Cross-ocean energy transmission is the only way to realize the optimal allocation of global energy, complement each other's energy resources on all continents, and efficiently utilize clean energy. It is an important link in building a global energy Internet that spans five continents, connects four oceans, traverses east and west, and runs through north and south.
Submarine cables, cross-sea bridge cables, submarine tunnel cables and cross-sea overhead lines are the main ways to achieve cross-sea interconnection and offshore wind power transmission. Submarine cables are the most commonly used.
Compared with overhead lines, submarine cable transmission accounts for less than 1% of the global power grid's transmission capacity. Large-scale offshore wind power development and extensive grid interconnection have provided direct impetus for the development of high-voltage DC submarine cable technology, which has great development potential in the future.
As of 2019, the total annual global electricity demand reached 28,000TWh, and the global power grid transmission scale exceeded 3,000GW, of which approximately 26GW was transmitted by submarine cable projects, accounting for less than 1%. As the first region to develop offshore wind power, Europe has become the region with the largest number of submarine cable projects and the largest construction scale in the world. The total length of submarine cables has exceeded 6,200km and the total transmission capacity has exceeded 22GW. With the rapid economic development, clean energy development and increasing demand for regional cross-sea networking, Asia is gradually growing into an important high-voltage submarine cable engineering application market.
By the end of 2019, the technology of ultra-high voltage DC submarine cables has gradually matured, mainly including two technical routes: sticky impregnated paper insulation and extruded insulation. The technical level can reach ±200kV~±600kV/1000MW~2500MW. Attachment technology is a key factor in realizing long-length submarine cables. The related materials and processes are extremely complex and are the biggest weak link.
With the rapid development of large-scale offshore clean energy development and cross-ocean interconnection of power grids, the demand for submarine cable project transmission capacity, distance and economic improvement has gradually increased, and the demand for the development of EHV DC submarine cable technology has continued to increase. It is estimated that in the next 30 years, the total transmission distance of cross-sea projects in Asia, Europe, North America and Africa will reach 10,000km, 9,000km, 5,000km and 4,000km respectively, and the total capacity will reach 120GW, 120GW, 40GW and 50GW respectively, of which the transmission capacity of most projects needs to reach 4000~8000MW, and some transmission distances can reach 2000~3000km.
It is technically difficult for ultra-high voltage DC submarine cables to meet future needs, so it is urgent to develop ±800kV and above ultra-high voltage DC submarine cable technology. In terms of economics, the comprehensive cost of ±200~±600kV ultra-high-voltage DC submarine cable bipolar cables is 1 million to 2.6 million US dollars/km, which is 5 to 10 times the cost of overhead lines of the same level, and is still at a relatively high price. But at the same time, as the voltage level and conductor cross-section increase, the unit capacity cost of DC submarine cables shows a downward trend. Therefore, in the future, EHV large-capacity DC submarine cables will be more economical than UHV DC submarine cables and will have better development prospect.
Compared with UHV DC submarine cables, EHV DC submarine cables have higher requirements in terms of key technologies and comprehensive economic indicators. In terms of technology, based on future capacity demand, production technology and equipment capabilities, it is expected that the unit voltage withstand capacity of the insulation materials of ±800kV~±1100kV/4000MW~12000MW EHV DC submarine cables (extruded insulation) in the future will need to be no less than 43~65kV/mm. In terms of economy, it is expected that in the future, ±500kV/2000MW~3000MW, ±600kV/4000MW, and ±800kV/8000MW DC submarine cables will need to cost less than 2.5 million, 3 million, and 7 million US dollars/km to have good market competitiveness.
The research and development of EHV DC submarine cables will face core technical bottlenecks such as voltage, capacity, distance, and sea depth improvements. Economic improvement is a key factor in the promotion and application of EHV DC submarine cables. Market and policy factors are catalysts to further promote the development of EHV DC submarine cables The development of EHV DC submarine cables requires breakthroughs in insulation materials, processing technology, accessory technology, construction technology and post-operation and maintenance technology.
In the short and medium term in the future, the technical level of ±800kV/4000MW EHV DC submarine cable can be achieved, and the technical level of ±800kV/8000MW is expected to be exceeded in the medium and long term. Mature and reliable impregnated paper insulation technology and rapidly developing extrusion insulation technology are the technical routes for realizing low-capacity EHV DC submarine cables in the short term in the future. With the further improvement of the heat resistance of insulation materials, it is expected to reach the ±800kV/8000MW level in 2035. It is expected that by 2050, under the conditions of major breakthroughs in the properties of conductors and insulating materials, it is expected to break through the ±1100kV voltage level technical level.
The research and development of EHV DC submarine cables is a step-by-step systematic project that requires breakthroughs in key technologies such as materials, design and processes in stages and steps.
Before 2025, focus on optimizing the insulation structure design of impregnated paper and extrusion technology, and construct the basic theory of DC submarine cable insulation structure design involving parameters such as space charge, temperature, electric field, conductivity, dielectric constant, etc., through nano-doping and base material purification and other methods are used to reduce the impact of space charge, study the electric field reversal mechanism and suppression methods, and improve the processing technology and operational reliability of the body and accessories; develop EHV grade test terminals and build EHV systematic test base; meet requirements for ±800kV/4000MW EHV DC submarine cable project.
From 2025 to 2035, improve the manufacturing level of insulation materials, develop high-purity and high-net insulation base materials, increase the long-term temperature resistance to 110°C, increase the insulation strength to 43kV/mm, and research matching shielding materials.
From 2035 to 2050, conduct in-depth and comprehensive research on potential insulation base materials, improve chemical synthesis capabilities, and develop new insulation materials with insulation strength as high as 65kV/mm; design the submarine cable insulation structure based on the performance characteristics of new materials to improve the water resistance, pressure resistance, and deformation resistance of the submarine cable body, and form a processing capability and production level for industrial mass production.
With the advancement and promotion of technology, the economics of EHV DC submarine cables will be greatly improved in the future and expected economic goals can be achieved. It is estimated that in 2050, the cost of ±800kV/4000MW and ±800kV/8000MW DC submarine cables will reach US$2.6 million/km and US$4.4 million/km, and the cost of ±1100kV/12000MW submarine cables is expected to reach US$5.8 million/km, with good economy and market competitiveness.
The development of EHV DC submarine cables will bring huge technical, economic, social environmental and political benefits. On the one hand, it can promote technological progress in materials, processes, control and other related industries, and drive investment in cross-sea DC power transmission projects of up to 150 billion US dollars. On the other hand, it can improve regional energy supply security and create approximately 14 million jobs in related industries. At the same time, it can promote the process of energy cleanliness, effectively address the problem of climate change, and reduce land resources by more than 100,000 km2.
In addition, the development of EHV DC submarine cables can also promote a new energy and power trading mechanism, accelerate the development of offshore clean energy and the construction of a global energy Internet, enhance the level of energy interconnection, improve the power supply level of island countries, and improve the security of energy transmission. Promote coordinated regional development.