Abstract
This article presents the results of comparative studies on streamer propagation and breakdown in a point-insulating plate electrode system in mineral oil and two ester liquids. The studies were performed for a 10-mm gap and a positive standard lightning impulse. The work was focused on the comparison of light waveforms registered using the photomultiplier technique. The results indicated that both esters demonstrate a lower resistance against the appearance of fast energetic streamers than mineral oil. The reason for such a conclusion is that the number of lightning impulses supplied to the electrode system for which the above-mentioned fast streamers appeared at a given voltage level was always higher in the case of ester liquids than mineral oil. In terms of breakdown, the esters tested were assessed as more susceptible to the appearance of breakdown in the investigated electrode system. The number of breakdowns recorded in the case of esters was always greater than the corresponding number of breakdowns in mineral oil. This may be supposed on the basis of the obtained results that imply that, in both synthetic and natural ester, the formed breakdown channel, which bridged the gap through the surface of pressboard plate, is characterized by higher energy than in the case of mineral oil.
Highlights
Synthetic and natural esters as alternative liquids for mineral oils have become more and more popular in transformer applications
Contrary to the results obtained for ester liquids in the case of mineral oil, only 11 of 20 oscillograms registered at 1.4 Vi concerned the propagation of fast streamers, 14 of 20 at only 11 of 20 oscillograms registered at 1.4 V i concerned the propagation of fast streamers, 14 of 20 voltage level equal to 1.6 Vi, and all for 1.8 Vi
In the point-insulating plate electrode systems investigated in these studies, the variety of phenomena which are possible to observe at a given voltage level is greater than that in classically applied electrode systems, including bare electrodes
Summary
Synthetic and natural esters as alternative liquids for mineral oils have become more and more popular in transformer applications. Among the features of the streamers developing in dielectric liquids, the most important are the streamer shapes, currents, and light waveforms, as well as propagation velocity All of these indicators change with the so-called propagation modes which occur together consecutively with voltage increases in the given electrode configuration. In the comparative assessment of the liquid behavior at lightning impulse stress, an especially important aspect is the determination of the value of voltage, at which a change in propagation mode from slow to fast occurs This value has been called the acceleration voltage (and marked commonly as V a ) and has been related to the value of inception or breakdown voltage for a given electrode setup. Energies 2016, 9, 467 liquids in the same testing conditions may allow for the assessment of the differences and similarities between them [14,15,17]
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