Improved feedthrough with recessed triple junction for reduction in surface PD

Surface PD (partial discharge) on an insulator from a TJ (triple junction) are reduced by recessing the TJ at the HV electrode. The insulators have an inner electrode connected to the ground potential. PD pulses whose magnitude is >10 pC are measured. The applied HV (50 Hz) is 1 to 1.5× higher than that at the initiation of the PD. The thickness of the insulator is in a range from 2 to 7.5 mm. For insulator thickness of 7.5 mm, the integrated magnitude and number of the PD pulses from the recessed TJ are reduced to ~20% and 50% of those from TJ without recess, respectively. The reduction in the pD by the recessed TJ is explained by the field conditions on the insulator near the TJ. It is considered that the reduction in the pD by the recessed TJ results mainly from the following three factors: high surface potential (V_s ) on the insulator, weak parallel component of electric field on the insulator surface, and strong normal component of the field on the surface. Propagation of the pD is restricted by these three factors. We have designed the improved configuration of recessed TJ in order to further emphasize these three factors. It is experimentally confirmed that the improved one reduces surface pD more efficiently than the recessed TJ before the improvement. At 7.5 mm insulator thickness, the integrated magnitude and number of pD pulses are reduced to ~5% and 15% of those for TJ without recess, respectively