Abstract
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.
Highlights
The main component of transformer internal insulation is oil-paper composite insulation system.At present in abroad and domestic, mineral oil and natural ester are two main kinds of insulating oil used in large power transformers
This paper investigated the particle motion characteristic in both mineral oil and natural ester, and compared the effect of particles on the conductivity and breakdown of oil under DC electric field
The motion and accumulation characteristics of fiber particles vary a lot from copper particles
Summary
The main component of transformer internal insulation is oil-paper composite insulation system. Compared with the research results related to mineral oil, the impact of impurities on the insulation performance of natural ester is still lacking. There are few researches focusing on the differences in the bridging characteristics of metal and non-metal particles in natural ester, as well as the influence of impurities accumulation on the breakdown of natural ester. This paper investigated the particle motion characteristic in both mineral oil and natural ester, and compared the effect of particles on the conductivity and breakdown of oil under DC electric field. The movement and accumulation behavior of fiber particles and copper particles in mineral oil and natural ester were recorded and simulated. The DC breakdown strength of mineral oil and natural ester with different particles concentration were compared
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