Test of a 400-kV Compact Bushing

A two-stage compact bushing has been developed by CEA-Cadarache and installed on its 1-MV testbed. This bushing acts as high-voltage feedthrough between a transmission line under pressurized nitrogen or sulfur hexafluoride (SF6) gas and a vacuum tank with a volume of 30 m3. The bushing has been successfully tested up to 420 kV. Electron field emission causes a dark current to flow between the electrodes in the vacuum, both inside or outside the bushing in the vacuum tank. High-voltage conditioning leads to a significant reduction in this dark current. The dark current can be further reduced or even suppressed by adding low-pressure (~1 mPa) nitrogen or SF6 gas to the vacuum.