Effect of Cellulose Particles on Breakdown Voltage in Wet FR3 Natural Ester

Due to the polar nature of natural ester, moisture content which may cause deterioration of electric strength is significantly higher in natural ester than that in mineral insulating oil. While the polarity of cellulose particles is stronger in terms of molecular structure than that of the natural ester, the breakdown voltage would be changed with increasing moisture and the cellulose particles. In light of this, the paper presents experimental studies on the effect of cellulose particle on breakdown voltage in wet FR3 natural ester. The DC breakdown tests are conducted and the breakdown voltages for three particle contamination levels, normal, marginal and high are investigated with five moisture content intervals, i.e., less than 100 mg/L, 100-200 mg/L, 200-300 mg/L, 300-400 mg/L and 400-500 mg/L respectively. The results show that the increase of cellulose particle contamination significantly accelerates the formation of cellulose bridge. Both the cellulose particles and moisture content have notable effects on the DC breakdown voltages. Higher cellulose contamination level in FR3 can lead to the decrease of DC breakdown voltage and more remarkable effect can be observed under the combined effect of the moisture. Analyses based on the relative dielectric constant and DC resistivity of the natural ester indicate that the reduction of the natural ester dielectric strength may be mainly attributed to the sharp decrease of the DC resistivity induced by the cellulose particles and the moisture. Also, the comparison of breakdown results between the DC and AC suggests the combined effect of particle and moisture in the natural ester breakdown behaviors.