Research on the process of the transformer winding inter-turn discharge caused by corrosive sulphur

Corrosive sulphur has been identified to cause unexpected failure in electric power transformer in recent years. In this paper, The winding samples were aged in 25# Kunlun transformer oil with sulphur powder to form sulphur corrosion. Then their partial discharge inception voltage and partial discharge developing process were researched by high voltage experiments. Tens samples were aged for 14, 24 and 30 days in oven of 130 ć with sulphur concentration of 0.3mg/g. Fourier infrared spectral analysis was done to confirm the cuprous sulfide has deposited on the surface of the insulation paper. Tearing strength of the paper bands of the windings was measured to analyze the influence of the corrosion to the paper. In order to research the influence of the sulphur corrosion to the dielectric characteristic of the windings, the partial discharge inception voltage and the developing process of the partial discharge were measured under step raising high voltage. The results show that the sulphur corrosion reduces the tearing strength and the partial discharge inception voltage so that the corroded transformers have more risk to partial discharge and short circuit failures. The PDs of the windings has a developing process. The average magnitude, maximum magnitude and PD numbers increase with high voltage or time obeyed an exponent rule, but the pattern of the phase distribution point graph of the PDs has almost no change. According to the increasing speed, the developing process has three stages. At the first stage, the PD phenomena were stable for very long time, the PD number and PD magnitude had not obvious increase. At the second stage, the PD number and PD magnitude suddenly increased with exponent rules. At the third stage, there were more and more very big PD pulses. The PDs can be divided into small ones and big ones. These research are valuable for the life evaluation and condition monitoring of large electrical power transformers.