Comparison between insulating properties of transformer oil and a low-flammability ester Midel 7131

The paper presents alternating and impulse breakdown voltages for paper, Nomex 410 and Nomex 411, impregnated with naphthenic transformer oil or Midel 7131. The samples contained an oil gap between a pair of coplanar rings, and the breakdown voltages are discussed in terms of the matching of liquid and impregnated tape permittivities which were also measured. The alternating breakdown voltages appear to depend on the degree of stress raising in the liquid gap, whereas the impulse voltages apparently do not. The measurements indicate that Midel could replace transformer oil to impregnate either Kraft paper or Nomex without detriment to alternating or impulse withstand voltages. Midel appears inferior to transformer oil in a large gap between spheres. Nomex gives a better AC performance than paper. The performance at elevated temperatures, AC discharge levels and insulation life were not investigated in this study.