Potential of cryogenic liquids for future power equipment insulation in the medium high voltage range

Cryogenic liquids show a noteworthy impact on the concept of improved future power equipment with superconductors. Superconducting materials eliminate any Joule heating but are based on very low operating temperatures. A high overall current density in the equipment may be presupposed. Cooling however must be guaranteed by cryogenic liquids, i.e. liquid helium or liquid nitrogen, respectively. The cooling liquids are interesting basic insulating fluids with a reasonable dielectric performance in the medium high voltage domain. Most exciting is the chemical inertness. Hazards such as fire risk in case of severe faults can be ruled out, thus reducing insurance cost drastically. Thermal aging of the liquids can also be disregarded because no chemical reaction must be taken into account during service, even in the case of temporary overload operation. Dielectric aging due to partial discharges may be of little concern because the liquids can be fully reprocessed easily by vaporizing and recondensing in a closed cooling circuit; no memory effects may degrade the liquid at all. On the other hand, overload operation may yield a large amount of thermal bubbles which can affect the dielectric strength seriously. Finally, the liquids are produced from gases which are found in the open atmosphere. There is no danger of any poisonous reaction in case of uncontrolled leakage, and the disposal of insulation systems at the end of operational life is straightforward without the need of costly processing.