Dependence of Uniform/Nonuniform Field Transformer Oil Breakdown on Oil Composition

The 60-Hz and impulse breakdown characteristics of two oils, A and B, which differed markedly in needle-sphere (N-S)impulse strength were determined in uniform and divergent fieldgeometries. Oil A, which had the higher N-S strength, was acommercial transformer oil, and Oil B was this same oil dopedwith anthracene. In the uniform field tests, the two oilshad essentially equal bare-electrode 60 Hz and impulsestrengths and covered-electrode 60 Hz strength. Oil B, thelower N-S strength oil, had a slightly higher covered-electrodeimpulse strength. In weakly divergent fields, the 60 Hzbreakdown and corona inception levels of Oil B were comparableor superior to those of Oil A. Impulse breakdown voltageswere found to depend on oil composition and impulse polarity; however, these dependences varied with field divergence.The data suggest that the anthracene additive modified thedivergent field breakdown initiation and the streamer propa-gation processes in oil, and indicate a lack of correlationbetween the highly divergent field N-S test and the insulatingcharacteristics of oils in uniform and weakly divergent fields.This work was part of an Electric Power Research Institutesponsored program to evaluate non-naphthenic transformer oils.