External flashovers, related insulation failures and corrective measures in converter stations of Nelson River bipole 1 and bipole 2

When Bipole 1 was placed in service in the 1970s at +450 kVdc, bushings were found to be vulnerable to external flashovers. Bipole 2 at +500 kVdc in the 1980s had bushings with increased strike distance of 1 m and creepage increased from 32.9 mm/kV to 43.6 mm/kV. This made the Bipole 2 bushings vulnerable to radial puncture under non-uniform wetting. Booster sheds were installed to greatly reduce external flashovers of porcelain bushings in both HVDC bipoles. Recently the porcelain clad, oil filled wall bushings of Bipole 1 have been replaced with oil-free composite wall bushings having silicone rubber sheds.