Safety and Reliability of 66-kV Class HTS Cable Systems in Short-Circuit Current Accidents-Experimental Results on 40-m Cable System

High-temperature superconducting (HTS) cables can transmit large amounts of electricity in a compact size with minimal losses. Therefore, it expects to save the construction cost of underground lines in urban areas and decrease transmission losses. The stability and reliability of HTS cable systems in normal operation has been verified by real grid interconnection tests around the world. While general practical use of HTS cable systems requires the verification of their stability not only in normal operation but also in the event of various accidents (ground fault, short-circuit current, liquid nitrogen leakage, etc.) and understanding these phenomena and their impact. However, a few studies have been conducted to investigate the safety of HTS cable systems in the event of the accidents so far. Regarding the short-circuit current, the maximum current depends on the voltage class and grid conditions, and it was set as 31.5 kA for the 66 kV systems in Japan. In order to verify the phenomena and impact to the cable systems, a 40-m cable system was constructed and cooled by circulated subcooled liquid nitrogen (LN₂). After the initial critical current (Ic) measurement of the cable system, the short-circuit current tests were conducted with ac 28.5 kA for 0.6 s and 16 kA for 8.4 s. The former testing condition is almost simulated as the current of 31.5 kA and the latter one is as equivalent heat generation in the HTS cable to 31.5 kA for 2 s. As the initial state of LN₂, temperature and pressure of the cable inlet were around 68 K and 0.26 MPaG. The measured temperature and pressure rises of the coolant were not so high to make some damage to the HTS cable. After these tests, the cable Ic was measured again and no damage was found.