Compared with AC superconducting cables, the power grid line loss is reduced by about 70%.
Recently, China's first high-temperature superconducting low-voltage DC cable has been officially put into commercial operation, filling the application gap of superconducting cables in China's low-voltage DC system. The application of DC superconducting transmission technology will inject strong impetus into the construction of new power systems and energy transformation and upgrading.
Superconducting cables are ideal cables for current transmission. Over the past 20 years, with the advancement of superconducting material technology, countries and regions such as China, the United States, Japan, South Korea, and Europe have maintained a high level of scientific research investment in the application research of high-temperature superconducting cables.
"Due to the absence of AC losses in DC power supply, DC superconducting cables will further reduce grid line losses compared to AC superconducting cables," said Gu Hongwei, member of the Superconducting Technology Professional Committee of the Chinese Electrotechnical Society. According to relevant literature data, the loss of AC high-temperature superconductors ranges from 3% to 4%, while the loss of DC superconducting cables is only 1% to 2%.
In June 2020, China Power Grid and JSYD Superconducting Technology Cooperation launched the construction of a high-temperature superconducting DC cable demonstration project. The project team constructed a high-temperature superconducting DC cable with a total length of 180 meters, connecting the 10 kV DC central station and the DC distribution room in the factory area.
This super cable is designed with a voltage of ± 375 volts, a current carrying capacity of 4500 amperes, and a conductor cross-sectional area of only 90 square millimeters. Compared to conventional PVC copper core cables of the same voltage level, its cross-sectional area is less than half of that of conventional cables. However, its transmission capacity is equivalent to 20 conventional cables, and the conductor loss is only one tenth of that of conventional cables. This greatly improves the transmission capacity while greatly reducing the loss during the power transmission process.
It is reported that the Yttrium Barium Copper Oxygen (YBCO) second-generation high-temperature superconducting tape used in this project is a domestically produced material, achieving the localization replacement of the core material for superconducting cable systems. Structurally, for the first time in China, this cable adopts a positive and negative pole coaxial method, which is equivalent to combining two cables into one. It is currently the most compact superconducting cable, which can make the power system more reliable.
The construction of demonstration projects has laid the foundation for the practical application of superconducting cables in urban distribution network systems, and has also provided typical sample experiences for the implementation of new power system construction and urban green and sustainable development.
With the maturity of superconducting application technology, the voltage level, capacity, and length of the constructed superconducting cables are constantly improving. However, due to factors such as the price of superconducting materials, vacuum insulation of cables, and refrigeration along the line, the economic and technological feasibility of using superconducting cables for long-distance transmission is not high, and it is necessary to find suitable application scenarios for this technology.
Industry insiders believe that although the cost-effectiveness of superconducting cables is still not comparable to conventional transmission lines at present, the high current carrying and large capacity characteristics of superconducting transmission technology can solve the transmission and distribution problems in large cities and high load density areas, and can be used in application scenarios such as distribution network renovation and upgrading and grid interconnection.
For example, the United States is researching the use of 13.8 kV three-phase AC cables to replace fully loaded conventional lines in cable trenches; South Korea uses superconducting cables to build power corridors and promote the upgrading of traditional power grids.