for differential thermal expansion occurring for example when the transmission line is first cooled down by sliding at the ends of the tubular superconductor portions the cooling tube 3 is preferably corrugated which imparts some flexibility to it the space between the tube 3 and the tubular elements 1 is filled with a filling material 11 procuring good thermal conduct between the tube 3 and the tubular superconductor elements one such material is epoxy resin or polyurethane resin for example or preferably polyamide 6 resin basf lutramid b4 one embodiment of the invention is now described this is a 1 gw three phase transmission line 400 meters long conveying a nominal current of 28 ka at a voltage of 20 kv each phase transmission line comprises the components described above namely a superconductor a cooling tube and an insulating tube the information given below concerns one phase transmission line the phase transmission line is made up of 40 tubular portions of bi 2 sr 2 cocu 2 o 8 ceramic each 10 meters long and 200 mm in diameter under superconducting conditions the current density is 100 a mm 2 the inside diameter of the copper cooling tube is on the order of 100 mm and its thickness is approximately 4 mm the liquid nitrogen used in normal operation is supercooled nitrogen injected at 65 u00b0 k at one end of the phase transmission line and collected at the other end at 75 u00b0 k the flowrate is approximately 1 8 kg s the outside diameter of the outside steel tube is in the order of 750 mm and its thickness is on the order of 5 mm the vacuum pressure is approximately 1 torr during normal operation the losses of the three phase transmission line.