DC Dielectric Strength of a SF6 Gas Insulated System

The breakdown characteristics of gas insulated systems were investigated for DC and switching impulse voltage superposed upon DC of opposite polarity in laboratory-clean conditions. The breakdown voltage of a SF6 gas gap for DC voltage is approximately 80 percent of that for AC crest value at pressure of 3.5kg/cm2 abs., while an epoxy cast gap withstands the DC voltage by 1.7 times of the AC crest value. However, when the short epoxy cast gap is subjected to the combined voltage of DC and switching impulse of reverse polarity, the switching impulse breakdown voltage is strongly affected by the preset level of DC voltage. On the other hand, the superposed switching impulse breakdown voltages of conical insulators, for supporting a high voltage conductor in a cylindrical enclosure, are only slightly influenced by the DC voltage of reverse polarity. Therefore the combination of SFb gas and epoxy resin may be used effectively as an insulation system of the HVDC gas insulated substation.