Akifumi Inui

Dielectric characteristics of static shield for coil-end of gas-insulated transformer

The impulse breakdown characteristics of insulation models simulating static shields for coil ends of SF/sub 6/ gas-insulated transformers are studied. The static shield is composed of a double-covered electrode and a filler between the coverings. If a filler made of elastomer material is fitted closely on the covered electrode to eliminate gas gaps between the filler and the covered electrode, the impulse breakdown voltage of the static shield is higher than with gas gaps. If a low-permittivity filler is used, the impulse breakdown voltage of the static shield is still higher. The breakdown voltage can be explained by considering that the breakdown is caused by small gas gaps between the filler and the covered electrode. The dielectric strength of the coil end can be increased by applying a static shield with an elastomer filler. >

Barrier effects of surface discharge propagation in SF/sub 6/ gas

The effects of a barrier with PET film on surface discharges in a composite insulating system in SF/sub 6/ gas were investigated. The relationship between surface discharge propagation characteristics and the protection effect of barriers was clarified. If a barrier has a right-angle bottom, the positive breakdown voltages are higher but the negative breakdown voltages remain unchanged. If a barrier having a bottom is installed near the high-voltage electrode, positive breakdown voltages are higher under a higher gas pressure, but are nearly the same under a lower gas pressure as without a barrier. If a barrier having a bottom is installed near the high-voltage electrode, a barrier effect is also observed with negative discharges, allowing breakdown voltage to rise.<<ETX>>

Static Electrification Phenomena on Dielectric Materials of SF6 Gas-Insulated Transformer

In Japan, large-capacity gas-insulated transformers (275kV, 300MVA) designed to be cooled by flowing SF6 gas have been developed and are operating in underground substations.1 They use insulating plastics with excellent heat resistance and insulation strength, in addition to cellulose materials such as insulating paper and pressboard used in conventional oil-immersed transformers. Such insulating plastics have a high surface resistance and can be electrified through contact with each other, delamination, or friction. Again, the resistivity of SF6 gas is so high that once the insulation material surface is electrified, charges are slow to leak out.