Partial discharge behavior of mineral oil and oil-board insulation systems at HVDC

Power transformers have been built successfully with oil-board insulation systems for many years. Oil has the function of being a coolant as well as an insulator, whereas the board serves for electrical and mechanical purposes. When dealing with solid insulating materials, special focus has to be set on partial discharges (PD), as they have a destructive nature. On the one hand they may supply charge carriers for breakdown mechanisms and on the other hand they can lead to a carbonization of the organic insulation. In the case of AC, phase-resolved patterns, so called finger prints, can be used to interpret the type of partial discharge. These methods are in use for many years and have proven successful in interpreting possible PD sources. However, in the case of DC, no phase information is available. Therefore other methods for interpreting the behavior of PD at HVDC have to be found. In this paper, a non-uniform test setup for mineral insulating oil and a small oil-board model was used. The different observations were evaluated to discuss the issues of PD measurements at HVDC. In addition to conventional electrical partial discharge measurements also optical detection of partial discharges was used. Furthermore, a scanning electron microscopy (SEM) was used for the high voltage electrodes to gain information about surface changes of the needle electrodes.