Development of a LN2 Impregnated High-Voltage Condenser Bushing for the Use in Liquid Nitrogen

Inhalt:
An initial development approach for cryogenic high-voltage condenser bushings is described which uses different insulation materials impregnated with liquid nitrogen (LN2) as insulation. The aim is to evaluate if it is possible to produce LN2 impregnated condenser cores that fulfill the basic requirements for the use in superconducting devices cooled with LN2. After carrying out electrical calculations, meeting at least the requirements of the operating voltage Um = 52 kV, three different condenser cores with conductive inserts were produced using insulating materials NKN (Nomex(r)-Kapton(r)-Nomex(r)), PPLP (polypropylene laminated paper) and a conventional insulating crepe paper. The NKN and the PPLP core were subjected to the rated AC withstand voltage test at 95 kV for 5 min, the NKN core to the rated lightning impulse (LI) withstand voltage test at 250 kV and all three to the partial discharge (PD) test. All tests were done in a cryogenic environment. The NKN core was investigated in a bath cryostat at 5 bar absolute, whereas the PPLP and the crepe paper core were investigated in open bath containers at ambient pressure. The NKN condenser core could successfully pass the AC withstand voltage test and was successfully tested up to a value of 208 kV without breakdown. The rated LI withstand voltage test could be successfully carried out, too. The PPLP condenser core was exposed to AC voltage that it could withstand. The limit value of the PD test of 5 pC at the required extinction voltage could not be met either by the NKN condenser core nor by the PPLP one. The crepe paper condenser core did not pass any of the tests.