The studies of electrical discharges developing in dielectric liquids under lightning impulse voltage have been conducted for many years. However, these studies have concerned mostly the systems of bare electrodes under non-uniform electric field distribution (point-plane or point-sphere configurations). In such systems, the influence of the type of liquid on the discharge development is quite well recognized. This concerns not only the discharges inception  but also the breakdown as well as the acceleration voltages. So far limited studies using insulated electrodes have been reported in the literature. Since bare electrodes practically do not exist in the actual insulating systems of transformers where the electrodes are always covered by paper,  it is therefore very important to investigate insulated electrode configurations. Hence, the authors propose in the paper, investigating the discharge inception and breakdown voltages of three type of liquid.  The HV electrodes consists in a conductor insulated by cellulose creep paper while the grounded electrode is made of aluminum plate covered with 1 mm thick cellulose pressboard. The gap between them was set to 5 mm. The considered liquids are inhibited naphthenic mineral oil, bio-based hydrocarbon, synthetic ester and natural ester.
The studies are focused on the inception and breakdown voltages under negative lightning impulse voltage. These parameters were determined using a step method.  A photomultiplier and a voltage divider were respectively used to monitor the inception and breakdown voltages. The results are analyzed statistically using the Weibull distribution based on 10 experimental values. The comparison is also performed in terms of registered light oscillograms related to the pre-discharges phenomena. It is important to pinpoint herein that the tested systems are always dried and impregnated with the liquid used further in the experiment.
The analysis of the results obtained indicates on similarity in discharge inception mechanisms in the liquids considered. The difference is lower than one voltage step (5 kV), which may be treated as negligible from statistical point of view. Similarly, the differences in breakdown voltages are marginal. Hence, a simple conclusion from the studies is that in the tested system of insulated electrodes there is no influence of type of liquid on inception and breakdown characteristics.

dielectric liquids;,breakdown voltage,lightning impulse voltage,insulated electrodes