Integration of High-Tc Superconducting Cables in the Dutch Power Grid of the Future

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Worldwide there is an increasing need for a more sustainable form of electrical power delivery with a growing share of renewable energy generation. In the distribution and transmission network, large-scale and small-scale wind and solar power plants will be introduced, in proportion to the annual economic growth. The transmission and distribution network will be expanded, focusing on the electricity transport, however, there will also be a need for exchanges with neighboring countries. Alternative solutions are needed in order to support the changes of the future grid. High temperature superconductors are an alternative to conventional conductors, due to their high current density and very low AC loss, and therefore deserve more attention. The purpose of this study is to explore ways to integrate high-temperature superconducting cables in a future network and to compare their favorable technical properties with, e.g., the conventional XLPE cable. The development of 2nd generation high temperature superconducting tapes results in a high tape quality, making it very attractive for use in superconducting power transmission cables. At the same time, the network requirements placed on the grid, based on society needs are changing, such as low magnetic field emissions, reducing space requirements, lower losses, minimizing visual intrusion, etc. Our study shows that superconducting cables compared to conventional cables score better on these societal requirements. From our comparison of three practical low and high temperature superconductors we can conclude that Yttrium Barium Copper Oxide is the most suitable superconductor for use in transmission cables. Our techno-economic analysis shows that superconducting cables become already competitive with conventional cable in the AC transmission, such as XLPE cables. Possible future problems concerning the transport capacity in the power grid where high temperature superconducting cables can offer a solution have been identified. For one promising location, we have formulated the requirements for the design of a high-temperature superconducting cable. Next, we propose two types of cable systems (with cold and warm dielectric). For both types we describe the core, the electrical insulation, the screen, the cryostat, the cooling system, etc. Also for the distribution grid a techno-economic investigation is conducted. To assess the feasibility of the application in distribution grids, we have experimentally demonstrated a reduction of AC conductor losses from 1 W/m to 0.1 W/m. We also carried out an experimental investigation to improve the developed cryostat design for a 6 km long cable connection. Despite these substantial technical improvements our economic study showed that the high-temperature superconducting distribution cable is not yet competitive with the present conventional distribution cable systems except for niche locations where additional advantages e.g. magnetic emission, reduced space usage, power density weighs more heavily. Besides the HVAC grid the Netherlands has HVDC interconnections with neighboring countries and there are initiatives for the use of DC high voltage connections to wind farms further out to sea. In our study, we make reference to a suitable location, where the above mentioned attractive features of the superconducting cable are applicable as well. We advised a modified design of a HTS HVDC cable which enables a possible upgrade of the transmission capacity of the HVDC link at such location. The main results from the investigation are that: • Based on our techno-economic analysis HTS cables offer the most competitive solution in transmission grids. Introduction of such cables will reduce HTS tape price, which in turn will stimulate further applications. • Conceptual designs of competitive HTS AC and DC transmission cables are formulated along with that for HTS AC distribution cable. Novel designs allow for much longer length between cooling stations. • Our experimental research has shown that HTS cable core losses may be reduced by a factor 10 (down to 0.11 W/m/phase at 3 kArms, 77 K, 50 Hz). • Dedicated low friction cable cryostat was developed and successfully tested for 47 meters length. Patented multi-layer thermal insulation improves the cable cryostat heat leak from 1 W/m to 0.1 W/m.