Overcurrent Characteristics on a 22 kV/12 kA HTS Model Cable

Generally, electric power is transmitted from a generator through a step-up transformer to high-voltage transmission lines. The feeder from the generator to the step-up transformer is often installed with an aluminum bus line and so on because the current through the feeder is quite large; likely more than 10 kA. Even a superconducting cable can transmit a large current in compact size, so this type of feeder from a generator is expected to be one of the applications of superconducting cables. Therefore, together with Tokyo Electric Power Company, we have been developing HTS cables for feeder lines and constructed a 10-m model cable with 22-kV/12 kA capacity. Several performance tests were conducted on the model cable, including critical current measurement, nominal current application, ac loss measurements, and ac voltage withstanding tests. Overcurrent tests were also conducted. In the TEPCO grid of 22 kV, a fault current is estimated to be 63 kA for 0.6 s. Because the fault current is about five times larger than the nominal current of 12 kA, when such a large fault current goes through an HTS cable, the superconductor is expected to return to the normal state to make a large amount of heat generation. These phenomena may cause a variety of problems. Due to the limitations of the current supply apparatus, the overcurrent tests were conducted with dc current instead of ac current at an LN2 temperature of 77 K and pressure of 0.2 MPaG. The critical current of HTS cable was 19 kA, and the dc overcurrent was 38 kA for 5 s, with the generated energy estimated to be greater than that from 63 kA for 0.6 s. Test results showed the maximum pressure increase to be approximately 20 kPa, which is not so large as to cause problems. The critical current tests of the HTS cable found no degradation between before and after condition. However, it is not certain whether significant pressure increases might happen in a longer HTS cable system, such as an actual case 100 m long. Therefore, we have also been developing a simulation code and will compare the experimental result with the analytical one.