Degradation and Partial Discharge Characteristics of Typical Defects in Converter Transformer Valve-side Bushings

The Insulation failures of dry-type bushing on the valve side of UHV converters often begin with manufacturing defects and progress to eventual insulation failure. In this paper, two types of specimens, sliced and scaled bushing, were designed to contain three types of typical defects: metal particles, air gaps, and foil breakage. An experimental platform for the diagnosis of electrical degradation and partial discharge of the specimens was built. The degradation channel morphology and partial discharge characteristics of the three types of defects were studied. The results showed that the electrical tree of the specimens with metal particles defects showed a branch-type morphology and contained round carbonized areas; the electrical tree of the specimens with air gaps defects showed the pine-type morphology with dark color; and the specimens with foil broken defect first showed a branchtype morphology and then broke down twice. The discharges of the metal particles and air gaps defects were concentrated in quadrants 1 and 3, and the foil-broken defects had lower partial discharges in the vicinity of the AC voltage over-zero point.