Insulating systems for HVDC power apparatus

Well-known design rule for AC power apparatus insulation cannot be applied to HVDC insulation design in many cases. It has to be taken into account that electric field distributions are quite different from AC distributions due to the resistivity 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 by means of two examples for insulating interfaces which are typical but very critical examples of highly stressed insulations