Dielectric measurements as quality criterion for liquid nitrogen

For the operation of high temperature superconducting (HTS) devices it is necessary to cool the HTS conductor down to the operating temperature of about 77 K. Depending on the application warm or cold dielectric insulation systems are used in electrical power systems and high voltage apparatus. Liquid nitrogen (LN₂) is an excellent dielectric liquid, but there is less knowledge about the quality criterions and their measuring methods. For the measurement of dielectric parameters (e.g. breakdown voltage, partial discharge, relative permittivity) the classical measuring methods have to be adapted. This paper gives a view to the results of dielectric measuring methods with LN₂. The measurements were done with different degrees of LN₂ pureness.