Geometric effects in the electrical breakdown of transformer oil

Since experimental breakdown studies of transformer oil are often carried out in different geometries, researchers often tend to correlate their results with other studies by means of an area or volume effect. Such correlations are also found within a given study. Yet, breakdown data also support a gap effect whenever this is a variable. Indeed, transformer designers routinely subdivide large oil gaps by means of pressboard barriers in order to achieve higher breakdown strengths, indicating a practical belief in a gap effect. A gap effect has usually been associated with a Townsend type of avalanche breakdown which, for a variety of reasons, has not found much favor as an explanation for oil breakdown. We explore such a Townsend-like breakdown process here for transformer oil and show how, when applied to different geometries, it can mimic an area or volume effect. Although an area effect may be present in terms of initiating a breakdown via localized field enhancement, it cannot account for the breakdown mechanism itself.