Influence of carbon particles on the breakdown voltage of transformer oil

This paper reports on experimental investigations dealing with the influence of carbon particles on the breakdown voltage of mineral based transformer oil in a temperature range between 20 and 110 degrees C. The oil was carbonized by stressing it with defined numbers of lightning impulses of a known energy. Special emphasis is laid on the dependence of the electric strength on the number of impulses, the impulses' energy, and the water content of the fluid. The results reveal that for temperatures above 50 degrees C the breakdown voltage of small oil gaps is predominantly governed by the strength reduction due to the presence of carbon particles. The humidity of the oil does no longer significantly influence the breakdown strength for high temperatures.