The heat leak of the cryogenic pipe constituting a transmission line of the superconducting power transmission depends on the surface temperature of the cryogenic pipe, because the heat leak is mainly caused by the radiative and conductive heat transfers. The heat leak of the cryogenic pipe for the superconducting power transmission has been measured with respect to its surface temperature ranging from -3 to 53 °C to investigate the effect of the surface temperature variation and to obtain data for the installation of cables in different places with different environmental conditions. The cryogenic pipe used for the measurements has two inner pipes, called cable pipe and return pipe, in a single outer pipe. For the cable pipe the heat leak of 0.82 W/m at -3 °C increases to 1.27 W/m at 51°C, while for the return pipe the heat leak of 0.46 W/m at -3 °C increases to 0.94 W/m at 51°C. The heat leak increases smoothly with the surface temperature, which can be well reproduced by a formula based on the temperature dependences of the radiative and conductive heat transfers. The heat leak by the conductive heat transfer is discussed based on the experiments and the calculations.

Abuttact-A "prospective power transmission model system integrated under superconducting environment", abbreviated to the PROMISE, has been constructed to verify a technical possibility of superconducting power transmission for next generation. The PROMISE is composed of a superconducting transformer, a superconducting fault current limiter and superconducting power cable of length 5m. Three superconducting apparatuses are enclosed all together in a long scale cryostat to be kept at the liquid helium temperature of 4.2K. The major insulation is provided by liquid helium. The PROMISE withstands AC voltage of 6kV for 2 minutes with quite low partial discharge. A voltage-current synthetic test has proved that the PROMISE has the transmission capability of 6,OOOV-1,OOOkVA. The fault current limiter actually limits overcurrent and prevents the other apparatuses from quenching. Quench current level coordination is actually realized in the PROMISE.