Large-scale transmission of clean energy is the primary problem to be solved in the new power system
There is a "Hu Huanyong line" in my country's clean energy distribution and electricity demand. Due to factors such as climate, productivity development, historical political economy and other factors, the economic development among regions in our country is unbalanced. The "Hu Huanyong Line" (also known as the Heihe-Tengchong Line) proposed in 1935 is a typical description of this phenomenon: the area east of the Hu Huanyong Line occupies about 36% of the country's land area and occupies more than 95% of country's population (in the 1930s data at that time). There is also a "Hu Huanyong line" in China's clean energy distribution and electricity demand. The east of the Hu Huanyong Line consumes 86.5% of electricity, while the west consumes only 13.5%. However, in terms of clean energy distribution, it can be seen from the distribution of wind resources and light resources in China that the west of the Hu Huanyong Line is much higher than the east of the Hu Huanyong Line. Except for offshore wind power resources, other high-quality wind resources are far away from load-intensive areas and have huge power deployment needs.
Offshore wind power is an important clean energy source along the coast, and it is the trend of the times to go offshore and scale up. China's offshore wind power is developing rapidly. In 2020, China's offshore wind power installed capacity will reach 3.1GW, surpassing Europe for the first time to become the world's largest offshore wind power market, with newly installed capacity exceeding half of the world's total. In 2021, China's newly installed offshore wind power capacity will be 16.9GW, a record high. However, with the withdrawal of state subsidies for offshore wind power in 2022, offshore wind power will enter the era of parity, and the installed capacity will return to normal levels. Offshore wind power is close to the load center, which is conducive to consumption, and the output of offshore wind power is relatively stable, and the utilization hours are high. It is the best clean energy in coastal areas. According to the offshore wind power planning of Guangdong, Jiangsu and other places, combined with the development trend of foreign offshore wind power, deep sea and large scale are the general trend.
UHV DC is the best solution for large-scale power transmission across regions
UHV includes UHV AC and UHV DC transmission. UHV AC refers to AC transmission projects with a voltage level of 1000kV, and UHVDC refers to DC transmission projects with a voltage level of ±800kV and above. The technical principles and development logic of the two are completely different. UHV DC is a typical point-to-grid power transmission project. Its basic principle is to use a converter valve to convert AC power into DC power, and then convert the DC power into AC power after being transported to the destination, and then connect it to the AC power grid. The main purpose is to transmit electric energy. In addition to transmitting electric energy, AC UHV also undertakes the role of improving the grid structure and enhancing the stability of the grid. DC transmission technology is a power transmission technology based on power electronics technology. Due to the advantages of simple topology, easy voltage transformation, and low equipment cost, AC transmission has become the most commonly used power transmission technology in countries around the world, and it is still the most important part of China's power grid. DC transmission technology is a technical route developed along with the birth of power electronics technology.
According to the different power electronic devices and functions, it can be divided into two routes: conventional direct current (LCC) and flexible direct current (VSC):
(1) Conventional direct current (LCC) is a direct current transmission technology that uses semi-controlled power electronic components such as thyristors as the core components of the converter valve. Its advantages are large transmission capacity and low cost, but it requires a strong AC grid support. The amount of harmonics is large, and reactive power needs to be absorbed from the grid, so a large number of DC filtering and AC filtering equipment must be configured.
(2) Flexible DC (VSC) is a DC transmission technology that uses fully-controlled power electronic components such as IGBTs as the core components of the converter valve. Its advantage is that it can form an alternating current very close to the standard sine wave through modular multi-level technology, and the active power and reactive power can be adjusted independently without filtering equipment or AC grid support. The disadvantage is that the cost is high and the delivery capacity is small.
From the perspective of long-distance power transmission, DC UHV has obvious advantages over AC UHV: the general pattern of partitioned operation of my country's power grid will not change. The operation of my country's power grid is undertaken by three main operators, the State Grid Corporation of China, the China Southern Power Grid Corporation and the Inner Mongolia Electric Power Company. There are 7 regional synchronous power grids, and there is only a weak connection between the regional power grids, and most of the electricity production and consumption are generated within the region.
