When installing a new substation, it is advantageous to construct it at the center of loads for loss minimization; however, it is not easy to secure a substation site in urban areas owing to high land prices, complex permission processes, and increasing social acceptability issues. The use of distribution superconducting cables has been actively promoted as a technological solution.
KEPCO initiated the Shingal Project to utilize the 23 kV triad-type HTS cable with an actual system, and it started its commercial operation in 2019.
However, the disadvantage of the triad type superconducting cable is that it is not easy to utilize it further owing to high investment costs. As an alternative, the development of tri-axial cables at the distribution voltage level without using HTS shield wires, such as those used in the Ampacity project in Germany, resulted in cost savings, but the limitations of the cooling system kept the superconducting cable from exceeding a length of 1 km.
This study shows that the distance can be increased up to 3 km by improving the cooling configuration of the tri-axial HTS cables from the two channels in the same cryostat to an external separate circulation channel for liquid nitrogen(LN2). With the use of 23 kV tri-axial HTS cables with radial power systems, the economic effects were compared with the conventional method. If the HTS cable is used, a 154 kV substation can be eliminated or relocated far from urban areas depending on how the 23 kV HTS cable connection is implemented into the existing power system. In addition, the higher the land price, the smaller the economic gap between the HTS
application and the conventional method.
Moreover, as a new configuration of an electric power grid for urban power supplies, an HTS power platform for using 23 kV tri-axial HTS power cables was presented. In an HTS power platform, two to three 23 kV switch stations are installed near a load’s center, and they are loop connected to the 154 kV transformer substations with 23 kV HTS cables to satisfy the N-1 reliability criteria. Individual loads can be fed through the 23 kV distribution feeders from the 23 kV switching station. It is appropriate for the HTS power platform to have a total load supply capacity of up to 300 MVA including the substation capacity, which is equivalent to two fully equipped 154 kV substations. Considering the load density of 4.5 MW/km2 in the newly developed area, it can supply an area of ∼80 km2.
Therefore, consideration should be given to the proper load supply capacity of 23 kV switching stations and the circuit configuration of 23 kV HTS cables that link the switching stations in urban areas with substations outside the load center. In particular, for determining economic feasibility, construction environments of power system, as well as HTS cables should be considered carefully among the various alternatives being compared. Construction environments include underground construction methods, land prices, and corridor constraints, which can affect economic feasibility. In view of these circumstances, the economic effects of the HTS power platform are compared with those of the conventional method. The HTS cable application can be more economical than the conventional method to a certain distance, although the economics, such as BCR and IRR, may show different values depending on the construction conditions. As a result, it was confirmed that the high investment cost of superconducting cables can be offset by the effects of land acquisition, cable tunnel construction, and the price reduction of the tri-axial HTS cable itself.
One cell of the 23 kV HTS power platform will be installed in an actual power system between KEPCO’s 154 kV Munsan and Sunyou substations. A 23 kV switching station with a 60 MVA capacity will be constructed in the
Chapter 6. Conclusions 134
middle of the cable route instead of a 154 kV substation, and it will be loop connected with the 23 kV tri-axial HTS cables to verify their performance, social, and economic effects. This study also showed the fault current issue of the loop-type HTS power platform can be addressed by installing SFCLs.
The author would like to explain some implications of this study. It was considered that the larger the capacity of the distribution voltage HTS cable, the better it was. However, it depends on their own configuration philosophy such as the number of substations, load supply capacity, the size of supply area, reliability level. Those HTS cables which blends harmoniously with conventional power facilities will not compromise the reliability and efficiency of the energy network. This innovation will have a great synergy effect when HTS technology works together with HVDC, energy storage technologies which are at the forefront of the paradigm change in energy. Additionally, HTS cable enables us to focus on eco-friendly, high-capacity transmission. However, the application of HTS cables can be feasible only for short distances for the time being. A significant price reduction in HTS wires and cooling system will further increase the feasible distance. As KEPCO has been increasing the efficiency of power facility investments and operations by standardizing the capacity of substations and power transmission lines, it is expected that standardization can be established to form an HTS power platform based on this study’s results.