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 "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 "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.

This paper reports on experimental investigations dealing with the influence of carbon particles on the lightning impulse flashover behavior of the interface between insulating rods of different permittivities and surface structures and transformer oil in a non-uniform electrical field. The results reveal that for negative lightning impulse stress carbon particles in the oil act as defects due to their positive polarity and thus reduce the breakdown and flashover strength of all investigated materials distinctly. For positive impulse polarity no significant detrimental impact of the carbon particles on the electrical strength can be made out. Furthermore the contribution displays that in clean oil as well as in carbonized oil the probability of a surface tracking is for all investigated spacer materials markedly lower at positive impulse polarity than at negative lightning impulse stress.

This paper reports on experimental investigations dealing with the impact of dissolved and disperse water on the breakdown strength of open oil gaps and the flashover behavior of solid/liquid interfaces in a uniform electrical AC field using clean as well as carbonized transformer oils. The measurements reveal that in technically clean oil with disperse water at small gap distances the breakdown strength can for subsequent voltage stresses be significantly higher after the first initiation of a breakdown than during the fmt readings while for larger gap spacings this effect cannot be noticed. Moreover, it is shown in this contribution that especially for practically relevant long gap distances carbonized oils can exhibit a better electrical strength in the presence of high absolute water contents than clean oil. This finding applies to the breakdown voltage of open oil gaps as well as to the flashover behavior of soliaquid interfaces.

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Relative permittivity E, at 20 "C Water solubility w, at 20 "C Loss factor tan 8 at 20 "C This paper reports on experimental investigations dealing with the impact of different water and carbon contents on the electrical strength of oil gaps without and with a solid/liquid interface in a uniform field. The results reveal that depending on the amount of carbon, the water content, and the gap distance the electrical strength of carbonized oils needs not necessarily to be lower than for clean oil under the same conditions. Moreover it will be shown that the combined impact of different carbon and water contents on the electrical strength of long oil gaps cannot in any case be deduced from results obtained at short gap spacing.2.2 2.4.10" 44 PPm