Technical report-particles in transformer oil

Attention is drawn to a parameter that plays a crucial role in determining the breakdown strength of transformer oil, yet which tends to be ignored. This parameter is the settling time, which can mean either the time interval between two consecutive measurements or the time for which an oil sample is stored before measurements begin. The term is used here in the former sense. Research carried out with Bruce profile electrodes and for gap spacings of 1 and 4 mm showed that settling times between measurements of 1 and 5 mm gave widely differing breakdown strength results. The breakdown strength with 1-min settling time was inferior to that for a settling time of 5 min. A particle counting analysis performed on two oil samples supports the hypothesis that short settling times do not allow either the bubbles from the previous breakdown to be dissolved or the decomposition products to settle under the influence of gravity, thereby increasing the probability of breakdown.<>