Effects of Fiber and Copper Particles on Conductivity and Breakdown Characteristics of Natural Ester and Mineral Oil under DC Voltage

The converter transformer is a key equipment in high voltage direct current (HVDC) transmission system. Its oil-paper insulation system in the valve winding and outlet bushing experiences AC, DC, AC/DC, and transient impulse voltages simultaneously. The oil contamination problem is more serious under DC electric field. Therefore, it is significant to investigate the characteristic of particles motion and accumulation under DC electric field. In this paper, first, the movement and accumulation behavior of fiber particles and copper particles in mineral oil and natural ester were recorded and simulated. Then, the influence of fiber and copper particles on the oil conductivity was analyzed. Finally, the DC breakdown strength of mineral oil and natural ester with different particles concentration was compared. Results show that the movement speed of copper particles was larger than that of fiber particles. Fiber impurities were easy to form bridges in mineral oil, while there was no impurity bridge in natural ester. The current density of mineral oil containing particles is larger than that of the natural ester at the same testing time. The DC 50% probability breakdown voltages of oil samples containing fiber and copper particles decreased linearly with the increase of particle concentration, and the decrease rate of DC 50% probability breakdown voltages of oil containing copper particles were faster than that of oil containing fiber particles. Compared to pure mineral oil, the DC breakdown voltages corresponding to 50% probability of contaminated mineral oil showed a decrease from 11.9% to 22.5% when the fiber particle concentration increased from 0.001% to 0.012%. The DC 50% probability breakdown voltages of contaminated mineral oil with copper particles decreased from 23.8% to 45.0% when the particle concentration increased from 0.1 g/L to 1.5g/L. However, the decline range of the figures for natural ester contaminated by fiber or copper particles showed a smaller drop.

[1]  H. A. Pohl.  The Motion and Precipitation of Suspensoids in Divergent Electric Fields , 1951 .

[2]  G. Molinari,et al.  Analytical evaluation of the electro-dielectrophoretic forces acting on spherical impurity particles in dielectric fluids , 1978 .

[3]  S. Birlasekaran.  The measurement of charge on single particles in transformer oil , 1991 .

[4]  Chen Chang-jie.  Characteristics of Suspended and Mobile Microbubble Partial Discharge in Insulation Oil , 2010 .

[5]  Lijun Yang,et al.  Comparison of ageing results for transformer oil-paper insulation subjected to thermal ageing in mineral oil and ageing in retardant oil , 2012, IEEE Transactions on Dielectrics and Electrical Insulation.

[6]  G. Wilson,et al.  Study of the charge dynamics in mineral oil under a non-homogeneous field , 2015, IEEE Transactions on Dielectrics and Electrical Insulation.

[7]  Q. Liu,et al.  Effect of cellulose particles on impulse breakdown in ester transformer liquids in uniform electric fields , 2015, IEEE Transactions on Dielectrics and Electrical Insulation.

[8]  G. Wilson,et al.  Experimental studies of influence of DC and AC electric fields on bridging in contaminated transformer oil , 2015, IEEE Transactions on Dielectrics and Electrical Insulation.

[9]  Yuan Li,et al.  Study on micro bridge impurities in oil-paper insulation at DC voltage: their generation, growth and interaction with partial discharge , 2016, IEEE Transactions on Dielectrics and Electrical Insulation.

[10]  Li Yuan,et al.  Generation Process of Impurity Bridges in Oil-paper Insulation Under DC Voltage , 2016 .

[11]  Q. Liao,et al.  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 , 2017 .

[12]  Jing Zhang,et al.  Copper Particle Effect on the Breakdown Strength of Insulating Oil at Combined AC and DC Voltage , 2017 .

[13]  Alan Sbravati,et al.  Increased loadability of transformers using natural ester and cellulosic materials as high temperature insulation systems , 2018, IEEE Electrical Insulation Magazine.

[14]  Lin Cheng,et al.  Accumulation Behaviors of Different Particles and Effects on the Breakdown Properties of Mineral Oil under DC Voltage , 2019, Energies.