Recently, a "underwater giant dragon" weighing 13700 tons and with a total length of over 180 kilometers sank steadily to the bottom of the sea in Yangjiang, Guangdong. The Three Gorges Yangjiang Qingzhou Five and Qingzhou Seven Sea Wind Farm Submarine Cable Concentrated Transmission Project, constructed by China Energy Engineering Guangdong Institute through EPC general contracting, has successfully completed the laying of the world's highest voltage level, largest transmission capacity, and largest conductor cross-section cross-linked polyethylene DC submarine cable.
Marching towards the deep sea
The Qingzhou Five and Seven Seas Wind Farm Cable Concentrated Transmission Project is located in Yangxi County, Yangjiang City, Guangdong Province. It is currently the world's highest voltage level and largest transmission capacity offshore wind power flexible DC transmission project, undertaking the important task of bundling and concentrating the clean electricity of 2000 MW offshore wind farms.
Construct a ± 500 kV/2000 MW offshore converter station, two ± 500 kV DC submarine cables, and an onshore centralized control center. Among them, the submarine cable line has a single length of over 90 kilometers and a total length of over 180 kilometers.
After the overall completion of the project, it can deliver 7.7 billion kilowatt hours of green electricity to the electricity load center of the Guangdong Hong Kong Macao Greater Bay Area every year, which is equivalent to reducing carbon dioxide emissions by about 6.3448 million tons per year. It will inject strong power into the transformation and upgrading of Guangdong Province's energy structure, and provide engineering demonstration for the development and transmission of large-scale and long-distance offshore wind power collection in China.
The birth of a "gentle, straightforward, and fast delivery"
The key to the development of large-scale offshore wind power in the deep sea lies in the transmission of electricity. The transmission distance of about 90 kilometers, voltage level of ± 500 kilovolts, and transmission capacity of 2000 megawatts pose a "world-class test" for the performance and manufacturing process of submarine cables.
Under the leadership of Three Gorges Group, Guangdong Institute, together with Ningbo Oriental Cable and other units, has formed a research and development team to carry out collaborative innovation covering the entire industry chain, including equipment research, design, manufacturing, laying, and operation. The team has successfully developed a ± 525 kV cross-linked polyethylene insulated optical fiber composite DC submarine cable.
This is the world's highest voltage level and largest transmission capacity cross-linked polyethylene insulated flexible DC submarine cable, with a copper core cross-section of 2500 square millimeters. It has made significant technological breakthroughs in structural performance, production and manufacturing, laying and operation.
Three core technologies
Ultra thick insulation and continuous manufacturing: The insulation layer is the key "armor" of submarine cables, and its insulation performance is crucial for the safe and reliable operation of submarine cables. The research and development team has innovatively applied ultra thick insulation and ultra clean extrusion technology, achieving continuous production of long lengths of thick cross-linked polyethylene insulation layers. At the same time, a special development of ± 525 kV DC submarine cable factory joints has successfully solved the problem of weak insulation at the joints of ultra-high voltage submarine cables, greatly improving the insulation performance and operational reliability of submarine cables.
Large capacity capacity expansion: In response to the bottleneck area of submarine cable current carrying capacity, the research team has improved the heat dissipation conditions by filling low thermal resistance materials with directional drilling and optimizing J-shaped pipe ventilation. The overall utilization rate of transmission capacity has been increased by about 10%, and the annual power transmission can meet the electricity needs of 1.2 million households. Through numerical simulation and other research, the theoretical capacity increase can reach 20%, leaving sufficient space for future offshore wind power expansion.
High resistance to external forces protection: The complex sea conditions in deep waters pose a high risk of damage from external forces such as ship anchors, and require a high level of protection. The research and development team has developed specialized software to accurately calculate the depth of the ship anchor into the soil. Through reverse calculation and verification, the safe burial depth of the submarine cable can be determined; Through simulation and experimentation, the high anti towing and anchoring capabilities of new protective measures such as concrete interlocking rows have been verified, laying a solid underwater barrier for the long-term safe operation of submarine cables.
This submarine cable system has been selected as the first major technical equipment in the energy field in the fifth batch by the National Energy Administration, marking a breakthrough in China's localization in the core technology field of ultra-high voltage flexible DC submarine cables.
In the submarine cable manufacturing process, Guangdong Institute dispatches a professional supervision team to go deep into the production line, track and control the entire chain of raw materials, core processes, and factory tests, and ensure the quality control of every meter of submarine cables.
Ultra long distance 'underwater embroidery'
The Yangjiang sea area has frequent monsoons, and the water depth of the submarine cable route corridor ranges from 5.5 meters to 49 meters, with a drop of nearly 45 meters. It also needs to pass through the waterway area and reef area. The underwater cannot be seen or touched, and it is as difficult as "underwater embroidery" to accurately lay this super heavy and ultra long distance "dragon" in place.
To this end, the project team conducted advance weather analysis, optimized key process flow operations, and used cable laying ships equipped with DP2 dynamic positioning systems, combined with advanced equipment such as electric pallets, buried plows, AIS ship automatic identification systems, and seabed detection systems, to precisely control every link from deployment, lifting, to recycling, from burial depth monitoring to route deviation control, making deepwater operations both "visible" and "stable".
The diameter of the submarine cable body is 176.5 millimeters, with a weight of 77.11 kilograms per meter. The project team strictly controlled the bending radius of all turning points and channels during construction to be 20 times or more the diameter of the submarine cable.
After entering the channel section, the submarine cable needs to be buried at a depth of 4 meters on the seabed. The dense traffic of ships in this sea area places extremely high demands on construction safety and navigation security. During construction, the project team will stably control the laying speed at 5~8 meters/minute, arrange the guard ship to be on duty within 500 meters of the construction ship, and realize real-time linkage with the maritime department through the VTS vessel traffic management system, so as to ensure that the construction operation and maritime navigation are both correct.
During the construction process, the built-in fiber optic detection of submarine cables cannot be ignored. The team conducts fiber OTDR testing every 24 hours and before and after key processes. After each turn and bend is laid, it is promptly inspected to ensure that the electrical and optical performance of each section can be traced and verified, keeping the "underwater nerve" of this "dragon" safe and stable.
From design and development to production and construction, from underwater laying to landing and connection, every inch of this' underwater dragon 'embodies the wisdom and sweat of engineers. The deep blue road has been built, and the future of the deep sea is sailing.
