Liquid nitrogen cooling system for superconducting electricity cables – experiences and advantages.

High temperature superconducting cables or bus bars are installed inside of vacuum-insulated cable cryostats through which subcooled liquid nitrogen is circulated to take out heat impact through the insulation or caused by AC-losses. The minimum cooling temperature is 64 K (-209°C). Lower temperatures are not practical, because at 63 K (-210°C) nitrogen becomes solid. A small temperature increase of the circulated nitrogen inside of the cable cryostat is acceptable, but the temperature must always stay several degrees below the boiling point for a reliable operation of the cable. An active cooled subcooler has to be integrated in the circulation system to compensate the heat impact of cable losses and circulation pumps. Furthermore the circulation needs a very stable pressure regulation system and an expansion vessel to compensate volume fluctuations. Messer has developed such a cooling- and pumping unit for operating a 10 kV concentric HTS cable (40 MVA) from Nexans which is integrated into the commercial electricity grid of Innogy, Germany. This worldwide longest HTS-cable with a length of 1,000 m is continuously in operation since March 2014 (AmpaCity-project) and delivers electricity into the city center of Essen. The subcooler is operated with liquid nitrogen from a storage vessel. Compared to cooling with mechanical cooling machines this has a lot of advantages. Reliability is very high because the unit can be supplied from several external nitrogen sources. The liquid nitrogen operated subcooler has very much cooling capacity reserve which is necessary for initial cooldown of the system to avoid cavitation of the circulation pumps during start up. Overall energy efficiency is high because the big industrial nitrogen liquefiers can be operated much more efficient than smaller on site cooling machines.