DC creepage breakdown characteristics of oil-immersed insulation

This paper deals with dc creepage breakdown characteristics of oil-immersed insulation in HVDC transformers. The authors investigated dc breakdown characteristics using element insulation models and a full-scale model with a high creepage electric field on the insulation surfaces. The results are as follows: The dielectric strength of creepage insulation in non-contact with electrodes was more than twice higher that of creepage insulation in contact with bare electrodes. The dielectric strength is dependent on the creepage distance of surface insulation and decreases to about 45% in case of surface insulation with ten times creepage distance.