and DC projects are both developed together. Why do we need to use ultra-high voltage for power transmission?

Recently, the State Grid's Sichuan-Chongqing 1,000 kV UHV AC project and the Jinsha River to Hubei ±800 kV UHV DC transmission project simultaneously completed laying across the same 500 kV transmission line. This is the first time in my country's UHV construction.

At the construction site, workers operated a drone, which slowly lifted off with a traction rope. At an altitude of 140 meters, it drove a six-ton ​​conductor, which slowly crossed the sky above the 500-kilovolt Jialin Line and was finally erected between two towers more than 170 meters high and 400 meters apart.

It is understood that this crossing construction took advantage of the power outage window period of the Sichuan Power Grid's 500 kV Jialin Line, realizing only one power outage and completing the simultaneous crossing and laying of lines in three operating sections of the Sichuan-Chongqing UHV and Jinshang to Hubei UHV projects. This can minimize the power outage time.

The Jinshang to Hubei UHV project is the first UHV project passing through the Qinghai-Tibet Plateau, and the Sichuan-Chongqing UHV project is the first UHV AC project in Southwest China. Both projects are scheduled to be put into operation in the summer of 2025. After completion, they will be able to transmit a total of approximately 75 billion kilowatt-hours of electricity per year .

What is UHV transmission?

As we all know, electricity is transmitted by wires. We use electricity in our homes, factories, shops, schools, and hospitals. All this electricity is transmitted through the power grid. The electricity in the power grid is generated by power plants. We may have seen power plants before, or we may have never seen them before. It doesn't matter because most power plants are built far away from us. The electricity generated by the power plant is transmitted to the power grid, and then transmitted to our homes, factories, shops, schools, and hospitals through the power grid. This is called "power transmission."

UHV is the abbreviation of Ultra High Voltage. The voltage symbol is U (V is used in some places). The unit of voltage is volt, and the unit symbol is also V. There are kV (kilovolts) larger than volt, mV (millivolts) and uV (microvolts) smaller than volt, and there are thousands of decimals between them. In China, UHV refers to the voltage level of direct current of ±800 kV and above and alternating current of 1000 kV and above .

Why use ultra-high voltage transmission?

In power engineering technology, there is a term called "economic transmission distance", which refers to the most economical transmission distance of a certain voltage level transmission line, because the transmission line also has losses while transmitting electricity, and it is not economically viable to have too long a line with too much loss. The economic transmission distance of a 500 kV ultra-high voltage transmission line is generally 600 to 800 kilometers, while the economic transmission distance of a 1000 kV ultra-high voltage transmission line is increased because the voltage is increased and the line loss is reduced.

The distance between China's large energy bases and load centers can reach 1,000 to 3,000 kilometers. Therefore, to ensure the intensive development of large energy bases and reliable power delivery, it is necessary to vigorously develop ultra-high voltage transmission technology with large transmission capacity, long distance, and high efficiency . Ultra-high voltage is the most advanced transmission technology in the world. It is developed on the basis of ultra-high voltage transmission. Its purpose is to improve transmission capacity, realize high-power medium and long-distance transmission, and realize the interconnection of long-distance power systems to build a joint power system.

In general, UHV power transmission has the following advantages:

1. It can meet the requirements of large-scale, long-distance and high-efficiency power transmission. At present, the price of power coal in the western and northern regions is 200 yuan/ton of standard coal. The coal is loaded from the local area, transported to Qinhuangdao Port by road and rail, and then transported to the east China by sea and road. The price of power coal increases to more than 1,000 yuan/ton of standard coal. After conversion, the fuel cost per kilowatt-hour of electricity is about 0.3 yuan. In the coal-producing area, the fuel cost is only 0.09 yuan/kilowatt-hour. Electricity is transmitted to the load center in the central and eastern regions through ultra-high voltage. After deducting the cost of the transmission link, the grid-connected electricity price is still lower than the average grid-connected electricity price of local coal-fired power, which is only 0.06-0.13 yuan/kilowatt-hour.

2. It is conducive to improving environmental quality. The use of UHV transmission can promote the intensive development and efficient use of clean energy, and transmit clean electricity such as hydropower in the southwest, wind power and solar power generation in the northwest and northern regions of my country to the load centers in the east, middle and southeast on a large scale and over long distances, so as to achieve "electricity from afar, and it is clean electricity", reduce fossil energy consumption and pollutant emissions, and have significant environmental benefits.

3. It is conducive to improving the safety of power grid operation. The use of "strong AC and strong DC" ultra-high voltage AC and DC hybrid power grid transmission can greatly reduce the problems of insufficient power flow transfer capacity and weak reactive voltage support of the 500 kW power grid in the case of DC system failure, reduce the risk of large-scale power outages in the power grid, and create conditions for the gradual layered and zoned operation of the next-level power grid, solve problems such as excessive short-circuit current that restrict the development of the power grid, and improve the flexibility and reliability of power grid operation.

4. The driving effect of UHV construction on the domestic equipment manufacturing industry is even more obvious. Almost all the equipment used in the three major UHV experimental projects in China is provided by domestic enterprises, with the localization rate of the projects reaching about 95% and the localization rate of the equipment reaching about 91%. Through the experimental projects, domestic equipment manufacturing companies have been trained and their scientific and technological research and development capabilities have been greatly improved. For example, Nantong Shenma Electric Power Technology Co., Ltd. has successfully overcome the world's difficult problem of UHV insulators.

5. Driving the development of scientific and technological innovation As a major scientific and technological innovation project, the company invested heavily to build four world-class UHV AC, DC, high altitude, and engineering mechanics test bases and two R&D centers for large power grid simulation and DC complete set design at the beginning of the conception and full launch, forming a large power grid experimental research system with complete functions and comprehensive indicators at the world's leading level. Over the past few years, State Grid Corporation of China has completed 310 major key technology studies around UHV projects, solved many world problems such as overvoltage and insulation coordination, external insulation design, electromagnetic environment control, system integration, and large power grid safe operation control, and gradually mastered the key core technologies of UHV transmission, which have been successfully applied in experimental projects.

Comprehensive sources: Science Encyclopedia, Jilin Science Popularization Micro Window, Science and Technology Daily, etc.