Review of requirements and diagnostics of oil/board DC-insulation systems

Since the beginning of the 20th century, the main insulation of power transformers has been built up efficiently with oil-impregnated cellulose, and since the 1960ies as layered oil/board-insulation. The latter represents a hybrid system, consisting of oil filled gaps subdivided by pressboard barriers. Different new mineral oils have differing values of conductivity (about a factor of 10), whereas the conductivity of solid insulation hardly changes. Due to this fact, designing HVDC converter transformers needs special attention. In addition, aging and water content also affect conductivity significantly. Furthermore, additional effects occur by application of an electrical DC field, which are in many cases not investigated or these effects are not considered. This paper deals with required parameters of oil/board insulating systems and methods for oil/board insulation diagnostics with special focus on HVDC applications.