According to data from the China Electricity Council, in 2021, 687.6 billion kilowatt-hours of electricity will be transmitted across regions across the country, accounting for only about 8.3% of the electricity consumption of the whole society, and the connections between regions are relatively weak. Expansion of the AC power grid may cause the risk of the power grid to rise instead of falling. According to the 2018 Chinese Academy of Engineering's "my country's Future Power Grid Pattern Research (2020) Advisory Opinion", we should continue to adhere to the structure with the six major regional power grids as the main body (the 2019 Chongqing-Hubei Project Investment Southwest Power Grid and Central China Power Grid will be separated after transportation). Therefore, AC UHV cannot transmit power across regions, and it can only play a role in specific situations, such as the presence of high-quality wind and solar resources and large demand for electricity in the same power grid, and the distance between the two is relatively long.
DC transmission is the best regional grid connection. However, due to differences in resource endowments across regions, my country has a relatively large demand for inter-regional power transmission. DC transmission has the following three advantages, making it the best solution for inter-regional power transmission:
(1) DC transmission has outstanding economy in long-distance power transmission. The cost of DC converter stations is higher than that of AC substations, but since DC transmission does not have skin effect and charging power, the utilization rate of transmission lines is higher. Therefore, when the transmission distance is long enough, its economy will surpass that of AC transmission.
(2) It can be used for asynchronous grid interconnection. The AC grid interconnection requires the frequency of the entire grid to be consistent, so it cannot be used for asynchronous grid interconnection. DC power transmission first rectifies AC power into DC power and then inverts it into AC power, which can be applied to asynchronous grid interconnection.
(3) It is conducive to the isolation of grid accidents and does not expand the risk of grid accidents. UHV DC transmission can be regarded as a long-distance controllable power source of the receiving end grid. The grids at both ends are not coupled, and the grids at both ends can be isolated. In the event of a serious power grid accident, UHV DC can isolate the accident without expanding the risk of power grid accidents. Another typical application scenario of UHV AC is to strengthen the power grid. As my country's large-scale direct current transmission enters North China, East China, Central China and Southwest China, the strength of the AC power grid determines the safety of the entire power system, and the demand for AC UHV increases accordingly.
The important role of flexible DC in new power systems
Flexible direct current is especially suitable for the transmission of large-scale offshore wind power in distant seas. At present, the mainstream offshore wind power transmission method is high-voltage AC transmission, that is, offshore wind turbines are connected to offshore booster stations, boosted to 220kV or higher voltage levels, and then sent to onshore power grids. Since DC transmission has no charging power, the investment and transmission efficiency of submarine cables are better than AC transmission. Generally speaking, when the transmission distance is greater than about 80km, the economy of DC transmission will exceed that of AC transmission. In addition, since conventional DC requires strong AC grid support, and offshore wind farms are weak AC systems composed of wind turbines, which cannot meet the power transmission requirements of conventional DC, flexible DC has become the only economical and feasible solution. The LCC-VSC hybrid technology route effectively solves the problem of UHV DC commutation failure in areas with dense DC drop points. After decades of construction, my country has built 32 direct current transmission projects with long-distance power transmission as the main function, of which more than 10 projects are located in the Yangtze River Delta or Guangdong Province, and the dense placement leads to direct current transmission between the two places. The risk of commutation failure increases, and the hidden danger of power grid accidents increases. The flexible DC can independently support the voltage without the risk of commutation failure, and is the best solution for continuing to feed DC to the above two places. At present, China Southern Power Grid has completed the Wudongde DC transmission project, and the State Grid is also building the Baihetan-Jiangsu UHV DC transmission project, both of which have applied flexible DC technology. But the technical solutions of the two projects are different.
Flexible DC interconnection increases the mutual aid capability of the power grid and improves the reliability and efficiency of power supply. In addition to the long-distance conventional power transmission scheme between regional power grids in my country, back-to-back flexible DC routes can also be used for interconnection at the junction of regional power grids. The so-called back-to-back flexible DC refers to the construction of a rectifier station and an inverter station together without a DC line. The back-to-back flexible DC technology can improve the mutual power capability between regional power grids without expanding the scope of power grid accidents. In addition, the 500kV power grids in Guangdong, Jiangsu and other places in China are already very large, with complex structures and prominent problems of excessive short-circuit current. Adding back-to-back flexible DC to "untie" the power grid can also effectively solve the above problems. The Chongqing-Hubei back-to-back project and the under-construction Fujian-Guangdong interconnection project are typical applications of flexible straight back-to-back projects.