Effect of cellulose particles on impulse breakdown in ester transformer liquids in uniform electric fields

Abstract

This paper presents experimental studies on the effect of cellulose particles on breakdown voltages and breakdown mechanisms of a synthetic ester and a natural ester under standard lightning impulse voltages. A mineral oil was also tested and used as the benchmark. In total, liquids with four particle contamination levels, i.e. low, normal, marginal and high; two moisture levels, i.e. dry and wet conditions, and three types of electrodes, i.e. plane-plane, rod-rod, and VDE electrodes were investigated. Results showed that the presence of cellulose particles reduces the 50% breakdown voltages of the ester liquids and the mineral oil in uniform fields, which is further enhanced when the particle is combined with the moisture. The particle effect on breakdown voltages also becomes more prominent when the effective stressed area/volume of the electrode configurations is increased. Nevertheless, at the same conditions, 50% lightning impulse breakdown voltages of the ester liquids are always comparable to that of the mineral oil. Linking with previously published results under AC voltages, it is summarised that the particle effect on liquid breakdown is associated with the effective stressed area, the duration of electric stress and the viscosity of the liquid itself. In addition, the streamer imaging tests revealed that the detrimental effect of cellulose particles on breakdowns in liquids is due to the promotion on streamer initiations rather than on streamer propagations.