Comparative study on the dynamic migration of cellulose particles and its effect on the conductivity in natural ester and mineral oil under DC electrical field

For a HVDC transformer, the cellulose particles bridging phenomenon under the DC electric field needed to be analyzed. This is useful for further understanding the insulation failure and partly guiding the transformer structure design. The images of cellulose particles accumulation and the conduction current for contaminated mineral oil and natural ester were compared under different DC electrical field strength and different electrode distance using plate–plate electrode system. The cellulose particles bridging model under the DC electrical stress was constructed. The bridging process of cellulose particles in mineral oil is much easier compared with that in natural ester, and the bridges in mineral oil were also thicker than that in natural ester. The contaminated mineral oil has much higher conduction current than the contaminated natural ester at the same electrical stress. The saturate conduction current of contaminated mineral oil decreases in a non‐linear way with the electrode distance increasing, while the saturate conduction current for contaminated natural ester increases in a linear way. The completed bridges contribute a lot to the conduction current. The effect of cellulose particles on the conduction property of natural ester is weaker than that of mineral oil..