Strength investigations on oil-board-arrangements in uniform electrical field

The oil-board insulation system is considered as competitionless standard insulation system in the high voltage technique for transformers, in particular to the high power range. The present knowledge status assumes, that for oil-board insulations always partial discharges occur, before breakdowns take place. In this paper it will be shown, that in uniform electrical field it is possible, that not always partial discharges can be measured before breakdown. Board has a specific resistance which is up to 100 times higher than that for oil. Charges can stick on the board surface and influence the electrical field and the electrical breakdown strength. Measuring this charging could be a new procedure for the non-destructive electrical strength limit recognition. This charging measuring technique is based on the fact, that board can be charged with an electrical field. The partial discharge measurement is a good method for failure recognition, but not always sensitive as indicator for the electrical strength recognition in oil-board insulation. In some cases the charging measurement method can be useful as method for determination of the electrical loading limit.