Insulating systems for HVDC power apparatus

It is pointed out that reliable HVDC insulation design has to take into account the fact that electric field distributions are different from AC distributions due to resistivities of the materials. Additionally, polarity reversals and transients result in field distributions different from steady state AC and DC fields. External effects such as wetting of surfaces result in dramatic and often unexpected changes. These effects, appropriate design rules, and technical solutions are discussed both for internal and external insulating systems for two examples for insulating interfaces: (1) the oil-end of a bushing in an oil immersed environment, and (2) the air-end of a bushing under quickly changing conditions (e.g. partial wetting). In the first case well designed barrier systems and in the second case hydrophobic surface properties will solve the associated problems