UHV AC transmission refers to AC transmission of 1000 kV and above, which has outstanding advantages such as large transmission capacity, long distance, low loss, and small footprint. The expansion of power system and transmission scale, and the development of high-tech in the world have promoted the research of UHV transmission technology. Since the 1960s, the former Soviet Union, the United States, Japan, Italy and other countries have successively carried out basic research, practical technology research and equipment development, and have achieved breakthrough research results and manufactured complete sets of UHV power transmission equipment. The former Soviet Union has built more than 1,900 kilometers of AC transmission lines with a rated voltage of 1,150kV (the highest operating voltage is l200kV), and 900 kilometers of them have been operated at the designed voltage; kilometer.
The single-circuit transmission capacity of the million-volt AC line exceeds 5000MW, and it has obvious economic benefits and reliability. As a backbone line for medium and long-distance power transmission, it will play an important role in the construction and development of the power grid.
1. Advantages of UHV AC transmission
Calculated by the natural transmission power, the transmission power of one UHV line is equivalent to the transmission power of four to five 500kV EHV lines (about 4000-5000MVA), which will save precious transmission corridors and greatly enhance the sustainable development of China's power industry Ability.
From a technical point of view, the use of UHV transmission technology is one of the main means to improve the transmission capacity of the power grid, and it can also obtain advantages in reducing the occupation of transmission corridors and improving the structure of the power grid; from an economic point of view, according to the research results, transmission Under the condition of 10GW hydropower, compared with other transmission methods, the competitive power transmission range of UHV AC transmission can reach 1000-1500 kilometers. If the transmission distance is short and the transmission capacity is large, the competitive advantage of UHV AC is more obvious.
2. Current status and development of UHV AC transmission
Since 2004, the State Grid Corporation of China has begun to contemplate and demonstrate UHV projects. In August 2006, China's first UHV AC power transmission and transformation project—Jindongnan-Henan Nanyang-Hubei Jingmen 1000 kV UHV AC test demonstration project was approved by the National Development and Reform Commission and started construction. It was completed and put into operation on December 30, 2008 Trial run, officially put into operation on January 6, 2009. The project has been in safe operation for nearly a year, with stable system operation and normal equipment status. The cumulative power transmission is 8 billion kilowatt-hours, which is equivalent to about 3.5 million tons of coal.
The UHV AC test and demonstration project from Southeast Shanxi to Jingmen is an AC transmission and transformation project with the highest operating voltage and the most advanced technology in the world, and China has completely independent intellectual property rights. The total length of the project is about 640 kilometers, and the dynamic investment of the project is 5.736 billion yuan, of which the investment in equipment accounts for about half, and the localization rate of equipment reaches 90%.
Starting from the UHV AC test and demonstration project, the State Grid Corporation of China is promoting the construction of UHV power grids "overall and rapidly". It is planned to basically form an UHV power grid covering North China, Central China, and East China around 2020 to realize "power transmission from west to east and mutual supply from north to south".
My country has mastered the core technology of UHV AC power transmission and established a complete technical standard system, laying the foundation for the popularization and application of UHV AC power transmission technology.
2.1 Determine the standard voltage of UHV AC transmission. Innovation has formed steady-state voltage control technology, transient overvoltage suppression and submerged arc suppression technology. For the first time in the world, the voltage optimization control of UHV system has been realized.
2.2 Reveal the nonlinear discharge characteristics of the external insulation of the UHV system in a complex environment, and develop the air gap, insulator configuration and lightning protection technology. The UHV AC test demonstration project has been in safe and stable operation for more than four years. For the first time in the world, the optimal configuration of the external insulation of UHV systems in complex environments has been realized.
2.3 The key technologies for the design, manufacture and testing of UHV power transmission and transformation equipment have been formed, and a complete technology industry system has been established. The successful independent research and development of a complete set of UHV AC power transmission and transformation equipment representing the highest level in the world has changed China's long-term development model of "importing technology from developed countries, digesting and absorbing" in the field of electrical equipment manufacturing, and realized Chinese manufacturing for the first time.
2.4 The environmental friendliness goal of the UHV project has been achieved, and the electromagnetic environment measurement of the UHV AC test demonstration project meets the national environmental protection requirements.
2.5 Proposed a complete set of design and construction methods for UHV power transmission and transformation projects, equipment field test schemes, and developed line live working tools and test equipment. The UHV AC test demonstration project has become the AC transmission project with the highest voltage level and the strongest transmission capacity in the world.
2.6 Proposed a high-voltage test method that comprehensively simulates environmental conditions such as high altitude, repeated ice, heavy pollution, etc., formed a high-voltage and high-current test detection capability with the highest testable parameters in the world, and established a complete UHV test research system.
3. UHV AC transmission technology
UHV transmission technology has the characteristics of long distance, large capacity, low loss, saving land occupation and economy. The research on UHV AC transmission technology mainly focuses on line parameter characteristics and transmission capacity, stability, economy, insulation and overvoltage, corona and power frequency electromagnetic field and so on.
3.1 Transmission parameters of UHV AC transmission technology
3.1.1 Transmission capacity
The transmission capacity of the transmission line is proportional to the square of the transmission voltage and inversely proportional to the line impedance. Generally speaking, the transmission capacity of 1100kV transmission lines is more than 4 times that of 500kV transmission lines, but the capacitive reactive power generated is also 4.4 times or more than that of 500kV transmission lines. Therefore, when the transmission power of the UHV transmission line is small, the voltage of the sending and receiving end systems will increase. In order to suppress the power frequency overvoltage of the UHV line, it is necessary to connect reactors in parallel at both ends of the line to compensate the capacitive reactive power generated by the line.
3.1.2 Line characteristics
The reactance and resistance per unit length of UHV transmission lines are generally about 85% and 25% of 500kV transmission lines, but the susceptance per unit length can be 1.2 times that of 500kV lines.
The transmission capacity of UHV transmission lines is largely determined by the stability of the power system. For medium and long distance transmission (300km and above), the transmission capacity of UHV transmission lines is mainly limited by power angle stability (including static stability, dynamic stability and transient stability); for medium and short distance transmission (80~300km) , it is mainly limited by voltage stability; for short-distance power transmission (below 80km), it is mainly limited by thermal stability limit.
3.1.4 Power loss
The power loss of a transmission line is proportional to the square of the transmission current and proportional to the line resistance. In the case of transmitting the same power, the line current of a 1000kV transmission line is about 1/2 of that of a 500kV transmission line, and its resistance is about 25% of that of a 500kV line. Therefore, the power loss per unit length of a 1000kV UHV transmission line is about 1/16 of that of a 500kV UHV transmission line.
Compared with EHV transmission, UHV transmission is superior to EHV transmission in terms of transmission cost, operational reliability, power loss, and line corridor width.
3.2 Matters needing attention in UHV AC transmission technology
The charging reactive power generated by UHV AC lines is about 5 times that of 500 kV. In order to suppress power frequency overvoltage, a shunt reactor must be installed on the line. When the transmission power of the line changes, the reactive power at the sending and receiving ends will change greatly. If the reactive power hierarchical partition balance of the receiving end grid is not appropriate, especially if the dynamic reactive power reserve capacity is insufficient, voltage stability may become a major stability problem under severe working conditions and severe fault conditions.