Influence of Specimen Size on the Dielectric Strength of Transformer Oil

The results of some specimen size effect experiments on technical grade transformer oil are reported. Four electrode areas (1.77, 8.19, 37.9 and 177 cm2) and four gap spacings (0 .215 0.464, 1.0 and 2.15 mm) were investigated using a fast ramp (5.5 x 1012 V.cm-1.sec-1) and an ac ramp (12 kV. cm-1.sec-1). Whilst the results are in general agreement with weak link theories, it is suggested that physical specimen size factors play a significant part. Examples of physical size factors are spark conditioning, electrode profile, and liquid motion.