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Energy is the cornerstone of a country's economic and social development and the lifeblood of national security. The development of human society has put forward new requirements for energy supply, energy structure and energy utilization mode. Especially since the beginning of the new century, problems such as the shortage of fossil energy, serious environmental pollution and global climate change have become increasingly prominent, making the demand for large-scale clean energy power transmission and wide-area balance between energy supply and demand increasingly strong.

Facing the severe challenges of global energy security, environmental pollution and climate change, the State Grid Corporation has put forward the major strategy of developing the "global energy Internet" by relying on UHV AC, DC and smart grid technologies. It aims to make overall development of energy in the context of global economy, society and environment, coordinate the development, allocation and utilization of global energy resources, and implement clean substitution and electric energy substitution. The establishment of a new global energy development, allocation and utilization system led by clean energy can greatly promote the development and consumption of renewable energy, integrate "one pole, one pole", large energy bases of various continents and countries and various distributed power sources, enhance international and regional cooperation, promote world peace and development, and promote the safe, clean, efficient and sustainable development of world energy.

The global energy Internet has a great demand for DC transmission technology

The transmission and exchange of large capacity electric energy will be one of the main characteristics of trans-continental and transnational power grid interconnection in the future. According to the demand analysis, it is expected that in 2050, the power sent out through the Arctic channel will reach 3 trillion KWH/year, and the power sent out from the equatorial region will reach 9 trillion KWH/year, which together will account for 16% of the global electricity demand. At the same time, transcontinental and transnational power transmission channels are usually thousands of kilometers long. The distance between the Kara Sea wind power base in the Arctic region and North China is about 4,400 kilometers. The Bering Strait wind base is about 5, 000 kilometers away from North China, Japan and South Korea, and about 4, 000 kilometers away from load centers in the western United States. Thus, it is an inevitable trend of the global energy Internet to develop transmission technology with longer transmission distance, larger transmission capacity and higher transmission efficiency.

Ultra-high voltage Direct current transmission technology (UHVDC) has the characteristics of long transmission distance, large transmission capacity, low loss, small converter station area, small transmission corridor and so on, especially for long-distance large capacity power transmission, has significant advantages. In the process of building a global energy interconnection grid, UHVDC will be mainly used for ultra-long distance and ultra-large capacity power transmission of large energy bases and the construction of transnational and trans-continental backbone channels. At present, China's UHV AC/DC projects have a maximum transmission distance of more than 2,000 kilometers and a transmission capacity of 8 million kW. With the comprehensive breakthrough of ±1100 kilovolt UHVDC transmission technology, the transmission distance will exceed 5,000 kilometers and the transmission capacity will reach 12 million kilowatts. The results show that with ±1100 kV UHVDC transmission, the development cost difference between the transmitting and receiving ends reaches US $0.042 per KWH, regardless of cross-border tariffs, even considering the high investment cost of lines and converter stations, and the economic transmission distance can reach 5000 km. This cost can already support the long-distance economic transmission needs of the world's large clean energy bases.

Clean substitution is an important concept of global energy Internet. By 2013, the global installed capacity of wind power and photovoltaic power was 320 million kW and 140 million kW respectively, accounting for about 5.6 percent and 2.5 percent of the total installed power capacity. It is expected that by 2020, the cumulative installed capacity of wind power and photovoltaic power will reach 700 million kW and 490 million kW. However, different from traditional hydropower and coal power, wind power, photovoltaic and other energy generation is characterized by intermittenity, volatility, randomness and non-storage, while wind power output characteristics show obvious anti-peak regulation characteristics. Its large-scale access will bring major challenges to the acceptance level and means of power grid.

Flexible Direct current transmission technology (VSC-HVDC) can improve the grid-connected efficiency of clean energy such as wind power and alleviate the impact of voltage fluctuation on the power grid. It has significant technical advantages, especially for remote land and offshore wind farms, and is an important technical means for large-scale clean energy bases to be connected to the power grid in the future. The DC power grid technology based on flexible DC can smooth the fluctuation and randomness of clean energy generation in a large range. In the field of power transmission and distribution, more and more attention has been paid. With the continuous innovation and maturity of technology, it is expected to become one of the key technologies of global energy Internet backbone grid in the future.

Worldwide power grid planning and DC project construction

From the actual distribution of global energy resources and load centers, in order to realize rational and efficient utilization and consumption of energy resources, especially renewable energy resources, it is necessary to build a large number of UHVDC transmission projects to realize long-distance transmission of large-scale electric energy.

For example, about 80 percent of coal resources and 70 percent of clean energy are concentrated in the western and northern regions of China, while the eastern and central regions, which are the power load centers, have scarce energy resources.

From the perspective of the distribution of clean energy resources in the world, the Arctic Circle and its surrounding areas (" one pole ") are rich in wind energy resources and the equator and nearby areas (" one pole ") are rich in solar energy resources. It will be an important direction of energy development in the future to develop Arctic wind energy and equatorial solar energy resources and send them to the load centers of all continents through UHV and other transmission technologies, support each other with large energy bases and distributed power sources on all continents, and provide safer and more reliable clean energy supply.

Up to now, there are 14 UHVDC power transmission projects with voltage levels of ±800kV or above that have been put into operation or are under construction worldwide. In the next 10-15 years, construction of 2-3 UHVDC transmission lines will begin every year, with direct investment of 50 billion to 100 billion yuan, according to the plan.

The rapid progress of flexible DC technology has promoted its wide application in wind power grid connection and power grid interconnection, and the development of the market has in turn promoted the improvement of the technical level. From the current application demand at home and abroad, the future development direction of flexible DC technology includes: high voltage and large capacity flexible DC transmission technology and long distance overhead line flexible DC transmission technology.

In November 2008, EU countries officially launched the Super Grid project, which aims to build a multi-terminal DC transmission network connecting Europe, North Africa and the Middle East based on high-voltage and large-capacity flexible DC transmission technology. By connecting wind power in the North Sea and the Baltic Sea, and solar power in North Africa and the Middle East, the supergrid can provide power from multiple sources and supply power to the European continent in the form of multiple stops, ensuring that the European grid has a good capacity to accept renewable energy.

Since 2010, international conference on power grid (CIGRE) and European electrotechnical standardization committee (EuropeanCommitteeforElectromechnicalStandardization, CENELEC has set up a task force to conduct a series of studies on supergrid technology. In 2011 CIGRE established the B4-52 "DC Grid Feasibility Study" working group to discuss the feasibility of DC grid construction from a number of aspects. Six working groups, B4-56 to B4-60 and B4-65, have been set up successively to carry out research work in DC power network planning, DC converter model, topology, power flow control, control protection, reliability and voltage level, etc. At the same time, CENELEC working group also carried out the preliminary research work of DC power grid.

Advanced DC transmission technology is an important basis for building the future global energy Internet. Among them, UHV DC transmission technology will be the main solution to solve the problem of long-distance large-capacity electric energy transmission across countries and continents, and the AC-DC interconnection grid with strong and rigid alternating current will become the main form of the backbone of global energy interconnection in the future. The continuous improvement and large-scale development of flexible DC power transmission technology and DC power grid technology will bring profound influence and profound reform to large-scale regional new energy access and delivery, as well as future power grid form.